Inheritance and fitness costs of Cry1A.105 resistance in two strains of Spodoptera frugiperda (J.E. Smith)

Cry1A.105 is a Bacillus thuringiensis (Bt) protein expressed in many Bt corn hybrids for controlling moth pests including the fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith). The objective of this study was to compare the inheritance and fitness costs of two Cry1A.105-resistant FAW strains (RR32 and RR67) from Florida, U.S. To assess inheritance of the resistance, larval mortality of one susceptible strain (SS), RR32, RR67, and eight FAW strains generated from various genetic crosses was assayed on Cry1A.105-treated diet and Cry1A.105 corn leaf tissues. In the fitness cost study, survival, growth, development, and fecundity of SS, RR32, RR67, and two F1 heterozygous-resistant FAW strains were evaluated on non-Bt corn leaf tissue. The Cry1A.105 resistance in both RR32 and RR67 was likely inherited as a single autosomal gene with no fitness costs. The dominance levels differed between the two strains and varied from incompletely recessive to incompletely dominant, depending on Cry1A.105 concentrations and assay materials. The results of this study indicate a relatively high risk of resistance development to single-gene Cry1A.105 corn in FAW. Together with published data, the results also show that non-recessive resistance and lack of fitness costs in resistance to Bt crops are not uncommon, which provides a cautionary evidence against the assumption that resistance is recessive in an resistance management plan for Bt crops.