Evaluating the potential benefits and long-term sustainability of neonicotinoid seed treatments in mid-Atlantic grain crop production
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Neonicotinoid insecticide seed treatments (NSTs) are heavily used in US grain production; nearly all corn and over a third of soybeans grown are treated. However, NSTs primarily provide protection against occasional early-season soil and seedling pests and rarely improve yield. Additionally, the active ingredients from NSTs can spread and persist in the environment where they can impact various non-target organisms including beneficial arthropods and soil microorganisms. To determine the costs and benefits of NSTs in Maryland grain crops, I evaluated the impacts of two popular NSTs, imidacloprid and thiamethoxam, and their associated seed applied fungicides on insect pest suppression, yield, non-target arthropods, and soil health in a three-year rotation of full-season soybean, winter wheat, double-cropped soybean, and corn. Pest pressure was low throughout the study, as is typical for Maryland, and the NSTs did not provide any yield benefits. Treatments variably impacted non-target arthropods, reducing the abundance of some predators and parasitoids. Seed applied fungicides also impacted non-target arthropods. Because parasitoid wasps were disrupted in winter wheat up to 32 weeks after planting, I conducted a laboratory study to better understand NST suppression of cereal aphids and the mechanisms by which they affect cereal aphid parasitoids. Neonicotinoid seed treatments may not be effective enough to maintain aphids below the economic threshold in winter wheat; thus, they may negatively impact parasitoids through contaminated hosts. In my study, NSTs did not detectably affect soil health or the soil microbial community; however, they have the potential to harm aquatic communities through leaching and runoff. Given the lack of pest pressure and yield benefits, as well as the potential for non-target impacts, my research suggests that the use of NSTs in Maryland grain crops is neither warranted nor sustainable. It also highlights the need for further evaluation of the non-target impacts of seed applied fungicides, and of the effects of NSTs on water bodies within the Chesapeake Bay watershed.