Comparative insecticidal properties of two nucleopolyhedrovirus vectors encoding a similar toxin gene chimer

Laboratory, greenhouse and field studies were conducted to characterize the insecticidal properties of genetically altered forms of Autographa califor nica (Speyer) nucleopolyhedrovirus (AcNPV) and Helicoverpa tea (Boddie) NPV (HzNPV) against selected heliothine species. The altered viruses each cont ained a chimeric 0.8-kb fragment encoding the insect-specific, sodium chann el neurotoxin from the Algerian scorpion Androctonus australis Hector (AaIT , hence recombinant viruses designated Ac-AaIT and Hz-AaIT). Based on LD50 values, results from diet-overlay bioassays showed Ac-AaIT and Hz-AaIT to b e equally virulent against larval tobacco budworm, Heliothis virescens (F.) , but Hz-AaIT averaged 1,335-fold greater bioactivity than Ac-AaIT against larval cotton bollworm, Helicoverpa zea (Boddie). Hz-AaIT killed larvae of both heliothine species at rates significantly faster than those imparted b y HzNPV (viral LT50 values averaged 2.5 and 5.6 d, respectively). In greenh ouse studies, foliar sprays of Ac-AaIT and Hz-AaIT were equally effective i n controlling H. virescens on cotton; however, Hz-AaIT provided control of II. ten on cotton at a level superior to that of Ac-AaIT. For example, afte r three weekly sessions of foliar application and H. tea artificial infesta tion, cotton treated with Ac-AaIT or Hz-AaIT at 10 x 10(11) occulsion bodie s (OB) /ha averaged 2.5 and 16.2 nondamaged flower buds per plant, respecti vely. Another greenhouse study conducted against heliothine species on cott on showed that the quicker killing speed exhibited by Hz-AaIT led to improv ed plant protection versus HzNPV. Finally, results from three field trials demonstrated that Hz-AaIT at 5-12 x 10(11) OB/ha provided control of the he liothine complex in cotton at levels slightly better than Bacillus thuringi ensis, equal to the macrolide, spinosad and only slightly less than that of selected pyrethroid and carbamate insecticides. Overall, results from thes e studies indicate that, because of host range differences between the two wild-type viruses, HzNPV is the Letter vectoring agent (versus AcNPV) for d esigning recombinant clones as insecticides targeted at the multi-species h eliothine complex. Further, these studies suggest that if appropriately tai lored for the pest complex, recombinant NPVs may be very effective, insect- specific approaches to managing pests in many cropping scenarios. Possible Hz-AaIT deployment strategies for control of heliothine species on conventi onal and transgenic cotton varieties are discussed.