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 (LD 50 from 3 to 10mug/larva) to implicate these compounds as resistance f actors in sesquiterpene-producing accessions of Lycopersicon hirsutum f. typicum Humb. and Bonpl. (hir). Three sesquiterpenes (zingiberene a nd two isomers of elemene) were present at 33 mug/cm2 on intact leaf s urfaces 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 sesquiterp enes 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. exigu a survival on plants from an F2 and backcross population from an hir x L. esculentum cross was independent of the concentration of sesquiter penes on the leaf surface. Sesquiterpene concentrations on foliage of the interspecific populations were only 10% as great as on the resista nt parent and were perhaps too low to cause detectable effects on S. e xigua larvae.