Comparing actual and perceived soybean defoliation with field surveys

The economic threshold for defoliating insects during the reproductive stages of soybean varies among U.S. 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 accuracy to estimate defoliation has not been documented. We surveyed 303 growers, consultants, extension personnel, as well as other occupations in North Carolina and Missouri. We asked them to estimate defoliation based on trifoliate photographs. We also asked growers and consultants the area they typically grow soybeans, and if they scout for defoliation or not. We hypothesized that people with different backgrounds would estimate defoliation differently based on their experience with soybeans. Additionally, we hypothesized that growers with more area destined to grow soybeans would estimate defoliation better than growers with less area destined to grow soybeans. Finally, we hypothesized that growers who scouted their fields would better estimate defoliation compared to those who did not scout their fields. We analyzed the data using logistic regression. Defoliation estimates were not different depending on occupations. Additionally, the amount of soybean a grower planted did not influence defoliation estimates. However, growers who scouted their fields better estimate defoliation compared to those who do not. Our results highlight the importance of experience for more accurate visual estimation of soybean defoliation.