controlling twospotted spider mite (tetranychus...

CONTROLLING TWOSPOTTED SPIDER MITE (Tetranychus urticae Koch) IN

FLORIDA STRAWBERRIES WITH SINGLE AND COMBINATION TREATMENTS OF Phytoseiulus persimilis Athias-Henriot, Neoseiulus californicus (McGregor), AND

ACRAMITE

By

ELENA M. RHODES

A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE

DEGREE OF MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2005

Copyright 2005

by

Elena M. Rhodes

This thesis is dedicated to the Lord and to the ministry of Chapel House.

ACKNOWLEDGMENTS

I thank Dr. Oscar Liburd, my major professor, for allowing me to be his student

and for all of his help on my projects and with the final write up of this thesis. I thank

Drs. Robert Meagher and Donald Dickson for all of their hard work to get this in on time.

I also thank Dr. William Crowe for sitting in for Dr. Dickson at my exit seminar.

My field work would not have been possible without the staff and workers of the

Citra Plant Science Research and Education Unit who took care of my strawberries in the

field and did the bulk of the harvesting. Many thanks go to them.

The staff and students of the Small Fruit and Vegetable IPM Laboratory also

deserve my thanks. Of special note are Crystal Kelts who assisted with a lot of the field

and microscope work in the 2003/2004 field season, Jeff White and Carolyn Mullin who

spent long hours helping me count mites, and Alejandro Arevalo for statistics help.

I thank Marinela Capana and Dr. Ramon Littell from IFAS statistics for helping me

with data analysis.

I thank my parents and all of my friends at Chapel House for support and for

putting up with my whining and frustration.

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TABLE OF CONTENTS page

ACKNOWLEDGMENTS ................................................................................................. iv

LIST OF FIGURES ......................................................................................................... viii

ABSTRACT.........................................................................................................................x

CHAPTER

1 INTRODUCTION ........................................................................................................1

2 LITERATURE REVIEW .............................................................................................6

Twospotted Spider Mite ...............................................................................................6 Biology ..................................................................................................................6 Management ..........................................................................................................7 Reduced-Risk Pesticides .......................................................................................9

Predatory Mites.............................................................................................................9 Phytoseiulus persimilis ........................................................................................11 Neoseiulus californicus .......................................................................................12 Potential as Biological Control Agents and Use in Integrated Pest

Management (IPM)..........................................................................................13 Hypothesis ..................................................................................................................15 Specific Objectives .....................................................................................................15

3 LABORATORY EXPERIMENTS ............................................................................17

Methods ......................................................................................................................18 Colony .................................................................................................................18 Experiment 1 .......................................................................................................18 Experiment 2 .......................................................................................................19 Data Analysis.......................................................................................................19

Results.........................................................................................................................20 Experiment 1 .......................................................................................................20 Experiment 2 .......................................................................................................20

Discussion...................................................................................................................21

4 SINGLE TREATMENT EFFECTS ON TWOSPOTTED SPIDER MITE CONTROL .................................................................................................................26

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Methods ......................................................................................................................26 Sampling..............................................................................................................27 Data Analysis.......................................................................................................27

Results.........................................................................................................................28 2003/2004 Field Season ......................................................................................28 2004/2005 Field Season ......................................................................................30

Discussion...................................................................................................................31

5 TREATMENT COMBINATION EFFECTS ON TWOSPOTTED SPIDER MITE AND PREDATORY MITE SPECIES .......................................................................46

Methods ......................................................................................................................47 Experiment 1 (2003/2004 Field Season) .............................................................47

Sampling.......................................................................................................47 Data analysis ................................................................................................47

Experiment 2 (2003/2004 Field Season) .............................................................48 Experiment 3 (2004/2005 Field Season) .............................................................49

Sampling.......................................................................................................49 Data analysis ................................................................................................49

Results.........................................................................................................................50 Experiment 1 .......................................................................................................50 Experiment 2 .......................................................................................................52 Experiment 3 .......................................................................................................52

Discussion...................................................................................................................54

6 CONCLUSIONS ........................................................................................................68

The Cost of Control ....................................................................................................68 Future Directions ........................................................................................................69

APPENDIX

BEHAVIORAL STUDIES.........................................................................................71

Protocol for Laboratory Bioassay...............................................................................71 Preliminary Experiments ............................................................................................72

Methods ...............................................................................................................72 Results .................................................................................................................72

Egg Consumption Over Time by Predatory Mites .....................................................73 Methods ...............................................................................................................73 Results .................................................................................................................73

Adult Consumption Over Time by Predatory Mites...................................................74 Methods ...............................................................................................................74 Results .................................................................................................................74

Discussion...................................................................................................................75

LIST OF REFERENCES...................................................................................................79

vi

BIOGRAPHICAL SKETCH .............................................................................................85

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LIST OF FIGURES

Figure ............................................................................................................................page

2-1 Twospotted spider mite and the damage it causes ...................................................16

2-2 Phytoseiulus persimilis and eggs..............................................................................16

2-3 Neoseiulus californicus and eggs .............................................................................16

3-1 Twospotted spider mite colony ................................................................................23

3-2 Cage construction.....................................................................................................23

3-3 Greenhouse experimental setup ...............................................................................23

3-4 Weekly average TSSM per leaflet in each treatment for Experiment 1...................24

3-5 Weekly average TSSM per leaflet in each treatment for Experiment 2...................25

4-1 Field experiment setup .............................................................................................35

4-2 Weighing the harvest................................................................................................36

4-3 Average TSSM per leaflet for 5 periods of the 2003/2004 season ..........................37

4-4 Weekly average number of TSSM per leaflet in each treatment during the 2003/2004 season .....................................................................................................38

4-5 Weekly Average TSSM and predatory mite populations in each treatment during the 2003/2004 season ...............................................................................................39

4-6 Average strawberry yield from each treatment for the 2003/2004 season...............41

4-7 Average TSSM per leaflet for 5 periods during the 2004/2005 season ...................42

4-8 Average number of TSSM per leaflet in each treatment for each week in the 2004/2005 season .....................................................................................................43

4-9 Weekly average TSSM and predatory mite populations in each treatment during the 2004/2005 season ...............................................................................................44

5-1 Treatment layout for 2003/2004 field season...........................................................56

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5-2 Treatment layout for 2004/2005 field season...........................................................56

5-3 Average TSSM per leaflet for 5 periods of the 2003/2004 season ..........................57




5-7 Weekly average TSSM and N. californicus populations each week in the Acramite/N. californicus treatment during the 2003/2004 season ...........................61

5-8 Average TSSM per leaflet for 5 periods of the 2004/2005 season .........................62




A-1 Experimental arena..................................................................................................76

A-2 Experimental setup..................................................................................................77

A-3 Number of eggs laid in each m arena ......................................................................77

A-4 Average percent reduction of TSSM adults in each treatment................................78

A-5 Cumulative average percent reduction of TSSM adults in each treatment .............78

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Abstract of Thesis Presented to the Graduate School

of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science

CONTROLLING TWOSPOTTED SPIDER MITE (Tetranychus urticae Koch) IN FLORIDA STRAWBERRIES WITH SINGLE AND COMBINATION TREATMENTS OF Phytoseiulus persimilis Athias-Henriot, Neoseiulus californicus (McGregor), AND

ACRAMITE

By

Elena M. Rhodes

August 2005

Chair: Oscar Liburd Major Department: Entomology and Nematology

Laboratory and field experiments were conducted from 2003 to 2005 to determine

the effectiveness of single and combination treatments of 2 predatory mite species,

Phytoseiulus persimilis Athias-Henriot and Neoseiulus californicus (McGregor), and a

reduced risk miticide, Acramite 50WP® (bifenazate), for control of twospotted spider

mite (TSSM) (Tetranychus urticae Koch) in Florida strawberry fields. In one set of

laboratory tests, 15 mite-free strawberry plants were selected randomly and infested with

known numbers of TSSM. After 1-2 weeks, 10 predatory mites from each species were

released onto each plant. Twospotted spider mite populations were recorded before

release of predatory mites and once a week for 4 weeks after release of predatory mites.

Both species significantly reduced TSSM numbers to below those found in the control.

However, TSSM numbers on the P. persimilis plants increased at week four.

x

In the second set of laboratory tests, two other treatments, application of Acramite

at the recommended rate and a combination release of P. persimilis/N. californicus, were

added to the experiment. Both significantly reduced TSSM numbers below levels found

in the control. No differences were recorded between predatory mite species except at

week four when there was a significant increase in numbers of TSSM in the P. persimilis

treatment.

Field studies employed two setups: one looking at only single treatment

applications of P. persimilis, N. californicus, and Acramite and the other adding

combination treatments of P. persimilis/N. californicus, Acramite/N. californicus, and

Acramite/P. persimilis. Both N. californicus and P. persimilis significantly reduced

populations of TSSM below numbers recorded in the control plots. In 2003/2004 P.

persimilis took longer than N. californicus to bring the TSSM population under control.

Acramite was very effective in reducing TSSM populations below 10 mites per leaflet in

2003/2004 but not in 2004/2005, possibly due to late application.

Among the combination treatments, the P. persimilis/N. californicus treatment

significantly reduced TSSM numbers to below levels found in the control, but was not as

effective as N. californicus alone in 2003/2004. Also, the Acramite/N. californicus, and

Acramite/P. persimilis combination treatments significantly reduced TSSM populations

to below the control.

These findings indicate that N. californicus releases, properly timed Acramite

applications, and combinations of Acramite applications and releases of either predatory

mite species are promising options for TSSM control in strawberry for growers in north

Florida and other areas of the southeast.

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CHAPTER 1 INTRODUCTION

Florida produces 100% of the domestically grown winter strawberry crop and ranks

second in the United States behind California in terms of the quantity of strawberries

produced (Mossler and Nesheim, 2002). During 2004, the total value of strawberries

produced in Florida was approximately $178 million (Florida Agricultural Statistics

Service, 2004). Approximately 95% of Florida’s strawberries are grown in Manatee and

Hillsborough County with the remaining being grown in Alachua, Miami-Dade, and

several other counties (Mossler and Nesheim, 2002).

In Florida, strawberries are planted as an annual crop using a raised bed system.

The beds are fumigated with a combination methyl bromide + chloropicrin 2 weeks

before planting and immediately covered with plastic mulch (Mossler and Nesheim,

2002). Transplants are planted in late September through early November. The common

strawberry varieties that are grown include Camerosa, Florida Festival, Sweet Charlie,

Oso Grande, Rosa Linda, and Selva. The first five are short-day or June bearing; they

need temperatures below 60ºF and/or photoperiods under 14 h to initiate flower

production (Rieger, 2005). Selva is an ever-bearing day-neutral; photoperiod has no

effect on flowering (Rieger, 2005). Overhead irrigation is used for the first 3 weeks after

transplanting to establish the strawberry plants. After this period, drip irrigation is used.

Fertilizer is usually applied through the drip irrigation system, a process often called

fertigation (Hochmuth and Albregts, 1994). The average harvest period in Florida runs

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from late November to early April. Harvesting occurs every 3 to 5 days. The harvest

period ends when California strawberries begin to dominate the market.

Strawberries are susceptible to many pests and soil borne pathogens. Bed

fumigation is used to manage plant parasitic nematodes and many weed species.

However, herbicides are used to control weeds in between rows. Most can only be

applied twice in a season and cannot be used once harvesting begins because the post-

harvest interval (PHI) is too long (Mossler and Nesheim, 2003). Removal of weeds by

cultivation is often necessary (Stall, 2003). Birds can also be a problem in some years

(Mossler and Nesheim, 2003).

Most of the foliar diseases affecting Florida strawberries are caused by fungi.

Anthracnose rots (Colletotrichum acutatum Simmonds, C. fragariae Brooks, and C.

gloeosporioides (Penz.) Penz. and Sacc.), gray mold (Botrytis cinerea Pers.),

phytophthora crown rot (Phytophthora cactorum (Lebert et Cohn) Schröter and P.

citricola Sawada), and powdery mildew (Sphaerotheca macularis (Wallr.:Fr.) Lind.) are

the main fungal diseases affecting strawberries in Florida, although others can also occur

(Mossler and Nesheim, 2003; Roberts, 2003). The principle fungicides used are Captan

and Thiram and a rotation of the two is the backbone of all strawberry growers’ fungus

control program (Mossler and Nesheim, 2003). Angular leaf spot, Xanthomonas

fragariae Kennedy and King, is the only major disease not caused by a fungus. This

bacterium causes an infection on plants that flourishes under cold, wet conditions (White

and Liburd, 2005; Mossler and Nesheim, 2003).

Insect pests of Florida strawberries include several Lepidoptera species namely:

fall armyworm, Spodoptera frugiperda (J. E. Smith), southern armyworm, S. eridania

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Cramer, and budworm, Helicoverpa zea (Boddie), sap beetles, flower thrips, and aphids.

The Lepidoptera species are early season pests. If present during the establishment

period, these insects grow to mature larvae before any treatment can be applied. Since

Bacillus thuringiensis (Bt) (Dipel®) is ineffective against mature larvae, growers must

resort to methomyl (Lannate®), a compound highly toxic to beneficial organisms

including predatory mites, for control. However, many growers now apply spinosad

(SpinTor®), which is much less toxic to predatory mites and other beneficials (Mossler

and Nesheim, 2003). Many species of sap beetles (family Nitidulidae) feed on

strawberries (Mossler and Nesheim, 2003; Mossler and Nesheim, 2002). They prefer

rotting fruit, but they will sometimes lay their eggs in fresh fruit. Sap beetles are only a

problem in fields where it is not possible to remove all fresh and rotting fruit from the

fields (Mossler and Nesheim, 2003). Western flower thrips (Frankliniella occidentalis

(Pergande), and strawberry and melon aphids {Chaetosiphon fragaefolli (Cockerell) and

Aphis gossypii Glover} are sporadic pests often controlled by natural enemies if toxic

broad-spectrum insecticides are not used continually (Mossler and Nesheim, 2003).

Insecticides are used to control these insects if outbreaks occur.

In Florida, twospotted spider mite (TSSM) Tetranychus urticae Koch is the key

arthropod pest effecting strawberries. High densities of TSSM significantly reduce

photosynthesis, transpiration, productivity, and vegetative growth (Sances et al., 1982).

Twospotted spider mite feeding causes injury to chlorophyll containing mesophyll cells

within the leaf tissue and increases stomatal closure, which results in a decrease of

photosynthetic capacities of infested leaves (Sances et al., 1982). Electron micrograph

pictures of tissues heavily injured by spider mites show misshapen cells that contain

4

homogeneous protoplasts with only vestiges of necrotic chloroplasts visible within them

(Kielkiewicz, 1985).

Twospotted spider mite populations can build up to damaging levels very quickly.

Development from egg to mature adult takes about 19 days and females can lay up to 100

eggs (Mitchell, 1973). This high fecundity also allows TSSM to quickly become resistant

to acaracides traditionally used to control them (Trumble and Morse, 1993).

Traditional control strategies for TSSM have required several applications of key

pesticides (= acaricides) during the strawberry production season. In many cases,

miticides have been applied as a preventative measure or on a calendar basis, resulting in

high control costs of up to $989 per hectare (Prevatt, 1991). This has resulted in a

significant reduction in efficacy due to development of acaricide resistance in the TSSM

population (Trumble and Morse, 1993). In addition, widespread concerns over food

safety, human health, and the environment have led the U. S. Environmental Protection

Agency (EPA) to announce cancellations, restrictions, and tolerance reassessments on

many major pesticides available for strawberry pest management.

Control of TSSM with releases of Phytoseiulus persimilis Athias-Henriot has

been fairly successful in south-central Florida (Decou, 1994). However, the introduction

of P. persimilis has not sufficiently suppressed high populations of TSSM in more

northern areas of the state. As a result, growers have relied on a conventional miticide

program. There is also evidence that P. persimilis may be less tolerant to cooler

temperatures that northern Florida growers experience during the winter months (White

and Liburd, 2005).

5

Another predatory mite that has been released in Florida to control TSSM in

strawberries is Neoseiulus californicus (McGregor) (Acari: Phtoseiidae). Neoseiulus

californicus has traits of both a type II selective predator and a Type III generalist

predator (Croft et al., 1998). It has also been reported to be more cold tolerant than P.

persimilis (White, 2003). These characteristics may allow N. californicus to provide

better control of TSSM in north Florida where the more specialized P. persimilis is

ineffective.

The objectives of this study were 1) to determine if N. californicus can provide

effective control of TSSM in north Florida strawberry fields and 2) to compare the

effectiveness of N. californicus with P. persimilis as well as a reduced-risk miticide for

control of twospotted spider mite.

CHAPTER 2 LITERATURE REVIEW

Twospotted Spider Mite

Early season infestations of twospotted spider mites (TSSM) cause reductions in

photosynthesis and transpiration at a much lower population level than the population

level that causes the same level of injury later in the season (Sances et al., 1981). The

high levels of stress caused by large populations of TSSM decrease the quality and

quantity of mature fruit and can lead to a reduction in flower development and vegetative

growth (Sances et al., 1982).

Biology

The TSSM life cycle progresses through five stages: egg, six-legged larvae,

protonymph, deutonymph, and adult. Each of the three intermediate stages feed and grow

for only a short time before entering a quiescent state (Mitchell, 1973). Development

from egg to mature adult takes about 19 days (Mitchell, 1973), although this time can be

as short as 5 days (Krantz, 1978). Optimal conditions for development are high

temperatures (up to 38°C) and low humidity (Krantz, 1978). Satoh et al., (2000) noted

that male TSSM use both precopulatory and postcopulatory guarding to increase their

paternity.

Twospotted spider mite adults are oval and 0.5 mm in length. They are usually light

greenish-yellow in color with two large, dark spots on their abdomens. However, their

coloration may include brown, red, orange, and darker green forms (Figure 2-1).

Although T. urticae is made up of two distinct lineages, both the red and green forms are

6

7

found in both lineages, which indicates that they are the same species (Hinomoto et al.,

2001). Navajas et al., (2000) notes that T. urticae is a relatively homogeneous species,

although reproductive incompatibility does occur between conspecific populations

coming from different host plants or locations.

The eggs, which are spherical and clear to tan in color, are usually laid on the

underside of leaves (Figure 2-1). They are approximately 0.2 mm in diameter.

Twospotted spider mites usually infest the undersides of leaves. They spin a fine

web on the leaves of strawberries and other host plants (Mossler and Nesheim, 2002).

This webbing provides protection from predators. It may also help to maintain favorable

environmental conditions for mite development on the leaf surface (Krantz, 1978).

Twospotted spider mites collect in large numbers on leaf edges as a response to

overcrowding or poor host plant condition and are dispersed by wind (Boykin and

Campbell, 1984). Even at low infestation levels, TSSM can be dispersed by winds of

only 8 km/h (Boykin and Campbell, 1984). Twospotted spider mites can also disperse by

walking (ambulatory dispersal), but they cannot cover great distances in this manner.

Management

Three cultural control practices that are important in the management of TSSM in

strawberries are: 1) close monitoring of transplants, 2) sanitation, and 3) irrigation

techniques. Planting transplants that are as mite-free as possible is extremely important

because chemical sprays are washed off of the plant by the overhead irrigation that is

used to establish strawberry transplants. In addition, predatory mites can also be washed

off the plants by overhead mists. Sanitation practices such as removing clipped runners

and other debris from the field eliminates potential reservoirs of TSSM.

8

Managing soil moisture level is also critical in regulating insect pests and diseases.

Recent investigation by White and Liburd (2005) found that low soil moisture promoted

TSSM development in the laboratory. Similarly, in field studies TSSM numbers were

significantly higher in low soil moisture treatments. Further investigation showed that

excessive irrigation involving drip and overhead was found to increase the incidence of

angular leaf spot, Xanthomonas fragaria disease in strawberries (White and Liburd,

2005).

There has been no economic threshold established for TSSM in north-central

Florida. Twospotted spider mite is usually monitored by collecting a known number of

leaf samples and counting the number of mites and eggs present on the leaves (White,

2003). In south-central Florida, a control action (release of predatory mites or use of a

miticide) is taken when 5% of the sampled leaves are infested with TSSM (Vrie and

Price, 1994).

Abamectin (Agri-Mek®), a compound derived from a soil bacterium, and

hexythiazox (Savey®), a non-systemic miticide with ovicidal, larvicidal, and

nymphicidal activity, and (more recently) bifenazate (Acramite 50WP®) are the

predominant miticides used on Florida strawberries. Bifenthrin (Brigade®), a general

insecticide, and fenbutatin-oxide (Vendex®), an organotin compound are used to a lesser

extent (Mossler and Nesheim, 2002; Mossler and Nesheim, 2003).

Their high fecundity and short life cycle allow TSSM to quickly become resistant

to miticides. Using greenhouse and laboratory experiments, Price et al., (2002) found that

TSSM in Florida strawberry fields show a ten-fold resistance to abamectin when

9

compared with spider mites in a laboratory colony taken from the field two years prior to

the experiments.

Reduced-Risk Pesticides

Recently, several new classes of pesticides have been introduced for use in small

fruit crops, largely in response to the potential loss of and/or restrictions on older

pesticides due to the 1996 Food Quality and Protection Act (FQPA) regulations.

Applications of reduced-risk miticides may hold potential for control of TSSM. To be

classified as reduced-risk, a pesticide must have at least one of the following

characteristics: it must pose a low risk to human health, have low toxicity to non-target

organisms, have a low potential to contaminate the environment, and/or enhance the use

and reliability of integrated pest management (IPM) (Price, 2002b).

Acramite 50WP® is one such compound that has shown promising results in

strawberries. This miticide has a low toxicity toward beneficials and carries a “Caution”

precautionary statement. Acramite can only be applied twice in a season at 0.85 to 1.125

kg product per hectare with applications at least 21 days apart (Mossler and Nesheim,

2003; Price, 2002a). In laboratory studies using leaf disks, White (2003) recorded a

higher rate of TSSM mortality using Acramite 50WP® compared with the conventional

miticide Vendex®.

Predatory Mites

Many species of phytoseiid mites are important biological control agents and form

an integral part of pest management programs in strawberries and other crops (McMurtry

and Croft, 1997). Generally, phytoseids are larger than TSSM, pear shaped, and have

10

longer legs. They range in color from pale to reddish depending on species. Phytoseiid

eggs are larger than TSSM eggs and elliptical in shape (Henn et al., 1995).

Phytoseiid mites are attracted to host volatiles released by plants when they are

attacked by spider mites (Dicke and Sabelis, 1988). Margolies et al., (1997) found that

there is a genetic component in predator response to herbivore-induced plant volatiles by

testing selected and unselected phytoseiid mites in a Y-tube olfactometer. Predatory mites

use volatiles produced by adult prey (possibly alarm pheromones) to avoid patches with

conspecifics. This prevents the formation of predator aggregations on prey patches

(Janssen et al., 1997).

Phytoseiid mites can be placed into four categories based on their food habits that

include: Type I specialist predatory mites that feed exclusively on Tetranychus mites,

Type II specialists that consume Tetranychus mites and species in other genera that

produce webbing, Type III generalists that feed on species in many different tetranychid

genera and will also consume small insects and even pollen, and Type IV generalists that

feed on mites but prefer pollen (McMurtry and Croft, 1997). Larval feeding types

(nonfeeding, facultative-feeding, or obligatory-feeding) are not associated with the

degree of prey specialization (Schausberger and Croft, 1999). Schausberger and Croft

(2000) examined whether specialist and generalist phytoseiid mites differ in

aggressiveness and prey choice in intraguild predation. They found that aggressiveness in

intraguild predation, species recognition, and preferential consumption of heterospecifics

when given a choice is common in generalist but not in specialist phytoseiid mites.

Like TSSM, phytoseiid mites disperse using both ambulatory and aerial means.

Some phytoseiids have behavioral control of take-off but the aerial dispersal itself is

11

mostly passive and, as far as is known, they have no control of landing (Jung and Croft,

2001a). Auger et al., (1999) showed that food deprivation and high temperature increase

ambulatory dispersal in N. californicus. Generally, selective predatory mites tend to have

higher rates of dispersal then generalists (Jung and Croft, 2001b).

Phytoseiulus persimilis

Phytoseiulus persimilis is a Type I specialist that feeds exclusively on tetranychid

mites (Figure 2-2). Adult female P. persimilis prefer TSSM eggs to larvae, but will feed

on all life stages (Blackwood et al., 2001). Walzer and Schausberger, (1999a) found that

adult female P. persimilis can survive for a limited period by cannibalism and

interspecific predation, but they cannot reproduce on this diet of conspecifics and

heterospecifics. Juveniles, however, can reach adulthood when provided either

conspecifics or heterospecifics. They appear to lack the ability to distinguish between

conspecifics and heterospecifics and will feed equally on both if given a choice (Walzer

and Schausberger, 1999b).

Phytoseiulus persimilis has a short developmental time, a nonfeeding larval stage,

and a high rate of fecundity (McMurtry and Croft, 1997). Friese and Gilstrap (1982)

found that P. persimilis has a fixed potential fecundity, and that within this potential, as

the ovipositional rate increases the ovipostion period decreases.

In the presence of excess prey, P. persimilis has a shorter developmental time for

active stages, a greater reproductive longevity, and kills more prey and a greater number

of prey/h both as an immature and as a reproductive adult than N. californicus (Gilstrap

and Friese, 1985). This allows P. persimilis populations to increase very quickly. The

population increases (booms) when spider mites are abundant and crashes (busts) when

the spider mite populations have been decimated. P. persimilis often disperse in patches

12

and overexploit their prey before emigrating to a new area. These “boom-bust” cycles

often make several releases necessary to effectively manage TSSM.

Neoseiulus californicus

Neoseiulus californicus has traits of both a Type II specialist and a Type III

generalist. They prefer spider mites as food but can subsist on other sources of food such

as thrips and pollen when mite populations are low (Figure 2-3). They will also prey upon

other predatory mite species (Gerson et al., 2003). Palevsky et al., (1999) observed that in

adult females, egg predation on heterospecific eggs is significantly higher than

cannibalism of conspecific eggs implying that N. californicus can distinguish its eggs

from those of other species. When given a choice, N. californicus appear to be able to

distinguish between conspecifics and heterospecifics and prefer the latter (Walzer and

Schausberger, 1999b). Neoseiulus californicus females can sustain oviposition when

preying upon other phytoseiid mites, but not when preying upon conspecifics. Juveniles

can reach adulthood preying upon both conspecifics and heterospecifics (Walzer and

Schausberger, 1999a). Neoseiulus californicus larvae are facultative feeders

(Schausberger and Croft, 1999). The presence of spider mite prey increases larval

walking and intraspecific interactions of N. californicus larvae (Palevsky et al., 1999).

Overall, N. californicus has a longer development time and a lower dispersal rate

than P. persimilis (Jung and Croft, 2001b). Like P. persimilis, N. californicus has a fixed

potential fecundity, within which, as the ovipositional rate increases the oviposition

period decreases (Friese and Gilstrap, 1982).

Walzer et al. (2001) found that N. californicus reared on detached bean leaf arenas

with diminishing TSSM prey survived three to five times longer after prey depletion than

P. persimilis whether they were reared alone or in combination. When reared together, N.

13

californicus eventually displaced P. persimilis. This suggests that N. californicus can

persist in a field for a longer period of time when TSSM populations are low and may

give better season-long control when introduced into the field earlier in the season.

Potential as Biological Control Agents and Use in Integrated Pest Management (IPM)

Both P. persimilis and N. californicus have been used to control TSSM on

strawberry and other crops throughout the world. Phytoseiulus persimilis has been

successfully used to control TSSM on strawberries grown in greenhouses in Korea (Kim,

2001), in walk-in plastic tunnels in England (Cross, 1984; Port and Scopes, 1981), and in

the field in many parts of the world (Charles, 1988; Cross et al., 1996; Decou, 1994;

Easterbrook, 1992; Oatman et al., 1977a; Oatman et al., 1976; Oatman et al., 1968;

Oatman et al., 1967; Trumble and Morse, 1993; Waite, 1988). A combination of releases

of P. persimilis and N. fallacis (Garman), a type II specialist, controlled TSSM in

Taiwan (Lee and Lo, 1989). Phytoseiulus persimilis has also been used to control TSSM

on dwarf hops (Barber et al., 2003), on raspberry in New Zealand (Charles et al., 1985),

and on Rhubarb in California (Oatman, 1970).

Neoseiulus californicus has also been successfully used to control TSSM on

strawberry, although to a lesser extent than P. persimilis. Garcia-Mari and Gonzalez-

Zamora (1999) noted that N. californicus is the main predator controlling TSSM on

strawberries in Valencia, Spain. Neoseiulus californicus has also shown potential for use

in the UK; controlling TSSM on potted strawberry plants in a gauze-sided glasshouse

(Easterbrook et al., 2001). Greco et al., (1999) reported that N. californicus is a promising

established natural enemy for controlling TSSM on strawberry in greenhouses in

Argentina. In the U.S., N. californicus has also been used to successfully control TSSM

14

on strawberry in southern California (Oatman et al., 1977b). Liburd et al., (2003) also

recorded good results from preliminary studies involving releases of N. californicus in

strawberry fields in north Florida. Neoseiulus californicus has also been used to control

TSSM on other crops, for example on dwarf hops (Barber et al., 2003).

A combination of one or two miticide sprays and the release of predatory mites

may provide better control than either alone. Trumble and Morse (1993) used weekly

yields and control costs to calculate the economic benefit of controlling TSSM with

several chemicals, P. persimilis releases, and combinations of both. The best returns were

generated by abamectin in combination with releases of P. persimilis. Their data also

indicated that neither fenbutatin-oxide nor hexythiazox are compatible with P. persimilis.

Easterbrook (1992) noted that to prevent an early build up of TSSM it may be necessary

to reduce TSSM numbers with an application of a miticide early in the season before a

later release of P. persimilis.

The use of the Pest in First (PIF) technique may enhance the effectiveness of P.

persimilis releases. In PIF, a crop is artificially seeded with a pest so that there will be

sufficient prey present for the predator to establish and suppress developing populations

when it is introduced some time later. This technique has been used in strawberry fields

in Queensland, Australia for six seasons (1995-2000). The predatory mites provided

season-long control for a cost roughly equivalent to one spray of abamectin (Waite,

2002). Chemical control in these strawberry fields requires at least two sprays of

abamectin.

Phytoseiulus persimilis is used to control TSSM on only 30% of grower fields in

south Florida. There are two big impediments to predatory mite adoption. Many growers

15

desire “clean plants” (no mites at all). The second barrier is that the liquid formulation of

Captan, one of the main fungicides used on strawberries, is highly toxic to predatory

mites (Mossler and Nesheim, 2003).

Hypothesis

Our hypothesis is that N. californicus will provide better control of TSSM in

strawberries compared with P. persimilis. Furthermore, N. californicus and the reduced-

risk miticide, Acramite 50WP® will significantly reduce populations of TSSM in

strawberries below untreated (control) plots.

Specific Objectives

The objectives of this thesis are as follows:

1. To conduct controlled laboratory experiments comparing the effectiveness of the predatory mites P. persimilis and N. californicus as well as a combination of the two and Acramite for control of TSSM.

2. To conduct field experiments to compare and evaluate the effectiveness of P. persimilis and N. californicus for control of TSSM and to compare predatory mite releases with periodic applications of the reduced-risk miticide, Acramite for control of TSSM.

3. To study competition between the two predatory mite species as well as their interaction with Acramite and the effects of these treatment combinations on TSSM control.

4. To conduct behavioral studies of P. persimilis and N. californicus to determine the rate at which they consume both TSSM adults and eggs.

16

A B Figure 2-1. Twospotted spider mite and the damage it causes. A) twospotted spider mite

and eggs, B) strawberry plants damaged by twospotted spider mites

A B Figure 2-2. Phytoseiulus persimilis and eggs. A) P. persimilis and B) its eggs (larger,

ovoid eggs) shown with TSSM eggs (smaller, spherical eggs) for comparison.

A B Figure 2-3. Neoseiulus californicus and eggs. A) N. californicus adult female and B)

eggs.

CHAPTER 3 LABORATORY EXPERIMENTS

In the presence of excess prey, Phytoseiulus persimilis has a shorter developmental

time for active stages, a greater reproductive longevity, and kills more prey and a greater

number of prey/h both as an immature and as a reproductive adult than Neoseiulus

californicus (Gilstrap and Friese, 1985). Both predatory mite species have been found to

effectively control TSSM populations on strawberries and other crops in various parts of

the world. Phytoseiulus persimilis does not perform well in north Florida because of the

colder temperatures found there (White, 2003). The purpose of these experiments was to

compare the efficacy of both species of predatory mite on TSSM control under controlled

greenhouse conditions. Under such conditions, P. persimilis would be expected to

perform as well or better than N. californicus because temperature effects are not a factor.

In the first experiment, an untreated control was compared with treatments where

each individual predator was released. In the second experiment, releases of each

individual species were compared to two other treatments: Acramite applied at the

recommended rate and a combination release of P. persimilis and N. californicus.

Acramite is known to effectively control TSSM populations (White, 2004). Lee and Lo

(1989) found that a combination of P. persimilis and N. fallacies (Garman) released

weekly or biweekly in November effectively controlled TSSM on strawberry in Taiwan

for the season. The purpose of adding these two treatments was to compare the efficacy

of N. californicus and P. persimilis releases on TSSM control to Acramite application

17

18

and to determine whether using a combination of the two species is an effective control

strategy.

Methods

Colony

A TSSM colony reared on strawberries was maintained in the laboratory to ensure

that only TSSM predisposed to strawberries were used in the experiments (Fig. 3-1). The

colony consisted of mite-infested strawberry plants that were screened periodically for

the presence of TSSM. The colony was kept under 14:10 photoperiod at a temperature of

~27°C with 65% relative humidity. Plants were watered twice weekly.

Experiment 1

Fifteen mite-free strawberry plants var. Festival were placed into previously

constructed mite-free cages. Each plant was placed into an individual cage. Cages were

constructed of nylon fabric. Velcro was placed on three sides (Figure 3-2A). Each cage

was attached to a pot using a pull cord sown into the bottom (Figure 3-2B). Cages were

used to keep both TSSM and predatory mites from dispersing between plants. Ten TSSM

were released onto each plant and allowed to multiply for one to two weeks (This varied

depending on when the predatory mite shipment arrived).

Prior to the release of predatory mites, a leaflet was collected from each plant. The

number of TSSM motiles and eggs on each leaflet was counted and the average number

per leaflet calculated.

Experimental design was a completely randomized block with 3 treatments. Each

treatment was replicated 5 times. Treatments included: 1) 10 P. persimilis released per

infested plant, 2) 10 N. californicus released per infested plant, and 3) untreated (control)

plants (Fig. 3-3).

19

Each week the population of predators and TSSM was sampled by taking one

leaflet from each plant (5 leaflets from each individual treatment) and counting the

numbers of TSSM as well as predatory mites (motiles and eggs). Samples were recorded

for 4 weeks.

This experiment was repeated three times: once in Feb./Mar. 2004, again in Dec.

2004/Jan. 2005, and finally in Mar./Apr. 2005.

Experiment 2

The protocol for Experiment 2 resembles Experiment 1 except that 5 treatments

involving 25 mite-free strawberry plants were tested. Treatments included: 1) 10 P.

persimilis released per infested plant, 2) 10 N. californicus released per infested plant, 3)

5 P. persimilis and 5 N. californicus released per infested plant, 4) Acramite sprayed onto

each infested plant at the recommended rate, and 5) untreated (control) plants (Fig. 3-3).

Experimental design was a completely randomized block with 5 treatments. Each

treatment was replicated 5 times.

Each week the population of predators and TSSM was sampled by taking one

leaflet from each plant (5 leaflets from each individual treatment) and counting the

numbers of TSSM as well as predatory mites (motiles and eggs). Samples were taken for

4 weeks.

This experiment was repeated twice: once in Dec. 2004/Jan. 2005, and again in

Mar./Apr. 2005.

Data Analysis

Data were subjected to statistical analysis using the SAS program (SAS Institute,

2002). TSSM motile and egg data from both experiments were log transformed. Average

TSSM per leaflet was compared each week across treatments using an ANOVA and

20

means were separated using a LSD test. Predatory mite data from Experiment 2 were

compared using a Student’s t-test or a Satterthwaite t-test depending on whether or not

the variances were equal.

Results

Experiment 1

There were no significant differences in TSSM motile and egg populations

between treatments when the initial sample was taken (week 0) (motiles: F = 1.1, df =

2,24, p = 0.3638; eggs: F = 0.94, df = 2,24, p = 0.4046) and one week after predatory

mite release (motiles: F = 0.08, df = 2,24, p = 0.9257; eggs: F = 0.45, df = 2,24, p =

0.6441) (Figure 3-4). Two weeks after release, P. persimilis significantly reduced TSSM

motile and egg numbers below numbers in the control (motiles: F = 3.4, df = 2,24, p =

0.0513; eggs: F = 4.4, df = 2,24, p = 0.0230). However, TSSM numbers on plants where

N. californicus was released were intermediary (Figure 3-4). Both species of predatory

mite significantly reduced numbers of TSSM motiles and eggs below the control by week

3 (for motiles: F = 6.2, df = 2,24, p = 0.0068; for eggs: F = 6.0, df = 2,24, p = 0.0075).

TSSM motile and egg numbers on plants where P. persimilis was released began to

increase at week 4. At this time, there were significantly fewer motiles per leaflet in the

N. californicus treatment compared with the other two treatments (motiles: F = 4.3, df =

2,24, p = 0.0256), however, the difference in egg numbers was of low significance (eggs:

F = 1.9, df = 2,24, p = 0.1717).

Experiment 2

As in experiment 1, there were no significant differences in TSSM motile and egg

numbers amoung treatments at week 0 (initial sample) (motiles: F = 0.70, df = 4,20, p =

0.5983; eggs: F = 0.25, df = 4,20, p = 0.9056) or week 1 (motiles: F = 0.50, df = 4,20, p =

21

0.7393; eggs: F = 0.81, df = 4,20, p = 0.5309) (Figure 3-5). All four treatments

significantly reduced TSSM numbers at week 2 compared to the control (motiles: F =

5.8, df = 4,20, p = 0.0028; eggs: F = 5.1, df = 4,20, p = 0.0056) (Figure 3-5). Numbers of

TSSM motiles and eggs remained low in all four management treatments at week 3 and

all management treatments had significantly lower numbers of TSSM than did the control

(motiles: F = 5.5, df = 4,20, p = 0.0036; eggs: F = 3.8, df = 4,20, p = 0.0191). At the end

of the experiment, numbers of TSSM motiles in the N. californicus and Acramite

treatments were significantly lower than those in the control (F = 4.2, df = 4,20, p =

0.0130). Numbers of TSSM eggs were significantly lower than the control in the N.

californicus and P. persimilis/N. californicus treatments (F = 2.6, df = 4,20, p = 0.0696).

However, TSSM motile and egg numbers in the P. persimilis treatment increased greatly

at week 4. Both TSSM motile and egg numbers increased slightly but not significantly in

the Acramite treatment at week 4.

There were no significant differences in numbers of P. persimilis motiles and eggs

between the P. persimilis and P. persimilis/N. caifornicus treatments (motiles, t = 0.16, df

= 71, p = 0.8701; eggs, t = 0, df = 78, p = 1). There were also no significant differences in

numbers of N. californicus motiles and eggs between the N. californicus and P.

persimilis/N. caifornicus treatments (motiles, t = 0, df = 78, p = 1; eggs, t = -0.92, df =

63.1, p = 0.3619).

Discussion

Both experiments indicated that N. californicus controls TSSM more effectively

than P. persimilis, although both species significantly reduce TSSM numbers below those

found in the control. Neoseiulus californicus continuously suppressed TSSM populations.

Unlike N. californicus, there were a few failures for P. persimilis especially at week 4

22

where high numbers were recorded in two replicates of the Dec.04/Jan.05 repeat of each

experiment. Most likely, the P. persimilis adults did not establish on these two plants for

unknown reasons.

Acramite appears to be highly effective in controlling TSSM populations.

However, the slight increase in numbers at 4 weeks suggests that it does not kill 100% of

the TSSM and that its affects eventually wear off. Since it can only be sprayed twice in a

season, timing of Acramite applications is critical, especially if it is the only mite control

strategy employed. Also, the application of N. californicus or P. persimilis following

Acramite sprays may be an important management strategy for TSSM control in Florida.

The P. persimilis/N. californicus combination treatment significantly reduced

TSSM numbers. This strategy appears to be more effective than releasing P. persimilis

alone and slightly less effective than releasing N. californicus alone, although these

trends were not statistically significant. Releasing both species in combination does not

appear to be an economical strategy since it is not any better than using the predatory

mites individually.

Numbers of predators between the single and combination treatments were not

statistically different. This is due to the fact that so few numbers of predators were found.

Therefore, no firm conclusions can be drawn from my experiments on the effects of

competition when both species of predatory mite are released together.

Relatively low numbers of TSSM were recorded on many plants as well. Therefore,

experiments need to be repeated before any firm conclusions are drawn.

23

A B

Figure 3-1. Twospotted spider mite colony. A) a colony heavily infested with TSSM, B) close-up of a heavily infested strawberry plant.

A B Figure 3-2. Cage construction. A) diagram showing placement of Velcro and B) a cage

laid flat on a laboratory bench.

A B Figure 3-3. Greenhouse experimental setup. A) the greenhouse setup for a replicate of

experiment 2, B) close-up of several caged plants.

24

Average TSSM motiles per leaflet in each treatment

0

5

10

15

20

25

0 1 2 3 4

Week after predatory mite release

TSSM

mot

iles

per l

eafle

t

CNP

a

aa

a

bb

bb

ab

A

Average TSSM eggs per leaflet in each treatment

0

10

20

30

40

50

0 1 2 3 4


TSSM

egg

s pe

r lea

flet

CNPa

a

a

a

bb

b b

ab

B Figure 3-4. Weekly average TSSM per leaflet in each treatment for Experiment 1. A)

TSSM motiles per leaflet and B) TSSM eggs per leaflet. Week 0 is the initial sample taken before predatory mites were released. Error bars represent standard error of the mean. (C = control, P = P. persimilis, and N = N. californicus).

25

Weekly average TSSM motiles per leaflet

0

5

10

15

20

25

0 1 2 3 4


TSSM

mot

iles

per l

eafle

t

CPNP/NA

a

a a

a

b bbb

b

bb

abb bb

A

Weekly average TSSM eggs per leaflet

0

20

40

60

80

0 1 2 3 4


TSSM

egg

s pe

r lea

flet

CPNP/NA

a

a

a

ab

bb bb

bbab

bb

b

B Figure 3-5. Weekly average TSSM per leaflet in each treatment for Experiment 2. A)

TSSM motiles per leaflet and B) TSSM eggs per leaflet. Week 0 is the initial sample taken before predatory mites were released. Error bars represent standard error of the mean. (C = control, P = P. persimilis, N = N. californicus, P/N = P.persimilis/N. californicus combination, and A = Acramite).

CHAPTER 4 SINGLE TREATMENT EFFECTS ON TWOSPOTTED SPIDER MITE CONTROL

Both P. persimilis and N. californicus have been shown to effectively control

TSSM on strawberry and other crops in various parts of the world. In Florida, P.

persimilis has been fairly successful in controlling TSSM populations in south-central

Florida (Decou, 1994). However, releases of P. persimilis have not been effective in

northern Florida, possibly because it cannot survive colder temperatures during winters

found in this area. As a more generalist predator, N. californicus may be able to withstand

these conditions and control TSSM populations where P. persimilis cannot. Preliminary

studies by Liburd et al. (2003) indicate that N. californicus can effectively control TSSM

in north Florida strawberries. The purpose of this field study was to compare

management of TSSM using either releases of N. californicus or releases of P. persimilis

or applications of Acramite. Strawberry production was evaluated to determine if

management affected crop yield.

Methods

This experiment was conducted at the University of Florida, Plant Science

Research Unit, Citra. Strawberry plants (var. Festival) were planted in plots 7.3 m x 6.1

m consisting of six rows 0.5 m wide with 0.5 m row spacing. The 24 plots were arranged

in a 4 x 6 grid and were spaced 7.3 m apart (Figure 4-1A). The experiment was a

completely randomized block design with six replicates. Four treatments were evaluated

and included: 1) releases of P. persimilis (P), 2) releases of N. californicus (N), 3)

application of the reduced-risk miticide Acramite 50WP® at the rate of 1.125 kg product

26

27

per hectare (A), and 4) an untreated control (C) (Figure 4-1B,C). During both seasons,

each treatment was applied twice. In the 2003/2004 field season, predatory mites were

released on December 11 and February 11 and Acramite was sprayed on December 18

and February 14. In the 2004/2005 field season, all treatments were applied on December

9 and March 10.

Sampling

Sampling was initiated once the plants had established. Each week, 6 leaves per

plot (24 leaves per treatment) were collected and brought back to the laboratory where

the number of TSSM motiles and eggs on each leaf were counted under a dissecting

microscope. After predators were released, the numbers of predators and their eggs were

also counted.

Yield data were collected beginning in early January in both seasons. Strawberries

were harvested weekly, and those from the four inner rows were weighed. The two outer

rows served as border rows and the yield from these rows was discarded (Figure 4-2).

Data Analysis

Data were subjected to statistical analysis using the SAS program (SAS Institute,

2002). The TSSM motile and egg data were separated into 5 periods based on treatment

application dates and time during the season. In the 2003/2004 field season these periods

were: 1) pretreatment (3 weeks prior to any treatment), 2) early-season (post treatment to

week 7), 3) mid-season (week 8 to the second application at week 12), 4) early-late

season (week 13 to when the second treatment was applied in the 2004/2005 season at

week 16), and 5) late season (weeks 17-20). The late season was split into two periods

because of the difference in timing of the second application between seasons. For

instance, during the 2003/2004 season, the second application occurred at the beginning

28

of the late season whereas in the 2004/2005 field season, the second application occurred

in the middle of the late season.

In the 2004/2005 field season, the periods were: 1) pretreatment (3 weeks prior to

the first treatment), 2) early-season (weeks 4-8), 3) mid-season (week 9 to when the

second treatment had been applied in the previous field season at week 13), 4) early-late

season (week 14 to when the second treatment was applied on week 16), and 5) late

season (week 17 to the end of the season). The late season was split into two periods

because of reasons discussed above. In each period, data were log transformed and then

treatments were compared using an ANOVA and means were separated using a LSD test.

Yield data in each replicate were totaled over the season. The average total yields

for each treatment were compared using an ANOVA. A LSD test was used to separate

the means for the yield data. This was done for both seasons.

Results

2003/2004 Field Season

There were no significant differences in motile and egg numbers between

treatments in the pretreatment period (motiles: F = 0.8, df = 3,15, p = 0.5041; eggs F =

0.7, df = 3,15, p = 0.5805) or in the early-season (motiles: F = 0.7, df = 3,15, p = 0.5511;

eggs F = 1.4, df = 3,15, p = 0.2755) (Figure 4-3). The N. californicus and Acramite

treatments had significantly fewer motiles and eggs per leaflet than the control plots

during the mid-season (motiles: F = 3.9, df = 3,15, p = 0.0297; eggs F = 5.0, df = 3,15, p

= 0.0137) and during the early-late season (motiles: F = 5.1, df = 3,15, p = 0.0121; eggs

F = 4.0, df = 3,15, p = 0.0280) (Figure 4-3). During these two periods, TSSM numbers in

the P. persimilis treatment were fairly high but were not significantly different from

TSSM numbers in the N. californicus treatment. Also, numbers of TSSM in the P.

29

persimilis treatment were not significantly different from those in the control with the

exception of the early-late season in terms of the numbers of motiles (Figure 4-3). There

were no significant differences in TSSM motile and egg numbers between treatments in

the late season (motiles: F = 1.2, df = 3,15, p = 0.3461; eggs F = 0.68, df = 3,15, p =

0.5777).

The same trends can be observed looking at the TSSM motile and egg populations

on a weekly basis (Figure 4-4). From early January to mid-March (the mid and early-late

seasons), populations of TSSM motiles and eggs were consistently the highest in the

control, peaking at 112 ± 44 motiles and 207 ± 85 eggs per leaflet on Feb. 20 (early-late

season). The P. persimilis treatment had the second highest TSSM population during this

period, peaking at 35 ± 12 motiles on Feb. 11 and 104 ± 71 eggs on Jan. 20 (mid-season).

In the N. californicus treatment, TSSM motile populations never exceeded of 10 ± 7

motiles per leaflet or 37 ± 24 eggs per leaflet after the first application. The Acramite

treatment also had low TSSM populations, with no more than 11 ± 8 motiles per leaflet

and 33 ± 31 eggs per leaflet occurring after the first application.

Some interesting trends can be seen when treatments are examined individually.

Towards the end of the season predatory mites dispersed into the control plots, causing

the TSSM population in these plots to decline (Figure 4-5A). A few predator motiles

were found in the Acramite plots after each release, but no eggs were found (Figure 4-

5B). The P. persimilis population in the P. persimilis plots peaked at the same time as the

TSSM populations (Figure 4-5C). In contrast, the in the N. californicus plots, the

predatory mite population peaked about two weeks after the TSSM population (Figure 4-

5D).

30

The average yield from the P. persimilis treatment was not significantly different

than the control (Figure 4-6). The P. persimilis plots averaged 83 ± 5 kg and the control

plots averaged 83 ± 8 kg. The N. californicus plots averaged a significantly higher yield

of 98 ± 4 kg. The Acramite plots had an average yield between the two of 94 ± 5 kg.

2004/2005 Field Season

In the 2004/2005 field season, the TSSM population peaked much later. There were

no significant differences in TSSM motile and egg numbers in the pretreatment period

(motiles: F = 0, df = 3,15, p = 1; eggs F = 0, df = 3,15, p = 1) or in the early-season

(motiles: F = 0.47, df = 3,15, p = 0.7073; eggs F = 0.57, df = 3,15, p = 0.6432) (Figure 4-

7). There were also no significant differences in TSSM motile numbers in the mid-season

(F = 1.9, df = 3,15, p = 0.1699). However, TSSM egg numbers showed a trend of being

higher in the Acramite treatment when compared to the N. californicus treatment (F =

2.4, df = 3,15, p = 0.1129) during the mid-season (Figure 4-7). The control treatment had

significantly higher numbers of TSSM motiles and eggs than the N. californicus and P.

persimilis treatments in the early-late season (for motiles: F = 11.1, df = 3,15, p = 0.0004;

for eggs F = 13.0, df = 3,15, p = 0.0002) and the late season (motiles: F = 14.6, df = 3,15,

p = 0.0001; eggs F = 14.6, df = 3,15, p = 0.0001) (Figure 4-7). During the early-late

season, numbers of TSSM motiles in the Acramite treatment did not differ significantly

from those found in the control. However, there were significantly less TSSM eggs in the

Acramite treatment during this period than in the control. The Acramite treatment had

significantly higher numbers of TSSM motiles and eggs than both predatory mite

treatments in the early late season. In the late season, Acramite treatment had

significantly more TSSM motiles than both predatory mite treatments and significantly

more TSSM eggs than the N. californicus treatment (Figure 4-7).

31

As with the previous field season, similar trends can be seen when the TSSM

population levels are observed on a weekly basis (Figure 4-8). The population of TSSM

in the control peaked at 44 ± 23 motiles and 130 ± 47 eggs per leaflet on March 9, 2005

(early-late season). TSSM numbers in the Acramite treatment peaked at 38 ± 18 motiles

per leaflet on March 9 and 131 ± 56 eggs per leaflet on March 1 (early-late season). In

the P. persimilis treatment, TSSM numbers peaked at 7 ± 4 motiles per leaflet and 36 ±

28 eggs per leaflet on March 9 (early-late season). TSSM populations in the N.

californicus treatment never exceeded 1 ± 1 motile per leaflet or 7 ± 6 eggs per leaflet

after the first application.

Examining each treatment individually shows some similarities to and differences

from the previous season. Predatory mites again dispersed into the control plots towards

the end of the season (Figure 4-9A). This season, they also dispersed into the Acramite

plots (Figure 4-9B). As in the previous season, the P. persimilis population peaked at the

same time the TSSM population peaked (Figure 4-9C). This occurred in mid-March. A

peak in N. californicus population also occurred at this time (Figure 4-9D).

There were no significant differences in yield between the four treatments (F =

2.6 df = 3,15 p = 0.0907). Average yield per treatment was much lower than the previous

season. Yields averaged 40 ± 1 kg in the control plots, 39 ± 2 kg in the Acramite plots, 35

± 1 kg in the N. californicus plots, and 37 ± 1 kg in the P. persimilis plots.

Discussion

In both seasons there were fewer TSSM in the N. californicus treated plots than in

the P. persimilis treated plots, although the difference in the 2004/2005 season was not

significant. This suggests that N. californicus is better at suppressing populations of

32

TSSM than P. persimilis. Also, P. persimilis may effectively control TSSM when

released in seasons when initial TSSM numbers are relatively low.

Based on data obtained from the weekly averages, it appears that one release of N.

californicus would be sufficient to control TSSM populations, whereas two releases of P.

persimilis maybe required. Walzer et al. (2001) came to a similar conclusion when they

found that N. californicus survived longer than P. persimilis when kept on bean leaves

with diminishing prey. Further research would be needed to substantiate this hypothesis.

Acramite was highly effective in the 2003/2004 season but did not adequately

control TSSM numbers in the 2004/2005 season. This was primarily a result of timing. In

the 2003/2004 season, the first spray knocked the TSSM population down and the second

spray in early February kept the numbers low. During the 2004/2005 season, in contrast,

there were no detectable TSSM populations when Acramite was first sprayed. By the

time TSSM populations began to increase, there was little residual activity left in the

Acramite plots. It is possible that if the Acramite plots were sprayed again in February

like in the previous season, the results from 2004/2005 would have been similar to the

2003/2004 season. In applying Acramite the fact that only 2 applications can be made per

field season was taken into consideration. However, the second application of all

treatments was delayed until March so TSSM populations could have a chance to

increase. Since the Acramite plots were essentially controls (due to delayed application),

the TSSM population in these plots exploded and was so high that the second application

of Acramite could not reduce TSSM levels. This illustrates how important proper timing

of Acramite applications is for growers as well as the importance of using Acramite in

combination with other management tactics (predatory mites) to control TSSM.

33

There were several differences in the TSSM populations between the two field

seasons. In the 2003/2004 season, TSSM were present in the plots from the beginning,

began to increase when the weather got warmer, and died off in all plots by the last few

weeks of the season. In contrast, no detectable numbers of TSSM were found in the

2004/2005 season until mid-January and numbers did not increase greatly until late

February. Both TSSM and predatory mite numbers were much lower in the 2004/2005

season. White and Liburd (2004) found that TSSM prefer hot, dry conditions. Late

summer and fall of 2004 were incredibly wet because of Hurricanes Francis and Jeanne.

This could have knocked back the population of TSSM leaving smaller numbers to

disperse into the strawberries when they were planted.

There was one more difference in the TSSM populations between seasons. The

green form of the TSSM was much more prevalent than the red form in 2003/2004. This

trend reversed itself in 2004/2005. The red form may survive better in wetter conditions

than the green form. This would be an interesting topic to research further.

Yield was substantially lower in 2004/2005 than in 2003/2004 in all treatments.

This was primarily due to a greater amount of fungal problems, bird damage, and weed

infestation in 2004/2005. The difference in TSSM population trends between the two

seasons may explain why there was a significantly greater yield in the N. californicus

treatment in 2003/2004 but not in 2004/2005. Sances et al. (1981) found that higher

numbers of TSSM are needed to cause a similar level of damage when infestation occurs

later in the season. The TSSM population in 2004/2005 peaked at a much lower number

later in the season than the population in 2003/2004. I suspect that the numbers of TSSM

were not high enough to affect the quantity of strawberries produced, although I did

34

observe that the quality of fruit harvested from the predatory mite plots looked better than

that harvested from the control and Acramite plots. However, quality of marketable

yields was not measured in this study.

It appears that N. californicus is not as affected by the difference in TSSM

population trends as P. persimilis. This is not surprising because it can survive and

reproduce on other prey whereas P. persimilis cannot. Acramite is very effective if

applications are timed and applied properly. Combinations of Acramite and a release of

one of the two predatory mite species could be highly effective in managing populations

of TSSM in north Florida strawberries.

35

A

B

C Figure 4-1. Field experiment setup. A) Picture of several field plots, B) treatment layout

for 2003/2004 season, and C) treatment layout for 2004/2005 field season.

36

Figure 4-2. Weighing the harvest.

37

Average TSSM motiles for five periods during the 2003/2004 season

0

10

20

30

40

50

60

pretrt early mid early-late late

Period

TSSM

mot

iles

per l

eafle

t

CPNA

a

a

ab

cbc

bb

b

A

Average TSSM eggs for five periods during the 2003/2004 season

020406080

100120140160


Period

TSSM

egg

s pe

r lea

flet

CPNA

a

a

ab

ab

bc

bbc

B Figure 4-3. Average TSSM per leaflet for 5 periods of the 2003/2004 season. A) TSSM

motiles per leaflet and B) TSSM eggs per leaflet (C = control, P = P. persimilis, N = N. californicus, and A = Acramite).

38

Weekly average TSSM motiles in each treatment

0

20

40

60

80

100

120

11/24

/2003

12/8/

2003

12/22

/2003

1/5/20

04

1/19/2

004

2/2/20

04

2/16/2

004

3/1/20

04

3/15/2

004

3/29/2

004

Date

Num

ber o

f mot

iles

CAPN

A

Weekly average TSSM eggs in each treatment

0

50

100

150

200

250

11/24

/2003

12/8/

2003

12/22

/2003

1/5/20

04

1/19/2

004

2/2/20

04

2/16/2

004

3/1/20

04

3/15/2

004

3/29/2

004

Date

Num

ber o

f egg

s

CAPN

B Figure 4-4. Weekly average number of TSSM per leaflet in each treatment during the

2003/2004 season. A) TSSM motile populations, B) TSSM egg populations. Arrows indicate dates when predators were released; triangles indicate dates when Acramite was sprayed. (C = control, P = P. persimilis, N = N. californicus, and A = Acramite)

39

Mites in control plots

0

50

100

150

200

250

11/24

/2003

12/10

/2003

12/26

/2003

1/7/20

04

1/20/2

004

2/2/20

04

2/20/2

004

3/3/20

04

3/17/2

004

3/30/2

004

Date

Num

ber T

SSM

per

leaf

let

0

0.5

1

1.5

2

Num

ber p

reda

tory

mite

s pe

r lea

flet TSSMm

TSSMePredmPrede

A

Mites in Acramite treated plots

0

20

40

60

80

100

120

11/24

/2003

12/10

/2003

12/26

/2003

1/7/20

04

1/20/2

004

2/2/20

04

2/20/2

004

3/3/20

04

3/17/2

004

3/30/2

004

Date

Num

ber T

SSM

per

leaf

let

0

0.5

1

1.5

2

Num

ber p

reda

tory

mite

s pe

r lea

flet TSSMm

TSSMePredmPrede

B Figure 4-5. Weekly Average TSSM and predatory mite populations in each treatment

during the 2003/2004 season. A) In the control, B) in the Acramite treatment, C) in the P. persimilis treatment, D) in the N. californicus treatment. Arrows indicate predatory mite release dates, triangles indicate dates Acramite was sprayed. (TSSMm = TSSM motiles per leaflet, TSSMe = TSSM eggs, Predm = predatory mite motiles, Prede = predatory mite eggs, Pm = P. persimilis motiles, Pe = P. persimilis eggs, Nm = N. californicus motiles, Ne = N. californicus eggs)

40

Mites in P. persimilis treated plots

0

20

40

60

80

100

120

11/24

/2003

12/10

/2003

12/26

/2003

1/7/20

04

1/20/2

004

2/2/20

04

2/20/2

004

3/3/20

04

3/17/2

004

3/30/2

004

Date

Num

ber T

SSM

per

leaf

let

0

0.5

1

1.5

2

Num

ber

P. p

ersi

mili

s m

ites

per l

eafle

t

TSSMmTSSMePpmPpe

C

Mites in N. californicus treated plots

0

20

40

60

80

100

120

11/24

/2003

12/10

/2003

12/26

/2003

1/7/20

04

1/20/2

004

2/2/20

04

2/20/2

004

3/3/20

04

3/17/2

004

3/30/2

004

Date

Num

ber T

SSM

per

leaf

let

0

0.5

1

1.5

2 N

umbe

r N

. cal

iforn

icus

m

ites

per l

eafle

t

TSSMmTSSMeNcmNce

D Figure 4-5. Continued.

41

Average total yield per treatment

0.00

20.00

40.00

60.00

80.00

100.00

120.00

C A Nc Pp

Treatment

Yiel

d (k

gs)

b baba

Figure 4-6. Average strawberry yield from each treatment for the 2003/2004 season. (F =

3.36, df = 3, 15, p = 0.0376)

42

Average TSSM motiles in five periods during the 2004/2005 season

0

15

30

45


Period

TSSM

mot

iles

per l

eafle

t

CPNA

aa

a

b

b

b

cc

A

Average TSSM eggs in five periods during the 2004/2005 season

0

20

40

60

80

100


Period

TSSM

egg

s pe

r lea

flet

CPNA

a

a

a

ab

ab b

b

bc

b

c

c

c

B Figure 4-7. Average TSSM per leaflet for 5 periods during the 2004/2005 season. A)

TSSM motiles per leaflet and B) TSSM eggs per leaflet (C = control, P = P. persimilis, N = N. californicus, and A = Acramite).

43

Weekly average TSSM motiles per treatment

05

101520253035404550

11/22

/2004

12/6/

2004

12/20

/2004

1/3/20

05

1/17/2

005

1/31/2

005

2/14/2

005

2/28/2

005

3/14/2

005

3/28/2

005

Date

Num

ber o

f TSS

M m

otile

s pe

r le

afle

tCAPN

A

Weekly average TSSM eggs per treatment

020406080

100120140

11/22

/2004

12/6/

2004

12/20

/2004

1/3/20

05

1/17/2

005

1/31/2

005

2/14/2

005

2/28/2

005

3/14/2

005

3/28/2

005

Date

Num

ber o

f TSS

M e

ggs

per

leaf

let

CAPN

B Figure 4-8. Average number of TSSM per leaflet in each treatment for each week in the

2004/2005 season. A) TSSM motile populations, B) TSSM egg populations. Arrows indicate dates when treatments were applied. (C = control, P = P. persimilis, N = N. californicus, and A = Acramite).

44

Mites in control plots

0

20

40

60

80

100

120

140

11/22

/2004

12/6/

2004

12/20

/2004

1/5/20

05

1/19/2

005

2/2/20

05

2/16/2

005

3/1/20

05

3/16/2

005

3/30/2

005

Date

Num

ber T

SSM

per

leaf

let

00.050.10.150.20.250.30.350.40.450.5

Ave

rage

pre

dato

ry m

ites

per l

eafle

t TSSMmTSSMePredmPrede

A

Mites in Acramite treated plots

0

20

40

60

80

100

120

140

11/22

/2004

12/6/

2004

12/20

/2004

1/5/20

05

1/19/2

005

2/2/20

05

2/16/2

005

3/1/20

05

3/16/2

005

3/30/2

005

Date

Num

ber T

SSM

per

leaf

let

0

0.1

0.2

0.3

0.4

0.5A

vera

ge p

reda

tory

mite

s pe

r lea

flet TSSMm

TSSMePredmPrede

B Figure 4-9. Weekly average TSSM and predatory mite populations in each treatment

during the 2004/2005 season. A) In the control, B) in the Acramite treatment, C) in the P. persimilis treatment, D) in the N. californicus treatment. Arrows indicate dates treatments were applied (TSSMm = TSSM motiles per leaflet, TSSMe = TSSM eggs, Predm = predatory mite motiles, Prede = predatory mite eggs, Pm = P. persimilis motiles, Pe = P. persimilis eggs, Nm = N. californicus motiles, Ne = N. californicus eggs).

45

Mites in P. Persimilis treated plots

05

101520253035404550

11/22

/2004

12/6/

2004

12/20

/2004

1/5/20

05

1/19/2

005

2/2/20

05

2/16/2

005

3/1/20

05

3/16/2

005

3/30/2

005

Date

Num

ber o

f TSS

M p

er

leaf

let

00.050.10.150.20.250.30.350.40.450.5

Num

ber o

f P. p

ersi

mili

s pe

r lea

flet TSSMm

TSSMePpmPpe

C

Mites in N. californicus treated plots

0

10

20

30

40

50

11/22

/2004

12/6/

2004

12/20

/2004

1/5/20

05

1/19/2

005

2/2/20

05

2/16/2

005

3/1/20

05

3/16/2

005

3/30/2

005

Date

Num

ber o

f TSS

M p

er

leaf

let

0

0.1

0.2

0.3

0.4

0.5N

umbe

r of N

. cal

iforn

icus

pe

r lea

flet TSSMm

TSSMeNcmNce

D Figure 4-9. Continued.

CHAPTER 5 TREATMENT COMBINATION EFFECTS ON TWOSPOTTED SPIDER MITE AND

PREDATORY MITE SPECIES

Trumble and Morse (1993) found that the best economic returns were generated by

using applications of abamectin in combination with releases of Phytoseiulus persimilis.

Abamectin is a reduced-risk miticidal compound derived from the soil bacterium

Streptomyces avermitilis Kim and Goodfellow, which interferes with the nervous system

of mites. Similarly, Acramite is a reduced-risk miticide, which is much less toxic to

predatory mites than abamectin; therefore, combination treatments with either P.

persimilis or Neoseiulus californicus may be a highly effective control strategy. Also,

Acramite may be useful as a single treatment to knock down a high population of TSSM

before releasing predatory mites

A combination of releases of P. persimilis and N. fallacis controls TSSM on

strawberries in Taiwan (Lee and Lo, 1989). Therefore, it is possible that a combination of

P. persimilis and N. californicus could be an effective treatment strategy to control

TSSM. However, Walzer et al. (2001) found that when reared together on detached bean

leaf arenas, N. californicus eventually displaced P. persimilis.

The purpose of these field experiments was to examine the effectiveness of three

combination treatments including: P. persimilis/N. californicus, Acramite/N. californicus,

and Acramite/P.

controlling twospotted spider mite (tetranychus...

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