Repellency to two-spotted spider mite, Tetranychus urticae Koch, as related to leaf surface chemistry of Lycopersicon hirsutum accessions

Bioassays employing Tetranychus urticae Koch were used to examine repellency of leaves and trichome secretions of one cultivar of tomato, Lycopersicon esculentum, and 11 accessions of L. hirsutum consisting of seven accessions of L. hirsutum f. typicum and four accessions of L. hirsutum f. glabratum. Leaves of f. typicum were more repellent to mites than were those of f. glabratum or L. esculentum. Removal of trichomes and their secretions by wiping leaves of f. typicum accessions with 95% ethanol rendered them less repellent, indicating that trichomes or their secretions were potentially responsible for repellency. Gas and thin-layer chromatography of leaflet washes obtained by steeping leaflets in hexane indicated that the chemical composition of the washes differed among accessions. The main difference between botanical forms was the presence in leaflet washes from f. typicum accessions of compounds eluting late on the gas Chromatograph and migrating further on reverse-phase thin-layer chromatography; compounds having these characteristics were absent in washes of f. glabratum and L. esculentum. The compositions of secretions taken directly from type I, type IV, and type VI trichomes were similar to that of the leaflet wash obtained from the same f. typicum plant, indicating that leaflet washes contained mainly compounds of trichomal origin. Leaflet washes from f. typicum plants were generally more repellent to T. urticae than were washes from f. glabratum when tested by choice and nonchoice bioassays. Taken together, the data indicate that trichome secretions of f. typicum are likely responsible for potent repellency to two-spotted spider mites observed in this taxon; the likely active compounds are sesquiterpene acids or other polar constituents of trichome secretions.