Behaviour of wild-type and genetically modified baculoviruses in the Helicoverpa armigera - cotton system: a simulation approach

Keywords:   Helicoverpa armigera , baculovirus, genetic modification, cotton,transmission,persistence,biologicalcontrol, modelling.The bollworm, Helicoverpa armigera , is the key pest threatening the production of cotton inChina,the world'slargest cotton producer. The insectpathogen, H. armigerasingle nucleocapsid nucleopolyhedrovirus (HaSNPV), has been developed as a commercial biopesticide for controlofcotton bollworm. To enhance the efficacy of HaSNPV, a recombinant (HaSNPV-AaIT) was constructed byinsertion of a gene encodingan insect-selective scorpion neurotoxin (AaIT). Depending on the instar stage this recombinant virus incapacitates bollworm larvae 17-43% quicker than the wild-type virus (HaSNPV-WT). HaSNPV-AaIT also acts quicker than anHaSNPVegt (ecdysteroid UDP-glucosyltransferase)-deletion mutant(HaSNPV-EGTD). Field trials indicate that application of HaSNPV-AaIT over an entire cotton season results in a 22% higher cotton lint yield than when HaSNPV-WT is applied. Virus yield per larva inoculated with HaSNPV-AaIT in the 1 ^st to 5 ^th larval stage varied between 23% and 47% of the yield of HaSNPV-WT.Virus production in larvae was studied in the droplet-feeding bioassays. The results indicate that optimal production of recombinant virus for commercial purposes is achieved by inoculating the 4 ^th instar larvae. The optimal dose is indicated. In the field, there was no significant differenceininactivation rateamong the two recombinant HaSNPVs and HaSNPV-WT. In field cages and in the open field HaSNPV-AaIT exhibited a significantly lower rate of horizontal transmission than HaSNPV-WT. Vertical transmission of HaSNPV-AaIT was significantly lower than that of HaSNPV-WT,both in laboratory and field tests. On the basis of these data a comprehensive process-based model was developed to simulate the epizootiology of wild-type and genetically modified HaSNPV in cotton. The model integrates ecological information of interactions between HaSNPV, H. armigera and the cotton crop. The model is used to study aspects of the biocontrol efficacy of application regimes of HaSNPV variants with differential insecticidal properties and to identify key parameters to assess the biosafety of recombinant HaSNPV when used to control bollworm in cotton. Results show that genetically modified HaSNPV accrues greater benefits to cotton growers than wild-type HaSNPV and thatitsmultiplication in bollworm populations is impaired as compared to HaSNPV-WT. As there are no demonstrable risks to human and environmental health, recombinant HaSNPV hasclear potential to deliver effective and safe bollworm control.