The implications of homozygous vip3Aa20- and cry1Ab-maize on Spodoptera frugiperda control

Historically, Bt maize hybrids have been produced with the Bt alleles in a hemizygous state. Homozygous Bt transgenes increase the expression of Bt proteins in plants; however, recent studies have only briefly explored the zygosity of Bt transgenes in maize hybrids and its effects on the control of lepidopteran pests. In this study, we investigated whether an additional allelic dose of the single Bt events Bt11 and MIR162, and a pyramided version (Bt11 + MIR162 + GA21) would impact Bt protein expression in leaves and increase the control of fall armyworm, Spodoptera frugiperda, in leaves and grains of near-isogenic maize hybrids. Our results revealed that homozygous Bt hybrids had higher Cry1Ab and Vip3Aa20 protein concentrations in leaves than their hemizygous versions. However, higher concentrations of Bt proteins in homozygous versions did not increase the control of S. frugiperda. Hybrids expressing Cry1Ab, hemizygous and homozygous for the transgene, had similar mortality of S. frugiperda fed on leaves and grains as the non-Bt near-isogenic version, likely due to resistance of the pest to Cry1Ab. High susceptibility to Vip3Aa20 caused all larvae to die on hemizygous and homozygous MIR162 leaves. Also, larvae that survived on Vip3Aa20 grains did not gain weight after four days of feeding, regardless of the Bt zygosity of Vip3Aa20-expressing grains. Additional implications of Bt zygosity in maize hybrids on insect resistance management strategies are discussed.