Biological activity and field efficacy of a genetically modified Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus expressing an insect-selective toxin from a chimeric promoter

A recombinant baculovirus (HaSNPV-AaIT) with improved insecticidal properties was constructed for the control of the cotton bollworm (Helicoverpa armigera). A chimeric promoter of the p6.9 and polyhedrin gene of H. armigera single-nucleocapsid nucleopolyhedrovirus (HaSNPV) was used to drive the expression of an insect-selective scorpion toxin (AaIT) at the egt gene locus of HaSNPV. This chimeric promoter, denoted ph-p69p, was constructed by directional insertion of the p6.9 promoter downstream of the polyhedrin promoter. Laboratory bioassays indicate that the infectivity (LD50s) of this recombinant is unchanged, compared to one wild-type clone of HaSNPV (HaSNPV-WT), and an egt-deletion mutant (HaSNPV-EGTD). The median survival times (ST50s) of 1st to 5th instar H. armigera larvae were reduced 17–34% after infection with HaSNPV-AaIT in comparison to HaSNPV-WT. The median times of feeding cessation (FT50s) were 30–43% shorter for HaSNPV-AaIT than for HaSNPV-WT in the 3rd to 5th instar of this species. This virus acts also quicker than HaSNPV-EGTD. Field trials at two research sites in 2000 indicate that the number of larvae and the proportion of damaged squares, flowers, and bolls was significantly lower in cotton plots treated with HaSNPV-AaIT than in plots treated with HaSNPV-WT or HaSNPV-EGTD. When HaSNPV-AaIT was applied to control infestations of bollworm over an entire cotton season, yield of cotton lint in plots treated by this recombinant was 22.1% higher than that in HaSNPV-WT treated plots in 2001 (p<0.05) and 20.7% higher in 2002 (p=0.251). These results indicate that this novel HaSNPV recombinant HaSNPV-AaIT is a more effective biocontrol agent than its wild-type relative, at least in the short term, and suggest that it offers potential for practical use.