Quantifying Pesticide Exposure Risk for Monarch Caterpillars on Milkweeds Bordering Agricultural Land

This study set out to gain baseline data on the presence and concentration of 65 commonly used pesticides in the leaf tissue of common milkweed (Asclepias syriaca) plants growing adjacent to croplands. The study was conducted at seven sites in northwest Indiana. Samples were taken in June, July and August for two consecutive years (2015 and 2016). During the second year of study soil samples were also taken from some sites.

Fourteen pesticides were detected (4 insecticides, 4 herbicides and 6 fungicides) in milkweed tissue. Pesticide presence and concentrations varied widely. The herbicide atrazine was the most commonly detected pesticide. Fungicides were the most omnipresent pesticides detected. Thiamethoxam and deltamethrin, respectively found in 1.8 and 0 percent of samples in 2015, were both present in >75% of the samples taken in 2016. Clothianidin was found both years but only during June sampling. 15 to 25% of the plants collected in June contained clothianidin, with ~ 60% of plants contaminated at some sites. Imidacloprid was the fourth insecticide found with a 0.2 % detection in 2015 and no detections in 2016. Leaf concentrations of thiamethoxam and clothianidin were similar to findings in other plant species adjacent to agricultural fields, and in some cases similar to concentrations in crop leaves grown from treated seeds.

Thiamethoxam detection frequency and concentrations tended to be higher for milkweed grown closer to agricultural lands. No relationship between linear distance from field was found with either clothianidin or deltamethrin. Deltamethrin was detected in 98.9% of milkweed in 2016, researchers hypothesized that this could be due to it being applied aerially. Similarly, landscape composition surrounding milkweed (amount of corn and soybean within a 1 km radius) had weak predictive power to determine presence of pesticides. The findings regarding the relationship between contamination and linear distance to crop fields raise questions about the benefits and drawbacks of “no-spray” buffers to minimize off-site contamination.

The soil samples, taken in 2016, were tested for the 14 pesticides detected in leaf tissue. Seven of the 14 pesticides were detected. Clothianidin was the only insecticide detected and it was found in all samples throughout the season (whereas it was only found in leaf tissue during the June sampling period). Clothianidin soil detection was highly correlated with levels of co-occurring milkweed leaves. Three fungicides and three herbicides were also detected in soil. Soil concentrations of the chemicals were more stable than leaf detections.

This study provided initial data demonstrating that a number of pesticides contaminate milkweed throughout monarch breeding season. The potential impact of these exposures is still unknown. Only one of the pesticides, clothianidin, had toxicity data available for monarchs, and 5-8% of detections were at a level that may cause sublethal effects in monarch larvae (though the risk varies seasonally). Researchers highlight the need for greater study into the impact these exposures might cause to monarch. They also note the need to examine site-specific factors, such as wind direction and slope, that can lead to off-site contamination.

Olaya-Arenas, P.A., and I. Kaplan
Frontiers in Ecology and Evolution
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