We assessed the toxicity of permethrin residues from barrier treatments for mosquito control on monarch butterfly (Danaus plexippus L.) larvae and adults. The motivation for this study was an absence of work on the effects of this commonly used insecticide on nontarget herbivorous insects and the fact that monarchs’ host plant, milkweed (Asclepias spp.), is often found in treated areas. In one experiment, larvae fed leaves from naturally growing milkweeds in operationally treated areas were much less likely to survive than control larvae, even when treated leaves were collected up to 21 d after spraying. In a second experiment, larvae fed leaves from potted milkweeds sprayed with 0.5 and 0.1% dilutions of the operational dose had lower survival and longer development times than control larvae. In a third experiment, ovipositing females did not discriminate against permethrin-treated plants. Female survival in 0.66-m3 cages with plants sprayed 1 d earlier was very low, whereas survival was greater if plants were assayed 8 or 15 d later. In a fourth experiment, neither frequent overhead watering (versus watering from below) nor outdoor sunlight (versus in a glasshouse) over a 14-d period lessened the toxicity of sprayed plants to larvae. These findings indicate that monarch larvae and adults are likely to be killed if exposed to residues of permethrin after barrier treatments for mosquito control. Extent of mortality in a field population of monarch butterflies will depend, among other things, on the proportion of host plants that are treated in a given area.
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