Effects of pink bollworm resistance to Bt on phenoloxidase activity and susceptibility to entomopathogenic nematodes.

Authors: GASSMAN, AARON - University Of Arizona; Fabrick, Jeffrey; Sisterson, Mark; HANNON, EUGENE - University Of Arizona; STOCK, PATRICIA - University Of Arizona; CARRIERE, YVES - University Of Arizona; TABASHNIK, BRUCE - University Of Arizona

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2009
Publication Date: 5/28/2009
Citation: Gassman, A.J., Fabrick, J.A., Sisterson, M.S., Hannon, E.R., Stock, P.S., Carriere, Y., Tabashnik, B.E. 2009. Effects of pink bollworm resistance to Bt on phenoloxidase activity and susceptibility to entomopathogenic nematodes.. Journal of Economic Entomology. 102 (3): 1224-1232

Interpretive Summary: Planting of genetically modified crops that produce insecticidal proteins derived from the bacterium Bacillus thuringiensis (Bt) continues to increase rapidly. Some insect pests targeted by this technology are under intense selection pressure to evolve resistance. To counter the threat of pest resistance to Bt crops, the refuge strategy has been implanted and requires non-Bt host plants near Bt crops in order to produce large numbers of Bt-susceptible insects that ideally would mate with rare resistant individuals that emerge from Bt fields and produce offspring that are unable to survive on Bt host plants. Over time, however, Bt-resistance alleles may accumulate within refuge populations and lead to the eventual evolution of resistance. Fitness costs associated with Bt resistance alleles could further delay pest resistance to Bt crops. Therefore, two species of entomopathogenic nematodes were tested for their ability to impose fitness costs in two Bt-resistant strains of the cotton pest, Pectinophora gossypiella. Both nematodes imposed fitness costs, although the costs differed in mode of inheritance and the strain of P. gossypiella affected. We hypothesized that differences in immune responses between Bt-resistant and Bt-susceptible P. gossypiella may contribute to nematode-mediated fitness costs. However, no difference in phenoloxidase activity, an important component of the insect immune response, were found between Bt-resistant and Bt-susceptible larvae, suggesting that other mechanisms may cause mortality for pink bollworm with Bt resistance genes. Incorporation of nematode-imposed fitness costs into to a spatially explicit simulation model suggests that entomopathogenic nematodes in non-Bt refuges could delay resistance by P. gossypiella to Bt cotton. These results point to the potential opportunity to use biological control agents, such as entomopathogenic nematodes, to assist in managing pest populations in refuges with the added benefit of delaying resistance to Bt by magnifying resistance costs.

Technical Abstract: Widespread planting of crops genetically engineered to produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt) imposes selection on many key agricultural pests to evolve resistance to Bt. Fitness costs of Bt resistance can slow the evolution of Bt resistance. We examined effects of entomopathogenic nematodes on fitness costs of Bt resistance in the pink bollworm Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), a major pest of cotton in the southwestern United States that is currently controlled by cotton that produces Bt toxin Cry1Ac. We tested whether the entomopathogenic nematodes Steinernema riobrave (Rhabditida: Steinernematidae) and Heterorhabditis bacteriophora (Rhabditida: Heterorhabditidae) affected fitness costs of resistance to Cry1Ac in two laboratory-selected hybrid strains of pink bollworm reared on non-Bt cotton bolls. The nematode S. riobrave imposed a recessive fitness cost for the one strain and H. bacteriophora imposed a fitness cost affecting rs individuals for the other strain. Activity of phenoloxidase, an important component of insects’ immune response, did not differ between Bt-resistant and Bt-susceptible families. This suggests that other mechanisms cause higher nematode-imposed mortality for pink bollworm with Bt resistance genes. Incorporation of nematode-imposed fitness costs into to a spatially explicit simulation model suggests that entomopathogenic nematodes in non-Bt refuges could delay resistance by pink bollworm to Bt cotton.