Pesticide residues on milkweed and strawberry at small farms and non-target effects of two fungicides on monarch butterfly caterpillars

TL;DR

This study examines pesticide residues on crops and milkweed, and how two fungicides affect monarch caterpillars, showing negative impacts at high concentrations.

Contribution

The study reveals non-target effects of fungicides on monarch caterpillars and highlights the role of host plant quality in pesticide impacts.

Findings

Pesticide residues were more common on strawberries than milkweed, with higher levels early in the season.

High doses of cyprodinil and difenoconazole reduced monarch caterpillar feeding and growth, especially on low-quality host plants.

Fungicide effects on caterpillar growth efficiency varied by chemical and host plant species.

Abstract

Concern over insect declines has increased attention on the effects of pesticide residues on native insects. We collected strawberry (target) and common milkweed (non-target) foliage and flowers on two small Central New York farms, within crop fields as well as field margins, and analyzed the tissues for pesticide residues of 94 agrochemicals. We found quantifiable levels of 13 fungicides, herbicides, and insecticides, mostly at low concentrations (typically less than 200 ppb where detected), and more often on strawberry than milkweed. We generally found higher pesticide residue levels early in the season (June vs. July) and on leaves compared to flowers. Residue levels in fields did not differ strongly from margins but pesticide drift may have left low-level residues on milkweed leaves and flowers in margins. Given that non-target effects of fungicides are understudied, we selected two prevalent fungicides found in this study (cyprodinil and difenoconazole) and used them in laboratory assays to assess impacts on early instar monarch (Danaus plexippus) caterpillar feeding, growth, and growth efficiency on common milkweed, along with two other milkweed (Asclepias) species. We tested the hypothesis that exposure to pesticides may be most impactful on poor quality host plants. For both fungicides, exposure at the highest doses (>100,000 ppb) reduced feeding, with the strongest effect on Asclepias asperula, the lowest quality host plant. Effects on caterpillar growth were similarly negative and consistent across host plant species. Finally, effects of fungicides on gross growth efficiency of caterpillars were negative, but dependent on the fungicide. Effects of cyprodinil were stronger than difenoconazole, but at realistically low concentrations there was little effect of either fungicide. Nonetheless, higher concentrations of these chemicals, approximating those experienced directly after fungicide application, may impact non-target species. The observed interaction of fungicides with host plant species highlights the importance of considering resource quality in the assessment of non-target effects of pesticides.

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