Distance Diminishes the Effect of Deltamethrin Exposure on the Monarch Butterfly, Danaus plexippus.

The monarch butterfly, Danaus plexippus (Lepidoptera: Nymphalidae), is threatened by substantial loss of habitat, extreme weather events linked to global climate change, and nontarget impacts of broad-spectrum insecticides. To investigate the impact of chronic ingestion of pyrethroids on monarchs, wild-type Florida D. plexippus were reared on milkweed (Asclepias curassavica) that was exposed to ultra-low volume applications of DeltaGard by a truck-mounted fogger, at distances of 25 and 50 m. We observed significant negative impacts on monarchs reared on milkweed at 25 m from the DeltaGard spray route, including significant decreases in survival, and significantly longer development times, compared with untreated controls. Larvae reared on host plants closest to the truck spray route were 3 times more likely to experience a mortality event than the control cohort in trial 1 and 6 times in trial 2. Survival of monarch caterpillars reared on milkweed sprayed at 50 m was not significantly different from controls. For monarchs that survived to adulthood, we did not observe statistically significant differences among cohorts for variables measured. These data demonstrate that ultra-low volume treatments of pyrethroids can result in significant mortality in monarchs, but that the effects diminish with distance from the spray route.