Damage assessment of Helicoverpa armigera (Lepidoptera: Noctuidae) in soybean reproductive stages

Abstract

Quantification of damage by Helicoverpa armigera on soybean is a key factor in understanding the potential of this pest to cause losses in Brazil soybean fields and determining the timing of its control. The objective of this study was to characterize and quantify the damage of H. armigera at two reproductive stages of soybean in Brazil. Four experiments were conducted at two soybean phenological stages, full bloom (R2) and early grain filling (R5.1) over two years. A range of H. armigera larval densities up to 8 larvae m−2 were infested on plants using third instar larvae for 15 days. Number of pods m−2, seeds m−2, seeds pod−1 and seed yield were assessed in the upper, middle and lower third of soybean plants. H. armigera caused more injuries in the middle part of the plant, followed by the upper and lower third. The rate of seed-yield loss at the R2 growth stage was 7.7 g per larva, significantly lower than the rate of loss at the R5.1 stage (10.6 g per larva). At both crop growth stages, there were losses in total pod numbers and seeds/pod with increasing larval density, but sometimes with an initial period of compensation. The response of average seed weight to increasing infestation varied with plant growth stage, decreasing at the R2 stage, but increasing or not changing at R5.1. The results demonstrate that relatively low infestations of H. armigera can cause significant reductions for soybean in Brazil, and it is necessary to intensify monitoring and use of efficient and alternative control methods for the use of insecticide. In conclusion, our study shows that growth stage R5.1 is more sensitive to damage by H. armigera than the R2 stage. Further studies should be carried out to quantify the damage of H. armigera in the vegetative stage of the soybean crop in Brazil and also on determinate cultivars.