Neonicotinoid seed treatments and early soybean planting: quantifying pest abundance, crop protection, and effects on yield.

Soybean (Glycine max (L.) Merr.) yields have increased with advances in plant breeding and management practices, including earlier planting. Early planting in temperate climates maximizes yield potential but may increase vulnerability to certain pests and environmental stressors. Neonicotinoid seed treatments, particularly thiamethoxam, are widely marketed to protect soybean seedlings, yet their effectiveness under typical early-season field conditions in Indiana remains unexplored. This 2-year (2023 to 2024) study was conducted at 3 sites in Indiana. Soybeans were planted during 4 discrete periods (early April to late May). Seeds were treated as follows: (1) untreated seeds, (2) CruiserMaxx APX (thiamethoxam, mefenoxam, fludioxonil, picarbutrazox, and sedaxane), (3) Cruiser 5FS (thiamethoxam only), and (4) CruiserMaxx APX without thiamethoxam. Soybean pest populations and defoliation were monitored weekly. Plant stands, counts, and plot yields were also collected. Thiamethoxam seed treatments did not reduce pest populations but did lower defoliation slightly under low pest pressure. Later-May plantings reduced pest counts and damage compared to early-April plantings. Despite higher observed pest pressure and damage in the early season, early plantings produced higher yields than later plantings, with no differences between treated and untreated plots. Planting timing strongly influenced pest dynamics and yields. Although pest abundance and damage were heightened in early-season planting, observed defoliation remained well below economic thresholds and did not translate into yield loss. In our study, the minor reductions in damage observed were not associated with economic benefits, suggesting that routine insecticidal seed treatments may be unnecessary, even for early spring soybean planting.