Efecto de diferentes niveles de defoliación mecánica y causados por Rachiplusia nu en el rendimiento de soja

Soybean provides 60% of the plant-based protein consumed worldwide and is the main summer crop grown in Uruguay. Rachiplusia nu (Guenée) (Lepidoptera: Noctuidae) is a common defoliating pest in soybean crops, whose damage may affect yield due to the reduction of photosynthetically active leaf area. The objectives of this study were to: a) determine whether the effect of defoliation on soybean yield varies between damage caused by R. nu larvae and mechanical defoliation; b) quantify total yield, the number and weight of grains in plants subjected to different levels of defoliation caused by R. nu larvae at various phenological stages. A field experiment was conducted from November 2023 to May 2024 at the "Dr. Mario A. Cassinoni" Experimental Station on a soybean crop (GÉNESIS 6301). Following a completely randomized design, exclusion cages were placed on individual soybean plants, which reached different defoliation levels generated at different phenological stages (V5, V10, and R5) by infesting R. nu larvae or inflicting mechanical cutting with scissors. The achieved defoliation was visually estimated, and plants with a maximum of 2% defoliation were considered as controls. The origin of the defoliation (larvae or mechanical cutting) did not result in significant differences in yield within the evaluated ranges and timings (46–60% in V5 or 21–45% in R5). These results validate the economic injury thresholds established in previous studies using mechanical cuts, suggesting that there would be no specific response in plant yield to herbivory or its simulation through mechanical cutting. Regarding different levels of defoliation, at the V5 phenological stage, plants with defoliation levels greater than 45% showed reduced yield compared to control plants, having a yield loss of 22%. At V10, plants that experienced the maximum level of defoliation (80%) showed no yield differences compared to controls. At the R5, the number of grains was lower from the 10-20% defoliation range, while yield decreased from the 21-45% range, in which yield loss was 19%.. These findings highlight the high tolerance of soybean crops to defoliation under non-restrictive water conditions and justify the need to reduce insecticide, promoting an integrated and sustainable pest management approach.