Characterization of resistance to Bacillus thuringiensis toxin Cry1Ac in Crocidosema sp. (Lepidoptera: Tortricidae) from Brazil.

Background: Crocidosema aporema (Walsingham 1914) has historically been the main bud borer species in soybean in Brazil; however, a recent study reported that this species is not C. aporema but an undescribed species. In recent seasons, injury by Crocidosema sp. has been reported in soybean expressing the Bacillus thuringiensis Berliner (Bt) toxin Cry1Ac. In this study, we investigated the genetic basis of Cry1Ac resistance and assessed the fitness costs of resistance in a field-derived Cry1Ac-resistant Crocidosema sp. strain selected through an F screen.

Results: The Cry1Ac-resistant strain of Crocidosema sp. developed from neonate to pupae fed on Cry1Ac soybean leaves and adults produced viable offspring. The resistance ratio to Cry1Ac in this strain was >500 000-fold relative to the susceptible strain. Results from leaf and diet-overlay bioassays indicated that the inheritance pattern of Cry1Ac resistance in Crocidosema sp. is sex-linked, with a lethal dose compensation in the F progeny from R♂ × S♀ (making resistance incompletely recessive), whereas the F progeny from R♀ × S♂ had complete mortality on Cry1Ac soybean (making resistance completely recessive). The mortality of backcrosses on Cry1Ac soybean varied significantly, suggesting that the resistance is controlled by multiple genes. Life-history results revealed that the Cry1Ac-resistant strain had lower pupal weight, reproductive performance and population growth on non-Bt soybean compared with the susceptible strain.

Conclusions: The inheritance patterns of Cry1Ac resistance in Crocidosema sp. are sex-linked, incompletely or completely recessive depending on the direction of the reciprocal cross, polygenic and associated with fitness cost. © 2025 Society of Chemical Industry.