The Wheat-Hessian Fly Interaction: Co-Evolution and Ecology in an Economically Important Plant-Insect System

The study of wheat?s H-gene mediated resistance to Hessian flies examined the cost of the constitutively expressed H-gene which functions in the plant?s surveillance system and the cost of the downstream induced response. For the constitutively-expressed H-gene, some measures indicated costs, but a greater number indicated benefits. For the induced resistance, plants showed benefits of being attacked. It is expected that fitness costs play a role in determining the rate at which plant defense evolves, and it is important for agriculture as plant breeders decide whether to pyramid resistance genes into a single cultivar to prevent the evolution of pest virulence. Before plant breeders undertake the effort to transfer resistance into crop cultivars, it must be asked: is the pest a sufficient threat to warrant the effort? To answer this question, the recently discovered female-produced sex pheromone of the Hessian fly was used to explore the pest potential for populations in the Upper Great Plains. Methods for pheromone trapping were established and trapping data were used to explore geographic distribution, phenology, and insect density. It was concluded that the Hessian fly is a risk to wheat in the Upper Great Plains and it was predicted that global warming and the increased cultivation of winter wheat will add to this risk. If Hessian flies are a sufficient threat to the region?s wheat crop, which of the 33 known resistance gene(s) should be used? To answer this question, traditional biotyping and an assay of all available H-genes were used to provide information on the virulence of a population of Hessian flies from the Upper Great Plains. The results were surprising as far more virulence was encountered than was expected. Using traditional virulence testing thirteen of the 16 possible Hessian fly biotypes were present in the North Dakota population, and in the assay of all available H-genes few gave 100% protection. In addition to information on Hessian fly virulence, the studies explored aspects of the wheat-Hessian fly interaction providing details on the fate of the Hessian fly and the wheat plant that have not been examined by other research on Hessian fly virulence