Neonicotinoids are the most widely used insecticides worldwide and are typically deployed as seed treatments (hereafter NST) in many grain and oilseed crops, including soybeans. However, there is a surprising dearth of information regarding NST effectiveness in increasing soybean seed yield, and most published data suggest weak, or inconsistent yield benefit. The US is the key soybean-producing nation worldwide and this work includes soybean yield data from 194 randomized and replicated field studies conducted specifically to evaluate the effect of NSTs on soybean seed yield at sites within 14 states from 2006 through 2017. Here we show that across the principal soybean-growing region of the country, there are negligible and management-specific yield benefits attributed to NSTs. Across the entire region, the maximum observed yield benefits due to fungicide (FST = fungicide seed treatment) + neonicotinoid use (FST+NST) reached 0.13 Mg/ha. Across the entire region, combinations of management practices affected the effectiveness of FST+NST to increase yield but benefits were minimal ranging between 0.01 to 0.22 Mg/ha. Despite widespread use, this practice appears to have little benefit for most of soybean producers; across the entire region, a partial economic analysis further showed inconsistent evidence of a break-even cost of FST or FST+ NST. These results demonstrate that the current widespread prophylactic use of NST in the key soybean-producing areas of the US should be re-evaluated by producers and regulators alike.

In the US, the most recent published estimates reflect that approximately 34-44% of planted soybean acreage are treated with neonicotinoid seed treatments (NST) (Douglas & Tooker, 2015). Based upon trendlines shown in that work, the current estimate for NST use in soybeans is very likely to exceed 50%. Insecticidal seed treatments of soybean belong to the neonicotinoid class of insecticides that include the active ingredients clothianidin, imidacloprid, and thiamethoxam. Corn and soybean seed treatments represent the largest uses of neonicotinoids nationally, and the higher seeding rate of soybeans mean that they are responsible for the highest levels of active ingredient per unit area (USGS, 2014). It is notable that current NST use rates far exceed historic benchmarks for insecticide use in soybeans; in the decade prior to introduction of neonicotinoid seed treatments, only about 5% of soybean acres received insecticides (Fernandez-Cornejo et al., 2014). This benchmark reflects that the region where most of US soybeans are grown, the upper Midwest, benefits from a temperate climate and relatively few insect pests, particularly in the early season when NST would provide most crop protection. Recent reviews of insect pest abundance in soybean re-confirm this assessment – early season pests of soybean are still infrequently encountered across the region (Hesler et al., 2018; Papiernik et al., 2018). Soybean aphid, a relatively recent invader to US soybean production, is a notable exception, but it’s distribution and phenology are a poor fit for the earlygrowing season, when NST are most effective (Krupke et al., 2017).

Soybean (Glycine max Merr.) is a globally important oilseed crop. Use of seed treatments to avert yield loss by managing seedling pathogens, early-season insects, and nematodes has become more common. Seed treatments may effectively protect plant stand and plant health in the presence of pathogens and pests, but the profitability of prophylactic seed treatment use across diverse environments remains in question, particularly when pests and pathogen populations are low or absent. Seed treatments, including a non-treated control (NTC), fungicide (F), fungicide-insecticide (FI), and fungicide-insecticide-biological nematode protectant (FIN), were evaluated on four soybean varieties at seven field sites in 2013, 2014, and 2015. In 2013, yield data was collected, and in subsequent years additional parameters were measured. FIN significantly improved plant stand at two sites in 2014 and three sites in 2015. Scant soybean aphid (Aphis glycines Matsumura) numbers were found at four sites across 2014 and 2015. Soybean aphid populations in FI plots were lower relative to the NTC at two of the four sites. Soybean cyst nematode (Heterodera glycines Ichinohe) (SCN) was present in one field in 2015, but FIN treatment did not significantly reduce SCN reproduction or population relative to the NTC. Yield across soybean varieties was significantly improved by FIN at the Allegan county sites in 2013 and 2015. Across sites in 2013, no seed treatment significantly improved net returns relative to the NTC. Across sites in 2014 and 2015, FIN significantly reduced net returns relative to the NTC. The probability that a seed treatment would result in economically neutral or positive outcomes was estimated by using Maximum Likelihood Estimation. Results from these planting dates and seeding rates indicate that seed treatments may not benefit all soybean growers. Seed treatment benefits may be affected by soybean variety, soil, environmental conditions, planting population, and planting date. Early planting dates and reduced seeding rates may see an increase in seed treatment profitability.