| Abstract
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The Hessian fly, Mayetiola destructor (Say), is a destructive pest of wheat (Triticum species) throughout most of the wheat growing areas of the world. These studies were conducted (1) to quantify the survival of the Hessian fly larvae on plants having resistance genes and determine whether this survival is genetic or due to other factors, and (2) to determine the effect of the genotypic interaction between resistance genes (usdH3,\ H6,\ H7H8,\ H9usd) in wheat and virulence genes (usda3,\ a6,\ a7a8,\ a9usd) in the Hessian fly on the expression of resistance. The usdH1H2usd and usdH7H8usd genes allowed the highest larval survival on resistance plants. These larvae, which are genetically avirulent, had minimal negative effects on plant growth. In Morocco, usdH7H8usd genes allowed the highest larval survival. Wheats heterozygous for usdH3,\ H5,\ H7H8,\ H9,\ H10usd, and usdH11usd genes expressed a high level of resistance and allowed for high larval survival on resistance plants. Plants heterozygous for usdH10,\ H11usd, and usdH22usd had decreasing levels of resistance as egg density increased. Resistance of plants heterozygous for usdH22usd and usdH6usd genes was severely reduced at 28C. Larval survival on resistant plants heterozygous for resistance genes appears to be the best criterion to determine if a gene has complete or incomplete dominance. Plants having usdH1H2usd or usdH7H8usd genes had increasing larval survival on resistant plants as egg density and temperature increased. Increasing egg density to 40 and 60 eggs/plant reduced the expression of resistance of usdH1H2usd, usdH7H8usd, and usdH11usd genes. Resistance of the usdH5usd gene was highly expressed in both spring and winter wheats at all egg densities and temperatures, but larval survival was higher on spring wheat. Hessian fly virulence genes usda6usd and usda9usd appear to be sex-linked. Plants homozygous for usdH3,\ H6,\ H7H8usd, and usdH9usd allowed for high survival of larvae heterozygous for the corresponding virulence genes, which suggest that avirulence to these resistance genes is incompletely dominant. High survival of larvae homozygous for usdA3,\ A6,\ A7A8,\ A9usd on the heterozygous plants of the corresponding resistance genes suggests incomplete dominance of these wheat genes.
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