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.