effects of resmethrin spray on monarch (danaus plexippus) butterflies and larvae by rosemary lelich...
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Effects of Resmethrin Spray on Monarch (Danaus Plexippus)
Butterflies and Larvae
By Rosemary Lelich 2004-05
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Background: resmethrin
• Type 1 pyrethoid
• Synthetic version of natural insecticide produced by chrysanthemum flowers
• Nerve poison - acts on sodium ion channels in nerve cell membranes
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www.free.de/WiLa/derik/Resmethrin.gif
Background: resmethrin, cont.
• Used to control mosquito populations
• Sprayed in wooded areas and fields where milkweed can grow
• Recent use has increased in Minnesota due to spread of West Nile Virus
• No previous studies on effects of resmethrin on monarchs exist
Background: Larvae
• Monarch larvae (caterpillars) non-target insect for insecticide sprayings
• Terrestrial• Only eat milkweed (Asclepias)
http://www.mbsf.org/news.html
Previous study: permethrin
• Similar pyrethoid • Study done by Sara
Brinda (2003)• Estimated % mortality
for 1st, 2nd, and 3rd instars, respectively, to be 92%, 89%, 71%
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Photo used with permission of Sara Brinda
Goals and Hypotheses
1. Estimate the percent mortality of monarch larvae and adult monarchs due to resmethrin exposure
Hypothesis: Resmethrin is lethal to monarchs
2. Determine if there are sub-lethal effects of resmethrin exposure on monarch larval development time and adult size
Hypothesis: Sprayed larvae will develop more slowly and will be smaller adults
Procedure: Round 1
• Kept 8 cages of larvae, used as controls
• Set out cages of larvae and cages of mosquitoes
• Resmethrin only reached stations downwind side of spray path
Photo by Bruce Leventhal
Cage containing milkweed leaves and monarch larvae
Resmethrin Sprayer
Photo by Bruce Leventhal
Procedure
• Recorded each time a larva molted to the next instar (growth stage)
• Recorded all mortality • Continued until all
monarchs emerged as adults
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http://asclepias.homestead.com/files/xmonlarvae.JPG
Results: Round 1 larvae Significant effect of treatment. More dead larvae in downwind treatment than the control or
upwind treatments. Comparing downwind larvae to control, p = 0.0000.
0
10
20
30
40
50
60
70
80
90
Control Upwind Downwind
Percent in Category
Alive
Dead
Missing
Mortality rates for 1st and 3rd Beginning Instars (Downwind only) p = 0.0273. Assuming missing
larvae died, p = 0.4568.
0
10
20
30
40
50
60
70
80
1st 3rd
Proportion in Category
AliveDeadMissing
Mass of emerged adult butterflies that were sprayed as larvae (with s.d.), p = 0.2862.
0.43
0.44
0.45
0.46
0.47
0.48
0.49
0.5
0.51
0.52
0.53
0.54
Control Upwind Downwind
Mass (g)
Mass
Control upwind downwind
AOV for total development time for beginning 3rd instars only, p = 0.3008.
Position n (Sample Size)
Mean (days)
S.E.
Upwind 15 22.4 d 0.3344
Downwind 6 23.33 0.5287
Control 17 22.88 0.3141
Downwind larval mortality rates with respect to distance, p = 0.7541.
44
46
48
50
52
54
56
58
60
1 2 3
Distance
Percent Mortality
Procedure: Round 2• Redesigned field set up before spraying
• Added to each station– Horizontal cages of larvae– Milkweed plant with larvae on leaves (on ground)– Milkweed plant with no larvae (on ground)– Cage of adult butterflies (on ground)
• Wind variable; reached both up and down wind stations
Dead Larvae: downwind during round 2 spraying
Photos by Brij Basin and Rosemary Lelich
Results: Round 2 Larvae
2 comparing larval mortality rates
Test p-value
Control vs. Sprayed 0.0151
Upwind vs. Downwind 0.1655
• No downwind/upwind effect -- wind too variable• Significant mortality rates of sprayed larvae
Larval, adult monarch, and mosquito mortality rates by station, p = 0.0001.
0
20
40
60
80
100
1 2 3 4 5 6 7 8 9 10 11 12
Station Number
Percent Mortality
larvaeadult monarchmosquitoes
• Larvae placed directly on milkweed plants and unsprayed larvae fed sprayed leaves produced significantly high mortality
• These exposures most closely mimic larvae’s natural positioning
• This suggests that round 1 underestimates larval mortality
Mortality rates of exposure types compared to control mortality rates Exposure type
2 p-value Vertical Cage 0.41 0.0949
Horizontal Cage 1.48 0.1929 Larvae on leaf 2.33 0.0006
Sprayed leaf fed to larvae 0.13 0.014
Percent mortality of sprayed larvae compared to sprayed mosquitoes
0
10
20
30
40
50
60
70
80
90
100
Round 1 Larvae Round 2 Larvae Mosquitoes(Rounds 1 and
2)
Percent Dead
Discussion
• Accept hypothesis 1: Resmethrin is lethal to monarch species
• Reject hypothesis 2: Sprayed larvae did not emerge as significantly smaller adults
• Resmethrin is likely a factor responsible for decrease in monarch populations– Very important to understand consequences of
using resmethrin in order to preserve monarch species
Future Study
• Conduct another round of resmethrin spraying, again including larvae and adults.
• Position more larvae downwind, attempt to retain large sample sizes for more significant results
Acknowledgements• Dr. Karen Oberhauser• Everyone in the lab (especially Ben,
Alma, Reba, Erin, and Jolene)• Metropolitan Mosquito Control District,
especially Dr. Stephen Manwieler• Sara Brinda• Ms. Lois Fruen• Team Research
Photo courtesy of Lois Fruen
Effects of Resmethrin Spray on Monarch (Danaus Plexippus)
Butterflies and Larvae
By Rosemary Lelich, 2004-05
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