The potato leafhopper, Empoasca fabae (Harris), is one of the most important pests of alfalfa, Medicago sativa L., in the midwestern and eastern United States. Four years ago, leafhopper-resistant varieties of glandular-haired alfalfa were made available commercially. However, little is known of the mechanism of leafhopper resistance. Previous studies support that resistance is chemically-based and that biologically-active compounds are secreted by the glandular trichomes. To determine whether glandular trichomes house active compounds, trichome exudates were extracted, analyzed using gas chromatography, and bioassayed for activity. We used 4 leafhopper-resistant genotypes provided by Cal/West Seed Co., ‘G98A’, ‘G98C’, and ‘G98D’, and the susceptible nonglandular clone ‘Ranger’. The extraction procedure used was designed to collect nonvolatile compounds. Volatiles will be assessed in future studies. Surface extracts were prepared by dipping stems from G98A or Ranger into chloroform for 20 seconds. Also, trichomes alone were collected from sections of alfalfa stems using liquid nitrogen and a vortexer, followed by extraction with chloroform. Several major peaks were unique to the glandular-haired clones, compared to Ranger. These peaks were also present in the highest concentration in G98A. Preliminary gas chromatography-mass spectrometry revealed nonvolatile fatty acids (C14-C20) both saturated and unsaturated associated with the exudate, along with several yet-unidentified compounds. Behavioral bioassays of adult nonpreference were conducted by “painting” extracts over parafilm-covered artificial diets. Leafhoppers were then offered a choice of Ranger or G98A-“painted” diets. Time-course analysis showed that extracts from Ranger were significantly (P < 0.001) preferred over those from G98A, for both plant surface and trichome-only extracts. In conjunction with the chromatographic studies, the behavioral studies support the hypothesis that nonvolatile compounds associated with the glandular trichomes on G98A alfalfa are deterrent to the potato leafhopper and are related to resistance.
Species 1: Homoptera Cicadellidae Empoasca fabae (potato leafhopper)
Species 2: Medicago sativa (alfalfa)
Keywords: Plant Resistance
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