Characterization of resistance to the Cry1F toxin from Bacillus thuringiensis in resistant fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) from Puerto Rico

Transgenic corn expressing Cry1F protein from Bacillus thuringiensis Berliner has been registered for Spodoptera frugiperda control since 2003. Unexpected damage to Cry1F corn was reported in 2006 in Puerto Rico and Cry1F resistance in S. frugiperda was documented. The inheritance of Cry1F resistance was characterized in a S. frugiperda resistant strain from Puerto Rico which displayed \u3e387-fold resistance to Cry1F. Concentration-response bioassays of reciprocal crosses of resistant and susceptible parental populations indicated that resistance is recessive and autosomal. Bioassays of the backcross of the F1 generation crossed with the resistant parental strain suggest that a single locus is responsible for resistance. Cross-resistance experiments indicated no significant Cry1F cross-resistance to Cry1Aa, Cry1Ba and Cry2Aa. Vip3Aa was effective against both strains indicating no cross-resistance with Cry1F. In contrast, significant cross-resistance (\u3c 20-fold) was observed for Cry1Ab and Cry1Ac. An F1 screen was performed to measure the frequency of Cry1F resistant alleles in 2010 and 2011. A total frequency of resistant alleles of 0.13 and 0.02 was found for Florida and Texas populations respectively, indicating resistant alleles could be found in U.S. populations. Fitness cost estimates associated with resistance suggest that heterozygous and homozygous resistant insects are equally fit with susceptible insects. This may affect initial allele frequencies in field populations and persistence in resistant populations (e.g. Puerto Rico). Behavior experiments suggest that there is not a strong difference between resistant and susceptible phenotypes in S. frugiperda and O. nubilalis. However, behavioral differences were observed between species. O. nubilalis exhibited increased movement between leaf discs, with susceptible neonates avoiding Cry1F tissue. In contrast, S. frugiperda selected plant tissue regardless of the presence of Cry1F, suggesting that refuge in a bag might be a suitable strategy for this pest. Understanding resistance in field-derived resistant populations will provide information for better risk assessments, improve predictions of resistance and maximize the benefits of current and future generations of transgenic crops