Oviposition patterns of primary lepidopteran defoliators in soybean and the impact on structured refuge recommendations.

Background: Transgenic Bt technology in soybean, with plants expressing Cry1Ac, has been adopted as an insect pest management tool. It was first adopted in large areas of South America and Asia in 2013. The risk of resistance in target pests to this technology demands insect resistance management (IRM) programs. In Brazil, a structured refuge (area of non-Bt soybean) planted adjacent to the Bt soybean crop has been an important IRM recommendation, particularly for the primary lepidopteran defoliators Anticarsia gemmatalis (Lepidoptera: Erebidae) and Chrysodeixis includens (Lepidoptera: Noctuidae). The overall goal of this study was to validate IRM recommendations to Bt soybean. The objectives were to document the impact of soybean phenology, cultivar choice and non-Bt soybean defoliation on moth oviposition. In addition, a mark-release-recapture study estimated the dispersal capacity of these species. Five field experiments per species were performed for 3 years.

Results: Our results revealed an increase in A. gemmatalis and C. includens oviposition, respectively, on Bt plants as a consequence of the difference in plant growth stage at the time of oviposition. Defoliation of non-Bt plants significantly increased the oviposition preference of both moth species for Bt plants. The mark-release-recapture experiment indicated an average dispersal distance of ~300 m from the release point for A. gemmatalis, with maximum recapture at 1000 m.

Conclusion: Overall, our findings emphasize the importance of planting synchronization of Bt soybean and the structured refuge. In addition, when operational aspects in large soybean areas challenge this recommendation, the priority should be for planting the refuge area first. This approach will minimize the impact of selective oviposition of A. gemmatalis and C. includens. © 2024 Society of Chemical Industry.