Monarch butterflies avoid pesticide-treated milkweed to limit chemical exposure

monarch milkweed

Hundreds of recent studies have voiced concern over the negative impacts of non‐target pesticides on pollinator health. However, pesticide loads are highly variable across agricultural landscapes and it is unclear whether pollinators exhibit behavioral responses (e.g. aversion) that mediate their exposure risk under realistic foraging environments.

We tested whether monarch butterfly Danaus plexippus adults and larvae base their oviposition [egg laying] and foraging decisions, respectively, on the presence and concentration of pesticide residues on their milkweed host‐plant, Asclepias syriaca. To do so, we experimentally simulated field‐realistic exposure for six of the most commonly detected pesticides—one insecticide, two herbicides and three fungicides—either alone or in combination.

These laboratory and greenhouse manipulations experimentally paired an untreated control with the pesticides at their mean or maximum concentrations. Last, we used a two‐year field survey to correlate pesticide concentration on milkweed leaves with monarch oviposition.

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Our data provide evidence that monarchs are capable of adaptively adjusting their oviposition and foraging behaviors based on which pesticides are present on their host‐plants. Importantly, this preference was observed at field‐relevant concentrations, suggesting that monarchs behaviorally regulate pesticide exposure risk for their offspring by avoiding contaminated plants.

Variability in pesticide identity and/or load among milkweeds within/between habitat patches should be considered in future restoration efforts aimed at attracting butterflies for larval development ….

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