Effects of leaf color, epicuticular wax amount and gramine content in barley hybrids on cereal aphid populations

Using four marker genotypes of F₂ hybrids derived from crosses between aphid resistant and susceptible barley lines, the effects of leaf color, epicuticular wax and gramine indole alkaloid on aphid infestation were studied in natural populations of cereal aphids, mainly the corn leaf aphid (Rhopalosiphum maidis) during 1994-1995. Resistant lines, OUH603 and OUH689 (Hordeum vulgare subsp. spontaneum), were the normal type (green waxy) with a higher amount of gramine. However, a susceptible line, OUL117 (Hordeum vulgare subsp. vulgare), was the yellow waxless type with a lower content of gramine. In the F₂ populations of OUL117 × OUH689, OUH603 × OUL117 and OUL117 × OUH603, the aphid densities on the marker genotypes were low in the normal type, intermediate in the green waxless and yellow waxy types, and high in the yellow waxless type. Gramine content was low and did not differ significantly among the genotypes measured in 158 F₂ plants of OUL117 × OUH689 in 1994. Additionally, there was no significant correlation between aphid density and gramine content within each genotype in the F₂ populations. These results showed that the genes controlling color and wax types had additive effects on normal plant genotype resistance to aphids, but did not affect gramine content.