Re‐evaluation of the economic threshold for Crocidosema aporema injury to indeterminate Bt soybean cultivars

Abstract

The neotropical soybean budborer (Crocidosema aporema [Walsingham, 1914] [Lepidoptera: Tortricidae]) is classified as a major pest of soybean that is difficult to manage with insecticides because larvae cause leaf-rolling and can bore into the stem. Genetically modified Bt (Bacillus thuringiensis) soybean has been effective in managing C. aporema populations until recently when resistance has emerged and high infestation rates have been observed in Brazilian Bt soybean fields expressing Cry1Ac. The economic threshold (ET) for C. aporema was previously studied only in conventional (non-Bt) determinate varieties of soybean. The present work evaluated the potential of C. aporema to injure indeterminate Bt soybean cultivars in two independent field trials carried out in 2020-2021 and 2021-2022 growing seasons. One field trial compared the response of individual injured and noninjured plants in the same field. The number of pods per plant, number of seeds per plant, and seed mass per plant in 12 of 13 trials were similar. In the second field trial, areas with and without insecticide applications were compared. There was a reduction in C. aporema damage after three insecticide applications, but yields were similar between treatments. Based on these results, C. aporema had low potential to cause economic injury to indeterminant Bt soybean cultivars. Thus, the current ET for C. aporema of 25%-30% of infested plants can be increased to 50%.