Ecological impacts of pesticide seed treatments on arthropod communities in a grain crop rotation

1. While many studies have investigated non-target impacts of neonicotinoid seed treatments (NSTs), they usually take place within a single crop and focus on specific pest or beneficial arthropod taxa.

2. We compared the impacts of three seed treatments to an untreated control: imidacloprid + fungicide products, thiamethoxam + fungicide products, and fungicide products alone in a three-year crop rotation of full-season soybean, winter wheat, double-cropped soybean and maize. Specifically, we quantified neonicotinoid residues in the soil and in weedy winter annual flower buds and examined treatment impacts on soil and foliar arthropod communities as well as on plant growth and yield.

3. Unquantifiably low amounts of insecticide were found in winter annual flowers of one species in one site year, which did not correspond with our treatments. Although low levels of insecticide residues were present in the soil, residues were not persistent. Residues were highest in the final year of the study, suggesting some accumulation.

4. We observed variable impacts of NSTs on the arthropod community; principle response curve and redundancy analyses exhibited occasional treatment effects, with treatments impacting the abundance of various taxa, including predators and parasitoids. Overall, foliar taxa were impacted more than soil taxa, and the fungicides occasionally effected communities and individual taxa.

5. Pest pressure was low throughout the study, and although pest numbers were reduced by the insecticides, corresponding increases in yield were not observed.

6. Synthesis and applications. Pesticide seed treatments can impact arthropod taxa, including important natural enemies even when environmental persistence and active ingredient concentrations are low. The foliar community in winter wheat showed that in some cases, these impacts can last for several months after planting. Given the low pest pressure and lack of yield improvement in full-season and double-cropped soybean, winter wheat, and maize, we did not observe benefits that could justify the risks associated with neonicotinoid seed treatment (NST) use. Our results suggest that NSTs are not warranted in Maryland grain production, outside of specific instances of high pest pressure.