TOPICAL TOXICITY OF TOMATO SESQUITERPENES TO THE BEET ARMYWORM AND THE ROLE OF THESE COMPOUNDS IN RESISTANCE DERIVED FROM AN ACCESSION OF LYCOPERSICON-HIRSUTUM F TYPICUM

The topical toxicity of five sesquiterpenes to neonate beet armyworm (Spodoptera exigua [Hubner]) was determined to be sufficiently high (LD50 from 3 to 10mug/larva) to implicate these compounds as resistance factors in sesquiterpene-producing accessions of Lycopersicon hirsutum f. typicum Humb. and Bonpl. (hir). Three sesquiterpenes (zingiberene and two isomers of elemene) were present at 33 mug/cm2 on intact leaf surfaces of a genotype of hir accession PI 126445. Ten-day survival of S. exigua larvae on foliage of this genotype was 0%, but removal of 90% of the sesquiterpenes by wiping the foliage with methanol increased S. exigua survival to 65%. Although these data suggest that sesquiterpenes are resistance traits, other factors are also apparently involved. Ten-day larval weights on hir foliage wiped with methanol were still 8-fold lower than on susceptible commercial tomato plants, suggesting that lamellar resistance factors slowed larval growth. Also, S. exigua survival on plants from an F2 and backcross population from an hir x L. esculentum cross was independent of the concentration of sesquiterpenes on the leaf surface. Sesquiterpene concentrations on foliage of the interspecific populations were only 10% as great as on the resistant parent and were perhaps too low to cause detectable effects on S. exigua larvae.