Fitness and mating compatibility of Rachiplusia nu strains exposed to soybean expressing Cry1Ac in Argentina.

Rachiplusia nu (Guenée) (Lepidoptera: Noctuidae) is a significant agricultural pest in South America infesting several crops, including soybeans. Bacillus thuringiensis (Berliner) (Bt) soybean, expressing Cry1Ac protein, is widely planted as a control method for numerous lepidopteran pests. However, insect resistance to Bt proteins poses a threat to its sustainability. Recent field reports from Argentina, Uruguay, and Brazil have documented unexpected injury to Bt soybean caused by R. nu populations, which may indicate the development of resistance to Cry1Ac in this species. This study aimed to evaluate the biological performance, reproductive parameters, and reproductive compatibility of two R. nu strains, one susceptible (SS) and the other exhibiting reduced susceptibility to Bt toxin (RR), when reared on Bt and non-Bt soybean. Reproductive compatibility between strains was investigated through parental and hybrid crosses, evaluating fecundity, fertility, and mating success. SS larvae fed with Bt soybean failed to complete their life cycle, whereas RR larvae exhibited higher survival rates. Egg and larval stages of RR larvae were longer when reared on Bt soybean. Pupal mass was lower for Bt-fed resistant strain, although this did not reflect on fecundity and longevity. Results on parental crosses revealed that Bt-fed RR strain displayed reduced mating success, fecundity, and fertility, compared to the non-Bt treatment. Hybrid crosses showed evidence for prezygotic and postzygotic incompatibility. These results suggest a shift in susceptibility of R. nu to Cry1Ac protein and highlight the importance of implementing robust insect resistance management strategies to maintain the effectiveness of Bt crops.