Trichome Exudates and Resistance to Beet Armyworm (Lepidoptera: Noctuidae) in Lycopersicon hirsutum f. typicum Accessions

Abstract

Three accessions of Lycopersicon hirsutum f. typicum Humb. and Bonpl. were highly resistant to beet armyworm, Spodoptera exigua (Hubner), as indicated by reduced survival and growth on excised leaflets, compared with leaflets of a susceptible tomato variety. Removal of trichome exudates from LA 1777 and LA 2329 eliminated resistance affecting early instars ; but this treatment had no effect on the resistance of PI 126445. Primarily, lamellar factors must condition resistance in PI 126445 and contribute to resistance to later instars in LA 1777 and LA 2329. Type-VI trichome exudates of all 3 accessions were topically toxic to 1st-instar S. exigua, and this toxicity differed among the accessions (LA 1777 > LA 2329 > PI 126445). The lower toxicity of trichome exudates on PI 126445 apparently accounts for their minimal role in resistance in this accession. During feeding, 1st instars ruptured sufficient numbers of type-VI trichome glands to account for the observed mortality on both LA 1777 and LA 2329. Toxicities of trichome exudates from the accessions were not correlated with their sesquiterpene and total phenolic contents ; the most toxic trichomes (LA 1777) had the lowest concentrations of volatiles detectable by gas chromatography, and total phenolic compounds. The important toxins remain unidentified. Identification of the lamellar factors from the accessions, and the toxins in the trichome exudates of LA 1777 and LA 2329, will facilitate the use of these accessions in breeding programs for resistance to S. exigua and possibly other pests of tomatoes.