Genetic basis of resistance to Cry1Ac in Rachiplusia nu (Lepidoptera: Noctuidae): inheritance mode, cross‐resistance patterns and fitness cost

Abstract BACKGROUND Rachiplusia nu (Guenée) was historically a secondary soybean pest in Brazil, but a key soybean pest in Argentina. From 2021 onwards, injury caused by R. nu has been reported in soybean that expresses the Cry1Ac toxin from Bacillus thuringiensis (Berliner) in both countries. In this study, we selected resistant and susceptible strains of R. nu to Cry1Ac using Cry1Ac‐containing leaf tissue and characterized the inheritance of resistance, cross‐resistance patterns and fitness cost. RESULTS Neonates of the Cry1Ac‐resistant strain of R. nu were able to develop on Cry1Ac soybean leaves and emerge as fertile adults, while neonates from the susceptible and heterozygous strains did not survive beyond 10 days. The resistance ratio to Cry1Ac estimated in diet‐overlay bioassays in the resistant strain was > 736.92‐fold. The inheritance pattern of Cry1Ac resistance in R. nu was characterized as autosomal recessive and monogenic. The Cry1Ac‐resistant strain of R. nu also exhibited high resistance to Cry1A.105 (resistance ratio > 159.87‐fold), but negligible resistance to Cry2Ab2 (resistance ratio = 1.25‐fold). Life history data showed that the resistance to Cry1Ac in R. nu is not associated with a substantial fitness cost. CONCLUSIONS The inheritance pattern of Cry1Ac resistance in R. nu is autosomal recessive, monogenic and not associated with obvious fitness costs. Cross‐resistance occurred between Cry1Ac and Cry1A.105 in R. nu but not between Cry1Ac and Cry2Ab2, indicating that Cry1A.105/Cry2Ab2/Cry1Ac soybean is a valuable tool to manage Cry1Ac resistance in R. nu . This is the first study reporting the genetic basis of Cry1Ac resistance in R. nu . © 2024 Society of Chemical Industry.