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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www.mlmp.org