Following 30 years of research and development, seed companies are currently marketing varieties of glandular-haired alfalfa, Medicago sativa L., with resistance to the potato leafhopper, Empoasca fabae (Harris). The defensive glandular trichomes contribute to resistance through a combination of chemical and physical factors. To examine the chemical basis for resistance, we mechanically isolated glandular trichomes from the resistant genotype 'G98A' (Cal/West Seeds). Trichomes were then extracted and subsequently fractionated using flash chromatography. Fractions were assayed by applying aliquots to the surface of a parafilm-covered artificial diet sachet, and leafhoppers were then offered a choice between G98A fractions vs. solvent control and G98A fractions vs. crude G98A trichome extracts. Time-course analysis detected a fraction with strong deterrency to leafhopper settling, and this fraction also exhibited a dose response. GC-MS analysis of the highly deterrent fraction revealed the presence of nonvolatile lipophilic amides CnH2n+1NO (n=19-23). Synthetic amides with identical mass spectra also possessed similar deterrent properties. In addition, no-choice tests using the isolated amides showed leafhopper mortality was the result of starvation via deterrence, and not toxicity. These results indicate the importance of behavioral deterrence in resistance of glandular-haired alfalfa to the potato leafhopper.
Species 1: Homoptera Cicadellidae Empoasca fabae (potato leafhopper)
Keywords: glandular trichomes
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