Abstract The economic threshold for defoliating insects during the reproductive stages of soybean varies among US states, from 15% to 25%. Defoliation estimation is made by visually inspecting the soybean canopy. However, there is variation among individual estimates, resulting in over- and underestimations of defoliation that might lead to improper spray decisions. Growers, consultants, and extension personnel are usually responsible for insecticide decisions, but their ability to estimate defoliation has not been documented. We surveyed 303 growers, consultants, extension personnel, as well as other occupations. We asked them to estimate defoliation based on trifoliate photographs. We also asked growers and consultants about the amount of area they typically grow soybeans and if they scout for defoliation or not. We hypothesized that low levels of defoliation would be easier to estimate compared to intermediate level of defoliation, that people with different backgrounds would estimate defoliation differently, and that growers who scouted their fields would better estimate defoliation compared to those who did not. We analyzed the data using multinomial logistic regression. Estimates were more correct for low levels compared to intermediate levels of defoliation. Estimates were not different depending on occupations. Growers who scout their fields better estimate defoliation compared to those who do not. Our results highlight the importance of experience for more accurate visual estimations of soybean defoliation and point out the need for reference tools to aid estimates of thresholds above 20% defoliation.
Schardong et al. (2026) studied this question.