Effects of Resmethrin Spray on Monarch (Danaus Plexippus) Butterflies and Larvae By Rosemary Lelich

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

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)

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% 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 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 =

Mortality rates for 1st and 3rd Beginning Instars (Downwind only) p = Assuming missing larvae died, p =

Mass of emerged adult butterflies that were sprayed as larvae (with s.d.), p = Control upwind downwind

AOV for total development time for beginning 3 rd instars only, p = Positionn ( Sample Size) Mean (days) S.E. Upwind d Downwind Control

Downwind larval mortality rates with respect to distance, p =

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 Testp-value Control vs. Sprayed Upwind vs. Downwind No downwind/upwind effect -- wind too variable Significant mortality rates of sprayed larvae

Larval, adult monarch, and mosquito mortality rates by station, p =

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

Percent mortality of sprayed larvae compared to sprayed mosquitoes

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,