Many accessions of Lycopersicon hirsutum are highly resistant to insects. Trichomes and their secretions have been extensively indicated as factors of resistance. One mechanism of resistance mediated by secretions is repellency, a mechanism that is consistent with the observation that few insects visit plants of L. hirsutum. Trichome secretions from certain accessions of L. hirsutum f. typicum are repellent to spider mites. However, the composition of secretions from different accessions of f. typicum are chemically diverse. Sesquiterpene hydrocarbons are prevalent in secretions, but are structurally diverse. How structure may relate to repellency is of interest but difficult to address because isolation of pure sesquiterpene hydrocarbons from hydrocarbon mixtures is difficult. To begin examining relationships between structure and activity we determined how chain length of n-alkanes related to repellency of spider mites. n-Alkanes having chain lengths from 8 to 22 carbon atoms were assayed for repellency. The C16-C18 alkanes were most repellent. Smaller and larger hydrocarbons were less repellent. The EC50 for n-hexadecane was equal to that of the most repellent natural products we have isolated from trichome secretions of L. hirsutum.
Spider mites (Tetranychus urticae Koch) readily colonize the cultivated tomato. Lycopersicon esculentum L. However, mites have extreme difficulty colonizing the wild relative of tomato, L. hirsutum Humb. and Bonpl. When mites approach leaves of L. hirsutum, they often veer away, suggesting the presence of a deterrent or repellent. Initial experiments indicated that trichome secretions on leaflets of L. hirstum deterred mites. In vitro bioassays indicated that at least four distinct compounds present in these sequiterpenoid secretions of L. hirsutum P.I. 251303 were deterrent. At least two of the compounds were soluble in dilute NaOH. Based on mass spectra and 1H and 13C NMR the structure of two base soluble compounds were established as two related bisabolane derived carboxylic acids.