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Crop Protection 30 (2011) 468e475

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Crop Protection

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Single versus multiple releases of predatory mites combined with spinosad for themanagement of western flower thrips in strawberry

Touhidur Rahman a,*, Helen Spafford a,b, Sonya Broughton c

a School of Animal Biology, The University of Western Australia, 35 Stirling HWY, Crawley, WA 6009, AustraliabDepartment of Plant and Environmental Protection Sciences, University of Hawaii, Manoa, 3050 Maile Way, Gilmore Hall 310, Honolulu, HI 96822, USAc The Department of Agriculture and Food, 3 Baron-Hay Court, South Perth, WA 6151, Australia

a r t i c l e i n f o

Article history:Received 29 July 2010Received in revised form22 November 2010Accepted 29 November 2010

Keywords:Frankliniella occidentalisTyphlodromips montdorensisNeoseiulus cucumerisHypoaspis milesSpinosadSingle species releasesMultiple species releasesStrawberry

* Corresponding author. Tel.: þ61 08 6488 2976; faE-mail addresses: m.touhidur.rahman@gmail.co

hawaii.edu (H. Spafford), sonya.broughton@agric.wa.g

0261-2194/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.cropro.2010.11.027

a b s t r a c t

Western flower thrips, Frankliniella occidentalis (Pergande), is a major pest of strawberry and other horti-cultural and ornamental crops. Biological control of F. occidentaliswith predatory mites is recommended asan additional management strategy to chemical control in glasshouse and protected crops. However, it isnot known whether multiple (two or three) species releases of predatory mites are more effective thansingle species releases. The effect of an application of spinosad followed by mite releases could furtherincrease suppression of F. occidentalis. In a series of trials in the glasshouse, we evaluated three commer-cially available predatory mite species, Typhlodromips montdorensis (Schicha), Neoseiulus cucumeris(Oudemans) and Hypoaspis miles (Berlese). Strawberry plants were sprayed once with either spinosad atthe recommended rate or with water. F. occidentalis adults were released onto plants 24 h after spraying,and mites were released six days later. Spinosad significantly reduced F. occidentalis compared to thecontrol (water). T. montdorensis, N. cucumeris and H. miles significantly reduced F. occidentalis compared tothe ‘no mite’ treatment. Spinosad had no effect on T. montdorensis and N. cucumeris, as their numbers didnot differ between the spinosad and control treatments; H. miles was not recovered. When mites werereleased after an application of spinosad, greater suppression of F. occidentalis was achieved than withreleases of predatory mites alone. When released as a double species combination, ‘T. montdorensis andH. miles’ was the most effective combination. There was no difference in efficacy between releases of‘T. montdorensis and H. miles’ or ‘T. montdorensis, N. cucumeris and H. miles’. We conclude that multiplespecies releases are more effective than single species releases, and that biological control of F. occidentaliswith predatory mites can be used together with spinosad.

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1. Introduction

Western flower thrips, Frankliniella occidentalis (Pergande)(Thysanoptera: Thripidae), is an economic pest of horticultural cropsworldwide, causing extensive crop losses through direct and indi-rect damage (Brødsgaard and Albajes, 1999; Jones et al., 2002; Kirkand Terry, 2003). In strawberry, Fragaria x ananassa (Rosaceae), F.occidentalis damage to flowers is characterized by brown andwithered stigmaandanthers, slight necrotic spots on the calyxof theflower (Steiner andGoodwin, 2005a; Coll et al., 2006) and reductioninflower receptacle size at high thrips density (>25WFTperflower)(Coll et al., 2006). Larval and adult feeding by F. occidentalis on the

x: þ61 08 6488 2243.m (T. Rahman), hspaffor@ov.au (S. Broughton).

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fruit surface causes net-like russetting (Steiner and Goodwin,2005a; Coll et al., 2006), which reduces shelf-life and fruit appear-ance (Coll et al., 2006). Thrips feeding onpink fruits causes bronzing(Coll et al., 2005). On older fruit, adult and larval feeding causesrussetting around the seed (achene) (Steiner and Goodwin, 2005b;Coll et al., 2006). Damage to strawberry by F. occidentalis has beenrecorded from Argentina (Gambardella and Pertuzé, 2006),Australia (Steiner and Goodwin, 2005a), Brazil (Gambardella andPertuzé, 2006; Nondillo et al., 2008), Chile (Gambardella andPertuzé, 2006), the USA (Zalom et al., 2008), Israel (Coll et al.,2005, 2006) and central, southern and northern Europe (Crosset al., 2001; Gonzalez-Zamora and Garcia-Mari, 2003).

To control F. occidentalis in strawberry and other crops, growersoften use insecticides as the main control strategy (Cook, 2000;Contreras et al., 2001; Bielza, 2008; Broughton and Herron,2009a). However, because of its small size, secretive habit, andhigh reproductive potential, management of F. occidentalis withinsecticides can be difficult (Jensen, 2000). In addition, F. occidentalis

T. Rahman et al. / Crop Protection 30 (2011) 468e475 469

develops insecticide resistance, and glasshouse and field pop-ulations resistant to insecticides in the older chemical classes havebeen recorded in field and protected crops worldwide (Brødsgaard,1994; Broadbent and Pree, 1997; Jensen, 1998, 2000). Populationsresistant to the ‘newer’ chemistry insecticides imidacloprid, fipronil(Zhao et al., 1994, 1995; Herron and James, 2005), and spinosad(Herron and James, 2005; Loughner et al., 2005; Bielza et al., 2007;Zhang et al., 2008) have also been detected. For these reasons,diverse control strategies are required to ensure that growers areable to maintain control of F. occidentalis.

Biological control with predatory mites and bugs has been usedto suppress F. occidentalis in strawberry (Steiner, 2002; Steiner andMedhurst, 2003; Coll et al., 2005, 2006; Shakya et al., 2010), therebyreducing the risk of insecticide resistance. Of the natural enemiesused for biological control of thrips, predatory mites have beenused successfully in field and protected crops (Chant, 1985; vanLenteren and Woets, 1988; McMurtry and Croft, 1997). The use oftwo or more species to suppress insect pest populations may alsobe more effective than single species releases. By releasing multiplespecies, the pressure on the pest population is increased, particu-larly if they attack different developmental stages of the pest.However, whilst some studies are supportive of the notion thatmultiple biocontrol agents are compatible (Gillespie and Quiring,1992; Wittmann and Leather, 1997; Brødsgaard and Enkegaard,2005), others oppose this view (Magalh�aes et al., 2004;Sanderson et al., 2005). For example, interspecific competitionmay occur when different species of natural enemies are combinedtogether (Schausberger and Walzer, 2001), including predation ofother natural enemies in addition to the target prey (intraguildpredation) (Brødsgaard and Enkegaard, 2005; Hatherly et al., 2005).An ideal biological control strategy for F. occidentalis would targetthe foliage-inhabiting adult and larval stages, and the soil-dwellingpupal stage. Typhlodromips montdorensis Schicha (Phytoseiidae)predates on first and second instar larvae of F. occidentalis andforages on all parts of the host plant (Steiner and Goodwin, 2002;Hatherly et al., 2004). Neoseiulus cucumeris (Oudemans) (Phyto-seiidae) predates on first instar F. occidentalis larvae (Bakker andSabelis, 1989) and prefers the lower part of the host plant(Messelink et al., 2006). Hypoaspis miles (Berlese) (Laelapidae) isa soil-dwelling predatory mite that predates on the prepupal andpupal stages (Glockemann, 1992), although some studies suggestthat it may also prey on late second instar larvae (Berndt, 2003).Because of the differences in preferred position on the plant and theprey stages of F. occidentalis consumed by each species, it isexpected that they will be compatible and their effect will becomplementary.

However, as natural enemies may not always be sufficient tosuppress the pest population, particularly in crops with loweconomic thresholds (Gillespie and Ramey, 1988; Bakker andSabelis, 1989; Gillespie, 1989), biological control could be inte-grated with chemical control. The use of ‘softer’ insecticides hasbeen successfully integrated with mite releases (Reuveni, 1995;Kongchuensin and Takafuji, 2006), though there are concernsabout the detrimental effects of pesticides on predatory mites.Sequential applications of insecticide followed by releases ofnatural enemies is one proposed solution to this problem (Rhodesand Liburd, 2006). Spinosad is often used in integrated pestmanagement and organic production, since it is highly efficaciousagainst F. occidentalis in a range of crops including strawberry(Steiner and Medhurst, 2003; Broughton and Herron, 2009b), withlow to moderate toxicity to predatory mites (Pietrantonio andBenedict, 1999; Williams et al., 2003; Jones et al., 2005). Spinosadhas been integrated successfully with biological control to manageF. occidentalis in field peppers and glasshouse-grown marigolds(Funderburk et al., 2000; Ludwig and Oetting, 2001).

To develop an integrated approach to managing F. occidentalis instrawberry, we evaluated the effect of releases of T. montdorensis, N.cucumeris and H. miles on F. occidentalis, combined with an appli-cation of spinosad. Our specific objectives were to (i) evaluate theeffectiveness of single versus combined release of predatory mitesfor the management of F. occidentalis, and (ii) evaluate the effec-tiveness of predatorymites released after an application of spinosad.

2. Materials and methods

The experiment was conducted in a glasshouse (25 � 2 �C,50e60% RH, 16:8 L:D cycle) at the University of Western Australia(UWA) from November 2007 to January 2008. The above conditionswere considered most suitable for F. occidentalis oviposition(Marullo and Tremblay, 1993). At 16.6e36.6 �C the total develop-ment period from egg to adult of F. occidentalis on chrysanthemumis 13e7 days (Robb, 1989).

2.1. Insect and plant rearing

Strawberry, Fragaria x ananassa Duchesne (Rosaceae) cv CaminoReal short-day length cultivar, developed by the University of Cal-ifornia (Shaw and Larson, 2008), was used in all experiments.Runners of Camino Real were obtained from a commercial growerand propagated in pots (325 mm L � 325 mm W � 405 mm H)containing potting mix (Baileys Fertilizers, Rockingham, WA) inglasshouses at the Department of Agriculture and Food WesternAustralia (DAFWA) and at UWA. All pots were fitted with drippersoperated by an automatic timer to water plants every third day. Aliquid fertilizer (Thrive�, Yates, Australia; NPK: 12.4:3:6.2; rate:5 ml/2 l water) was applied once a week. Pots were covered ina thrips-proof mesh cage, 450 mm � 350 mm, made from 105 mmesh net (Sefar Filter Specialists Pty Ltd., Malaga, WA) fitted overa steel-rod stand.

F. occidentalis were initiated from individuals collected fromcalendula, Calendula officinalis L. (Asteraceae) at DAFWA, and werereared on calendula planted in potting mix in plastic pots(50 mm � 100 mm). Planted pots were placed in thrips - proofPerspex cages (500mm� 420mm� 400mm). The sides of the cagewere coveredwith105mmeshnet, and the frontof thecagewasfittedwith a removable cover, held in place with magnetic strips. The cagewas placed on top of a Nylex tote box (cage tray,432 mm � 320 mm � 127 mm, Blyth Enterprises Ptd Ltd, Australia).All pots were fitted with drippers with an automatic timer as previ-ously described. Cages were kept in tunnel houses (insect proof nethouse) atUWA. Every secondweek, adultswere collected fromcagedplants with an aspirator and released onto new potted calendulaplants to ensure the continuous availability of thrips for experiments.

To obtain uniformly aged thrips, adult females (20 individuals)were collected from the colony, released onto fresh plants, andallowed to lay eggs for 24 h. After 24 h, females were removed withan aspirator. The plants were checked daily for larval emergence.Newly hatched larvae were removed and released onto a straw-berry leaf on a moistened filter paper in a Petri dish (150 � 15 mm).The leaf petiole was covered with cotton, soaked in 10% sugarsolution to extend leaf life. The top of the Petri dish was coveredwith thrips-proof mesh (105 m) and the edges of the Petri dish andmesh were sealed with paraffin film (Parafilm M�, Micro AnalytixPty Ltd) and kept in a controlled temperature room (25 � 1 �C,50e60% RH, 16:8 h L:D regime). Larvae that hatched on the sameday were transferred to a new Petri dish as above and allowed topupate. Adults that emerged on the same day were used in trials.

Predatorymites (T. montdorensis, N. cucumeris andH. miles) usedin the study were sourced from commercial suppliers (BiologicalServices, South Australia; Chilman IPM Services, Western Australia;

Fig. 1. Mean number of F. occidentalis (A) adults and (B) larvae per plant sprayed witheither spinosad or water, in the presence of no mite or different mite combinations.Within each group, means were significantly different (a ¼ 0.05). Tm ¼ T. montdorensis,Nc ¼ N. cucumeris, Hm ¼ H. miles.

T. Rahman et al. / Crop Protection 30 (2011) 468e475470

and Beneficial Bug Company, New South Wales). Mites wereprovided in plastic buckets as a mix of adults and nymphs invermiculite. Trials were conducted immediately upon receipt ofmites.

2.2. Effect of single versus multiple species releases of mitescombined with spinosad on F. occidentalis

The study was conducted as a two-factor repeated measuresdesign with 10 plants per treatment (each plant as a replicate) ina glasshouse at UWA. One hundred and sixty potted strawberryplants, two to three weeks old with 2e3 leaves (excess leaves werepruned), and were randomly divided into two groups. One group ofplants (n¼ 80)was sprayedwith spinosad at the recommended rate(80 ml/100 l rate, 0.096 g a.i./l), the other (n ¼ 80) with distilledwater. Treatments were applied with a hand-held atomizer (HillsSprayers, BH220063) until run-off. Plants were immediatelycovered separatelywith thrips - proof cages as described above. Thebottom of the cage was secured around the base of the pot withsticky tape, and the top of the cagewas enclosedwith a rubber band.

Twenty-four hours after spraying, 15 female thrips adults (2d old) were released onto each plant. Plants were then randomlyassigned to one of eight treatments, with ten plants per treatment:(i) no mites, (ii) T. Montdorensis, (iii) N. cucumeris, (iv) H. miles, (v)T. montdorensis and N. cucumeris, (vi) T. montdorensis and H. miles,(vii) N. cucumeris and H. miles, and (viii) T. montdorensis, N. cucu-meris and H. miles. Six days after spraying (after Khan and Morse(2006)), six adult mites were released onto plants in the singlespecies treatment, three individual adult mites of either speciesonto plants in double species treatments, and two individual adultmites of either species onto plants in the three species treatment.Pots were fitted with drippers and an automatic timer as describedabove. A small hole was made in the cage just above the pot top andirrigation pipe was inserted through the hole. The side of the holewas sealed with sticky tape.

Thenumberof F. occidentalis adults and larvaeoneachplantwerecounted every third day for three weeks (from thrips release to theend of the third week). Between 06:00 to 08:00 h, all leaves of eachplant were examined with a battery-powered hand-held illumi-nated magnifying glass (50 mm diameter, with 4 � bifocal magni-fier) for thrips adults and larvae without touching the plant or thepot. If theywerenot disturbed, F. occidentalis individuals tend to stayat their position. At the end of the trial, plants were cut at soil level,andplaced into a container (500ml)with 80% ethyl alcohol and laterplant materials were checked to determine the number of T. mon-tdorensis andN. cucumeris. In the laboratory, plants were sieved intoa double mesh net (105 mm), and mesh was then examined undera stereomicroscope (20 � magnification) for mites. The number ofT. montdorensis and N. cucumeris per plant were recorded. Todetermine the number of H. miles (soil-dwelling), the top soil(20e30 mm) from pots was collected and placed separately intoa container with 80% ethyl alcohol. Initially tullgren funnel wasconsidered to count H. miles numbers, however, the requirednumber of tullgren funnels was unavailable. The soil was sieved asdescribed above, and checked under stereomicroscope for H. miles.

2.3. Data analysis

The effectiveness of mite treatments (single-, double- or triple-species releases) and spray treatment (spinosad, control) onF. occidentalis numbers over time, were analyzed with repeatedmeasures ANOVAs (Proc Mixed Procedure). Adults and larvae wereseparately analyzed (independent fixed variables: mites treat-ments, spray treatment and time; random variable: plantnumbers). Because, there was a significant three - way interaction

between the main effects of spray treatment, predatory mites andtime (days), additional ANOVAs were performed for each spraytreatment (Quinn and Keough, 2002). Due to the number of post hocmultiple tests, the significance level was adjusted (a ¼ 0.025 (0.05/2)). When significant differences among means were detected, themeans were separated using least square means.

H.mileswas not recovered from samples andwas not included inanalyses. The difference in number of predatory mites for singlespecies releases (T. montdorensis and N. cucumeris) and spraytreatment (spinosad andwater)was analyzedwith two-wayANOVA(Proc Mixed Procedure; independent fixed variables: spray, mitespecies; response variable: number of mites). Similarly, two-wayANOVAs were used to determine the differences in numbers ofT. montdorensis and N. cucumeris in double species (T. montdorensisand N. cucumeris) and triple-species (T. montdorensis, N. cucumerisand H. miles) combinations. A two-way ANOVA was also used todetermine the difference in numbers of T. montdorensis andN. cucumeris when released in double species combination withH. miles (‘T. montdorensis and H. miles’ and ‘N. cucumeris and H.miles’). Two-way ANOVAs were also used to determine the differ-ence in numbers of T. montdorensis in double species releases(combinations: ‘T. montdorensis and N. cucumeris’ and ‘T. montdor-ensis and H. miles’). Similarly, separate two-way ANOVAs were usedto evaluate the abundance ofN. cucumeris in double species releases(‘T. montdorensis and N. cucumeris’ and ‘N. cucumeris and H. miles’).

Square root transformationsappliedto rawdatawereappropriate,tomeet the assumption of homogeneity of variances (Zar,1999). Datawere reverse transformed for presentation. All statistical analyseswere computed using Statistical Package SAS 9.1 (SAS, 2002e2003).

3. Results

3.1. Frankliniella occidentalis

3.1.1. Adults per plantAcross all mite combinations, there were fewer adult thrips on

spinosad-treated plants than water-treated plants (F7, 144 ¼ 14.06,P < 0.0001; Fig. 1A).

T. Rahman et al. / Crop Protection 30 (2011) 468e475 471

When plants were sprayed with spinosad, the number of adultson plants treatedwith differentmite combinations varied over time(F35, 360 ¼ 23.94, P < 0.0001, Fig. 2A). Six and nine days after thripswere released, the number of thrips adults per plant did not differbetweenmite combinations. From days 12 to 21 however, the meannumber of thrips adults was lowest on plants that received‘T. montdorensis and H. miles’ (two-species combination), exceptfrom days 15 to 21, when there was no significant difference inthrips numbers between plants that received ‘T. montdorensis andH. miles’ combination and three-species combination.

When plants were sprayed with water (control), the number ofthrips adults on plants treated with different mite combinationsalso varied over time (F35, 360 ¼ 37.21, P < 0.0001, Fig. 2B). On daysix, the number of adults per plant did not differ significantlybetween mite combinations. From days nine to 21, plants with thetwo-species combination of T. montdorensis and H. miles had thelowest number of thrips adults. Plants that did not receive anymites had the highest number of thrips adults.

3.1.2. Larvae per plantSimilar to adults, there were fewer thrips larvae on plants

sprayed with spinosad compared to water (F7, 144 ¼ 46.62,P < 0.0001, Fig. 1B).

On plants sprayed with spinosad, the number of thrips larvaevaried with mite combination over time (F35, 360 ¼ 6.64, P< 0.0001,Fig. 3A). On days 6 and 9, fewer thrips larvae were found on plantsthat received ‘T. montdorensis and N. cucumeris’. However, fromdays 15 to 21, the number of larvae did not differ between plantstreated with either the two-species combination of ‘T. montdorensisand H. miles’ or the three-species combination. Highest larvalnumbers per plant were recorded from plants that did not receiveany mites, except on day six. On day six, there was no difference inthe number of larvae on plants that did not receive any mites andplants that received H. miles.

In the water (control) spray treatment, the number of larva perplant differed amongst mite combinations over time (F35, 360¼ 5.21,P< 0.0001, Fig. 3B). On day six, plants treated with ‘T. montdorensis,

Fig. 2. Effect of predatory mites on the mean number of F. occidentalis adults per plant sprrelease. Tm ¼ T. montdorensis, Nc ¼ N. cucumeris, Hm ¼ H. miles.

N. cucumeris and H. miles’ had the lowest number of thrips larvae,and numbers remained low throughout the remainder of the trial.The number of larvae per plant did not differ between plantsreceiving ‘T. montdorensis and H. miles’ or ‘T. montdorensis andN. cucumeris’, nor between plants that received ‘T. montdorensis andH.miles’or ‘T.montdorensis,N. cucumerisandH.miles’ (Fig. 3B). Plantsin the control treatment that did not receive any mites had thehighest number of larvae across all sampling days except day nine.

3.2. Predatory mites

3.2.1. Single species releasesThere was no significant interaction of spray treatment and

predatory mite species on the overall mean number of mites(P > 0.05). Similarly, there was no significant different between themean number of T. montdorensis (20.18 SE ¼ 0.86 mites/plant) andN. cucumeris (19.75 SE ¼ 0.88 mites/plant) (P > 0.5). The overallmean numbers of mites per plant did not differ with spray treat-ment (spinosad 19.55 SE ¼ 1.22; water 20.38 SE ¼ 0.88, P ¼ 0.36).

3.2.2. Double species releasesSimilar to single species releases, there was no interaction of

spray treatment and mite species on the total numbers of mites perplant (spinosad 12.00 SE ¼ 11.51 mites/plant, water 12.45SE ¼ 1.32 mites/plant, P ¼ 0.5). However, significantly moreT. montdorensis (10.50 SE ¼ 0.49 mites/plant) were found thanN. cucumeris (8.55 SE¼ 0.55 mites/plant; F1, 36 ¼ 15.29, P¼ 0.0004).When either T. montdorensis or N. cucumeris were released withH. miles, there was no significant interaction of spray treatment andmite combination on the total number of mites per plant (P¼ 0.10).There was also no difference between the mean number ofT. montdorensis (12.10 SE ¼ 0.91) or N. cucumeris (12.35 SE ¼ 0.52)per plant (P > 0.5).

When T. montdorensis was released with either N. cucumeris orH. miles, spray treatment andmite combination had no effect on themean number of T. montdorensis per plant (P ¼ 0.4). There was noeffect on the mean number of T. montdorensis per plant (P ¼ 0.1)

ayed with (A) spinosad or (B) water. X-axis represents days after initial F. occidentalis

Fig. 3. Effect of predatory mites on the mean number of F. occidentalis larvae per plant sprayed with (A) spinosad or (B) water. X-axis represents days after initial thrips release.Tm ¼ T. montdorensis, Nc ¼ N. cucumeris, Hm ¼ H. miles.

T. Rahman et al. / Crop Protection 30 (2011) 468e475472

when plants were treated with either spinosad (10.90 SE ¼ 0.46T. montdorensis/plant) or water (11.72 SE ¼ 0.55 T. montdorensis/plant). However, more T. montdorensis were recovered (F1, 36¼ 11.73, P ¼ 0.002) when released with H. miles (12.12 SE ¼ 0.51)than with N. cucumeris (10.51 SE ¼ 0.49).

When N. cucumeris was released with either T. montdorensis orH. miles, there was no significant interaction between spray treat-ment and mite combination on the mean number of N. cucumerisper plant (P > 0.5). Similarly there was no difference in the meannumber of N. cucumeris per plant when plants were treated witheither spinosad (10.19 SE¼ 0.59 N. cucumeris/plant) or water (10.70SE ¼ 0.48 N. cucumeris/plant, P > 0.1). However, significantly moreN. cucumeriswere recovered from plants (F1, 36 ¼ 49.51, P< 0.0001)when released with H. miles (12.35 SE ¼ 0.62) rather than T. mon-tdorensis (8.55 SE ¼ 0.55).

3.2.3. Three species releasesThere was no interaction between spray treatment and mite

combination (P> 0.5). Thoughmore T. montdorensiswere found perplant (8.35 SE ¼ 1.42 T. montdorensis/plant) than N. cucumeris (7.50SE ¼ 1.22 N. cucumeris/plant), this difference was not significant(P ¼ 0.2). The overall number of predatory mites did not differbetween plants treated with either spinosad (7.95 SE ¼ 0.99 mites/plant) or water (8.95 SE ¼ 1.31 mites/plant).

4. Discussion

Predatorymites have been used tomanage F. occidentalis in fieldand greenhouse crops (Chant, 1985; van Lenteren andWoets, 1988;McMurtry and Croft, 1997), with varying degrees of success

(Chambers and Sites,1989; Gillespie,1989; Brødsgaard, 2004; Shippand Ramakers, 2004). The results of the present study suggest thatpredatory mites can reduce the F. occidentalis population in straw-berry, and when released as a single species, T. montdorensis wasmore effective resulting in fewer F. occidentalis than either N. cucu-meris or H. miles. However, our results suggest that the two-speciescombination of T. montdorensis and H. miles, or the three-speciescombination provide better management of F. occidentalis thansingle species releases.

Single species releases may not be sufficient to maintainF. occidentalis populations below the economically damaging levels.The economic threshold for strawberry is based on the number ofthrips in the flowers (Steiner and Goodwin, 2005a). In this study,strawberries were kept flowerless; therefore, it was not possible todirectly assess the efficacy of predatory mites on lowering thethrips number below the economic threshold. It would be valuableto assess the performance of the mites in the presence of flowersbecause functional and numerical responses by the predators toF. occidentalis may also be affected by the addition of pollen inflowers (Wei andWalde,1997; van Rijn et al., 2002; Messelink et al.,2010; Nomikou and Sabelis, 2010).

We did find that the double species combination of ‘T. mon-tdorensis and N. cucumeris’ was not as effective in reducing thripsnumbers as the other multiple releases. Both T. montdorensis andN. cucumeris are foliage-inhabiting mites and both predate onthrips larvae, which might result in interspecific competition andpotentially intraguild predation between these two species. Intra-guild predation can have a significant effect on prey suppression(Losey and Denno, 1998). Although intraguild predation is commonin many predators (Vance-Chalcraft et al., 2007) including mites

T. Rahman et al. / Crop Protection 30 (2011) 468e475 473

(Schausberger and Walzer, 2001; Walzer and Schausberger, 2005),it is not known whether intraguild predation occurs amongT. montdorensis, N. cucumeris and H. miles. Wiethoff et al. (2004)reported that cucumber plants had fewer F. occidentalis whenN. cucumeris and H. miles were applied together, than if eitherN. cucumeris or H miles were applied singly. Premachandra et al.(2005) showed that when foliage-inhabiting (T. montdorensis) andsoil-dwelling (H. miles) mites were applied together, they providedthe highest suppression of F. occidentalis. Synergistic effects can beexpected if plant-dwelling predators can evoke the escape behaviorof the prey, making the prey available for ground-dwelling preda-tors (Losey and Denno, 1999). When T. montdorensis and N. cucu-meriswere released together, there were more T. montdorensis thanN. cucumeris, which did not occur when they were released withH. miles suggesting that intraguild predation might have beenoccurring. However, this difference was not statistically significant.However, when all three species were released together, thenumbers of T. montdorensis and N. cucumeris seemed to indicate nointeraction. It may be possible that in the two-species combination,plants had higher initial numbers of both T. montdorensis andN. cucumeris, which may have increased interspecific competition.In triple-species releases however, plants had initially less T. mon-tdorensis and N. cucumeris, and therefore perhaps less competitionfor F. occidentalis. For successful implementation of multiplereleases of predatory mites in a pest management program, furtherstudies need to be determined if cannibalism, intraguild predationand preference between primary prey (pest population) andsecondary prey (predator population) occurs. Furthermore, theoptimal release rate of mites needs to be determined that caneffectively reduce the pest population, without having any negativeimpact on each other.

Chemical control combined with biological control was mosteffective at reducing F. occidentalis numbers in our study. Predatorymites in any combination performed better against F. occidentaliswhen released after spinosad was applied. Reducing numbers ofthrips before release of predators increased the predatorepreyratio and it would therefore be expected to give a better reductionin thrips numbers. Previous studies have similarly shown thatF. occidentalis can be controlled with an application of spinosad,followed by the release of beneficial organisms (Ludwig andOetting, 2001; Ludwig, 2002; Thoeming and Poehling, 2006).Ludwig and Oetting (2001) and Ludwig (2002) suggest that appli-cations of spinosad and the predatory bug Orius insidiosus Say(Hemiptera: Anthocoridae) can significantly reduce F. occidentalison glasshouse-grown potted chrysanthemum and marigold thaneither spinosad or releases of O. insidiosus alone. In contrast, vanDriesche et al. (2006) found that ‘spinosad only’ and‘spinosad þ N. cucumeris’ treatments had a similar effect on thenumbers of F. occidentalis in ornamental flowers, indicating thattherewas no advantage in releasingmites where spinosad has beenapplied. However, this may be due to differences in controlbetween a short duration crop in their study versus longer durationcrops (>2e3 months).

Spinosad is reported to have no or little effect on O. insidiosus(Funderburk et al., 2000; Ludwig and Oetting, 2001; Ludwig, 2002),though other studies have shown that spinosad has low toxicity toO. insidiosus (Elzen et al., 1998; Pietrantonia and Benedict, 1999).Kongchuensin and Takafuji (2006) reported that fresh spinosadresidues (up to 48 h old) have significant, negative effect on eggsand the immature stage of the predatory mite, Neoseiulus long-ispinosus (Evans) (Acari: Phytoseiidae). However, spinosad was notharmful to N. longispinosus seven days after application(Kongchuensin and Takafuji, 2006). van Driesche et al. (2006)suggest that spinosad is detrimental to N. cucumeris, inhibitingpopulation increase by reducing oviposition. In the present study,

spinosad appears to pose no detrimental effect to T. montdorensis,N. cucumeris, or H. miles if mites are released six days after spraying.

Chemical control has been the primary control strategy forF. occidentalis in many crops. However, calendar application ofbroad-spectrum chemicals for control of F. occidentalis has resultedin a classic 3-R situation: resistance to insecticides includingreduced-risk chemicals (such as spinosad), resurgence of thripspopulations due to the killing of naturally occurring natural enemiesand competitor species of thrips, and replacement with variousother pests that are induced by the application of the broad-spec-trum insecticides (Frantz and Mellinger, 2009; Funderburk, 2009).Application of spinosad followed by releases of natural enemiescould be used as a resistancemanagement strategy as this approachcould slow the resistance development process by reducing theselection pressure. Furthermore, we evaluated an indirect andresidual effect of spinosad on thrips in the sense that we releasedF. occidentalis 24 h after spraying spinosad on the plants. We foundthat this chemical can reduce thrips numbers even if they are notdirectly sprayed. F. occidentalis management could be furtherimproved by reducing the interval between spray application andpredatory mite release. However, the residual toxicity of spinosadand the effect of spinosad on oviposition of these predatory miteswould first need to be determined.

Acknowledgements

We thank Anthony Yewers of Berry Sweet, Bullsbrook, forproviding strawberry runners used in this study. David Cousins, atthe Department of Agriculture and FoodWA, helped raise andmaintain strawberry runners. We also thank Kevin Murray, Schoolof Mathematics and Statistics and Rohan Sadler, School of Agri-cultural and Resource Economics, the University Western Australia,for their expertise and assistance with statistical analyses used inthis study.

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