Mechanisms of resistance in Lycopersicon germplasm to the whitefly Bemisia argentifolii

The silverleaf whitefly Bemisia argentifolii Bellows & Perring (Homoptera: Aleyrodidae) [also known as strain B of the sweetpotato whitefly B. tabaci (Gennadius)] is a major pest of tomatoes due to both feeding damage and transmission of plant viruses. Certain wild species of Lycopersicon have demonstrated high levels of resistance to the pest. Greenhouse studies were undertaken to quantify the effects on whitefly behavior and mortality of individual, resistant plants selected from three accessions of L. pennellii (Corr.) D'Arcy (LA 1340, LA 1674 and LA 2560), five accessions of L. hirsutum f. typicum Humb. & Bonpl. (LA 386, LA 1353, LA 1777, PI 127826 and PI 127827) and one accession of L. hirsutum f. glabratum C.H. Mull. (PI 126449). In no-choice experiments, fewer adults settled on leaflets of the wild species and deposited 75-100% fewer eggs compared to the cultivated tomato, L. esculentum Mill. Adult mortality ranged from 77-100% on wild accessions but was only 1% on L. esculentum. Most dead adults were trapped in glandular trichome exudates. The effects of these resistant accessions on B. argentifolii were mechanically transferable by appressing the trichome exudates onto the leaves of the susceptible tomato, indicating an association between the factors mediating the resistance and the glandular trichomes. Laboratory studies evaluated the repellent, fumigant and residual toxic effects of representative constituents of trichome exudates on B. argentifolii adults by using selected concentrations and probit analyses. RC₅₀ values (estimated concentration to repel 50% of the adults) and LC₅₀ values for fumigant and residual toxicity indicated that 2-tridecanone had low levels of repellent and residual toxicity activity; that 2-undecanone had high levels of repellent and fumigant activity; and that ginger oil (composed, in part, of sesquiterpene hydrocarbons) had high levels of repellent and residual toxicity activity. These studies suggest that multi-factor resistance exists in wild tomato germplasm. By combining genetically the observed chemical constituents of resistance into a single germplasm, the resulting resistance may be more difficult for B. argentifolii to overcome.