Non-target effects of neonicotinoid seed treatments on milkweeds and monarch butterflies

The population decline of monarch butterflies has been a major conservation concern in recent years. Although different factors are hypothesized to cause this decline, one potential factor impacting monarch health is exposure to agrochemicals because their milkweed host-plants grow in close proximity to agricultural fields throughout the Midwest. To test the effects of agricultural contamination on monarch ecology, we tagged, georeferenced, and collected milkweed leaves from plants at varying distances from corn fields and tested them for residues of insecticides, herbicides, and fungicides. Liquid chromatography was used to separate, identify and quantify the chemical components in the samples. Simultaneously we surveyed the milkweed-associated insect community on sampled plants. All samples were collected from five sites in Indiana over three sample periods in June, July, and August. In early season samples we detected residues of two insecticides (the neonicotinoids clothianidin and thiacloprid), two herbicides (atrazine and metolachlor), and two fungicides (azoxystrobin and pyraclostrobin). Neonicotinoids were detected in ca. 20% of sampled plants with clothianidin concentrations ranging from 0.24 – 14.0 ppb. The most prevalent pesticide was the herbicide atrazine, which was present in all samples with concentrations varying between 0.31 – 13.9 ppb. Importantly, none of the pesticides showed spatial associations with distance to nearest agricultural field, suggesting that coordinating monarch/milkweed restoration efforts to avoid cropland may be ineffective. While the range of insecticide values we recorded have been shown to cause sub-lethal and lethal effects on beneficial insects, it remains unclear what effect, if any, the non-insecticide pesticides have on insect conservation.