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 LC50 of XenTari in neonates of resistant strains ranged from 0.28 to 0.68 µg a.i./cm2, while for the susceptible reference strain (Sus), LC50 was 0.21 µg a.i./cm2. This indicated a resistance ratio lower than 3.2-fold. A similar variation in susceptibility was detected in EC50 values, which ranged from 0.04 to 0.13 µg a.i./cm2, demonstrating a maximum resistance ratio of 4.3-fold relative to the Sus strain (EC50 = 0.03 µg a.i./cm2). In the F1 progeny from reciprocal crosses, the LC50 ranged from 0.28 to 0.64 µg a.i./cm2 and EC50 from 0.03 to 0.18 µg a.i./cm2, 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, LC50 of XenTari ranged from 10.79 to 39.85 µg a.i./cm2, while for the Sus strain, LC50 was 9.25 µg a.i./cm2 (resistance ratio inferior to 4.3-fold). At the same stage, in heterozygous strains the LC50 ranged from 14.75 to 58.47 µg a.i./cm2 (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.

Effective dominance of resistance of Spodoptera frugiperda to Bt maize and cotton varieties: implications for resistance management.

The resistance of fall armyworm (FAW), Spodoptera frugiperda, has been characterized to some Cry and Vip3A proteins of Bacillus thuringiensis (Bt) expressed in transgenic maize in Brazil. Here we evaluated the effective dominance of resistance based on the survival of neonates from selected Bt-resistant, heterozygous, and susceptible (Sus) strains of FAW on different Bt maize and cotton varieties. High survival of strains resistant to the Cry1F (HX-R), Cry1A.105/Cry2Ab (VT-R) and Cry1A.105/Cry2Ab/Cry1F (PW-R) proteins was detected on Herculex, YieldGard VT PRO and PowerCore maize. Our Vip3A-resistant strain (Vip-R) exhibited high survival on Herculex, Agrisure Viptera and Agrisure Viptera 3 maize. However, the heterozygous from HX-R × Sus, VT-R × Sus, PW-R × Sus and Vip-R × Sus had complete mortality on YieldGard VT PRO, PowerCore, Agrisure Viptera, and Agrisure Viptera 3, whereas the HX-R × Sus and Vip-R × Sus strains survived on Herculex maize. On Bt cotton, the HX-R, VT-R and PW-R strains exhibited high survival on Bollgard II. All resistant strains survived on WideStrike, but only PW-R and Vip-R × Sus survived on TwinLink. Our study provides useful data to aid in the understanding of the effectiveness of the refuge strategy for Insect Resistance Management of Bt plants.