Re-evaluation of Late Season Action Thresholds and the Effect of Seed Treatments on Early Season Colonization of Threecornered Alfalfa Hopper, Spissistilus festinus (Say), in Louisiana Soybean, Glycine max (L) Merr

Soybean, Glycine max (L.) Merr, is considered the most economically important oil and feed crop in the US and is grown on nearly 35,000,000 ha on an annual basis. Factors that limit yield include weed pressure, disease infection, and of particular interest to entomologists, insect herbivory. Soybean is targeted by a range of above and below ground pests which are primarily controlled by insecticides. Neonicotinoids are a recently popularized insecticide class that when applied as a seed treatment, can provide protection from early season pests. The most economically significant early season pest is threecornered alfalfa hopper, Spissistilus festinus (Say), whose feeding habit causes the formation of necrotic tissue known as a girdle. To understand the effect of neonicotinoid seed treatments (NSTs) on S. festinus, one greenhouse and two field studies were conducted from 2020–2021. An additional study was done to re-evaluate the Louisiana state recommended late-season action threshold (1 adult/sweep) over the same period. Hand-treated neonicotinoid seed treatments were found to significantly reduce survivorship of S. festinus infested on soybean under greenhouse conditions. However, when the same seed treatments were taken to the field level, NSTs did not significantly impact S. festinus cumulative insect days (CID). In additional field trials using Maturity Group (MG) IV and V soybean varieties commercially treated with different insecticide + fungicide seed treatments, only MGV varieties significantly impacted S. festinus CID. Threshold re-evaluation field trials found no relationship between S. festinus CID and yield parameters (total seed and hundred seed weight (g), per five plants). Results of greenhouse and field trials suggest NSTs do not provide consistent control of S. festinus. Additionally, no evidence was found to change the Louisiana state recommended action threshold. It was concluded that rather than rely on NSTs, S. festinus is best managed according to traditional integrated pest management practices