Repellency to the potato leafhopper (Homoptera: Cicadellidae) by erect glandular trichomes on alfalfa

We sought to gain insights into the mechanism of resistance of glandular-haired alfalfa to the potato leafhopper, by testing the trichomes of proprietary genotypes for chemical versus morphological differences. Two-choice, noncontact tests were conducted using four glandular-haired and one glabrous genotype of alfalfa, representing a spectrum of resistance, to determine whether adult female leafhoppers could detect and be repelled by putative volatiles released front the trichomes. Settling patterns roughly agreed with the degree of resistance reported by industry scientists who provided the plants. Repellency was strong for the most resistant genotypes, and absent with the most susceptible genotype. The remaining, intermediate-to-low-resistance genotypes exhibited variable repellency. A no-choice, plant-contact preference test showed no significant differences in settling among stems left intact or denuded, neither within nor among all genotypes. In all cases, adult leafhoppers settled on glandular-haired alfalfa if given no alternative. Environmental scanning electron microscopy of erect, glandular trichomes of eight proprietary genotypes (including the four used above) showed no obvious morphological differences among genotypes, or between stems and leaves within each genotype, although morphological comparisons were not quantified due to the fragility of the specimens. Taken together, our results Suggest that volatile compounds contribute to variable levels of repellency to potato leafhoppers by glandular-haired alfalfa, but that such compounds cannot strongly prevent feeding at all costs.