Characterization and estimation of Cry1F resistance in fall armyworm, Spodoptera frugiperda (J. E. Smith)

Transgenic maize (Zea maize) expressing Cry1F protein from Bacillus thuringiensis has been registered for Spodoptera frugiperda (J. E. Smith) control since 2003. However, 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 determined in a resistant colony of S. frugiperda originating from Puerto Rico which displayed >300-fold resistance to purified Cry1F. Concentration-response bioassays of reciprocal crosses of resistant and susceptible parental populations indicated that the resistance is recessive and autosomal. Bioassays on the backcross of the F1 generation crossed with the resistant parental population suggest that a single locus or a set of tightly linked loci is responsible for the resistance. Because the resistance is recessive and conferred by a single locus, we employed an F1 screening approach to estimate allele frequencies from populations in South Florida and Texas in which field collected individuals of unknown genotype were crossed with the resistant colony, assumed to be homozygous for resistance. The offspring of these crosses were bioassayed using a discriminating Cry1F concentration and it was determined that resistance alleles could be detected from Florida and at much lower frequencies in Texas. In addition, cross-resistance to Cry1Aa, Cry1Ab, Cry1Ac, Cry1Ba and Cry2Aa was assessed in the Cry1F resistant colony. There was no significant cross-resistance to Cry1Aa, Cry1Ba and Cry2Aa, although activity of these toxins was limited even in the susceptible strain. In contrast significant cross-resistance (< 20-fold) was observed for both Cry1Ab and Cry1Ac.