Lack of relevant cross-resistance to Bt insecticide XenTari in strains of Spodoptera frugiperda (J. E. Smith) resistant to Bt maize.

The resistance of fall armyworm, Spodoptera frugiperda, has been characterized to Cry and Vip3A proteins of Bacillus thuringiensis (Bt) expressed in maize in Brazil. Here, we investigate the cross-resistance to Bt-insecticide XenTari in selected fall armyworm strains resistant to Bt maize varieties. The LC of XenTari in neonates of resistant strains ranged from 0.28 to 0.68 µg a.i./cm, while for the susceptible reference strain (Sus), LC was 0.21 µg a.i./cm. This indicated a resistance ratio lower than 3.2-fold. A similar variation in susceptibility was detected in EC values, which ranged from 0.04 to 0.13 µg a.i./cm, demonstrating a maximum resistance ratio of 4.3-fold relative to the Sus strain (EC = 0.03 µg a.i./cm). In the F progeny from reciprocal crosses, the LC ranged from 0.28 to 0.64 µg a.i./cm and EC from 0.03 to 0.18 µg a.i./cm, similar to the values verified in parental resistant strains and representing a maximum resistance ratio of 3.0 and 6.0-fold, respectively. We also determined that susceptibility of third instar larvae to XenTari decreased when compared to neonates, however the variation remained similar. For third instar larvae from resistant strains, LC of XenTari ranged from 10.79 to 39.85 µg a.i./cm, while for the Sus strain, LC was 9.25 µg a.i./cm (resistance ratio inferior to 4.3-fold). At the same stage, in heterozygous strains the LC ranged from 14.75 to 58.47 µg a.i./cm (resistance ratio inferior to 6.3-fold). Our data demonstrate a lack of significant cross-resistance to Bt-based insecticide XenTari in fall armyworm strains with resistance to Bt maize varieties.