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Sanford D. Eigenbrode and John T. Trumble

Four accessions of Lycopersicon hirsutum f. glabratum Mull. and eight accessions of L. hirsutum f. typicum Humb. & Bonpl. were evaluated for their resistance to the beet armyworm [Spodoptera exigua (Hübner)]. Survival of S. exigua neonate larvae for up to 96 hours on foliage of all these accessions differed significantly from their survival on a susceptible tomato cultivar. Spodoptera exigua survival did not differ significantly between the two forms of L. hirsutum. Antibiosis to S. exigua in L. hirsutum f. glabratum appears similar to the levels found to other Lepidoptera. Accessions of L. hirsutum f. typicum included the very susceptible PI 199381 and the two most resistant accessions, LA 2329 and LA 1777. Insect resistance had not been reported previously in four of the L. hirsutum f. typicum accessions. Spodoptera exigua survival was significantly negatively correlated with the density of type IV glandular trichomes on the leaf surfaces, calculated across all 12 accessions. This relationship did not occur within L. hirsutum f. glabratumor L. hirsutum f. typicum accessions, nor was it significant if PI 199381 was excluded from the analysis. Leaf-surface exudates of L. hirsutum f. glabratum accessions were dominated by the methylketones 2-undecanone and 2-tridecanone. Leaf-surface exudates of L. hirsutum f. typicum were dominated by three sesquiterpenes-zingiberene, δ elemene, and γ elemene. Resistance was not correlated with the amounts of these specific compounds within or across botanical form. Spodoptera exigua survival in L. hirsutum f. typicum (excluding PI 199381) correlated negatively with the total estimated amount of leaf-surface volatiles extracted. PI 199381 may be useful as a susceptible parent for intraspecific crosses to examine S. exigua resistance in L. hirsutum.

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Stuart R. Reitz and John T. Trumble

We examined two aspects of treating plants with a cytokinin-containing seaweed extract (SWE). In the first series of experiments, we tested the hypothesis that immature lima bean (Phaseolus lunatus L.) and tomato (Lycopersicon esculentum Mill.) plants provided with exogenous cytokinins could recover from defoliation by a generalist insect herbivore, Spodoptera exigua (Hübner), more rapidly than plants without cytokinin supplements. However, the SWE inhibited growth of lima beans at all levels of herbivore damage. The SWE neither inhibited nor stimulated growth of tomatoes following defoliation. Because SWE effects largely were neutral for tomato growth, we conducted a second series of experiments to test the hypothesis that SWE treatments alter the attractiveness of tomato foliage to S. exigua larvae. In these experiments, we determined consumption of, and preference for, SWE-treated tomato foliage by S. exigua larvae. Repeated root applications of SWE led to increased consumption and preference by S. exigua. Repeated foliar applications did not alter consumption or preference compared with controls. Spodoptera exigua larvae gained significantly more mass when feeding on SWE-treated foliage compared with controls. While these data indicate that plant responses to exogenous cytokinin-containing materials depend on taxa and application method, the practical uses of SWE appear limited given the negative effects on plant growth and increased attractiveness of treated foliage to herbivores.

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Sanford D. Eigenbrode, John T. Trumble, and Richard A. Jones

Accessions of Lycopersicon esculentum var. cerasiforme (Dun.) A. Gray (cer) and L. pimpinellifolium (Jusl.) Mill. (pimp), sustained significantly less damage to fruit by beet armyworm [Spodoptera exigua (Hiibner)] than standard cultivars and breeding lines of L. esculentum Mill. (esc) under natural infestations in 1990 and 1991 in southern California. The dwarf vine cherry cultivar Tiny Tim also sustained less damage than the standards. Accessions of esc with various monogenic mutations sustained at least as much beet armyworm damage as did standard cultivars. The percentage of fruit damaged was significantly correlated with vine weight, weight per fruit, number of fruit, and the fruit-foliage weight ratio (Pearson's coefficients, respectively: -0.533, 0.450, -0.483, 0.390, n = 37). In laboratory assays, survival of beet armyworm was significantly lower (5% of susceptible& growth rates were significantly lower, and development time was significantly longer on the fruit of resistant `Tiny Tim' and LA 1320 cer than the fruit of 11 other test lines. There were no substantial differences in beet armyworm survival on the foliage of the test lines. In the field trials, there were also significant differences among the test lines in damage by Liriomyza species and hemipteran pests. Lines with genes for increased densities of nonglandular leaf trichomes (especially LA 1663) were generally less damaged by Liriomyza than other lines. Damage by hemipterans was correlated with vine and fruit size, fruit count, and fruit-foliage weight ratio in 1991, but high intraseason variability prevented clear identification of test lines resistant to these pests.

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Barbara E. Liedl, Darlene M. Lawson, Kris K. White, Joseph A. Shapiro, William G. Carson, John T. Trumble, and Martha A. Mutschler

Acylsugars, the primary components of the exudate secreted by type IV trichomes of Lycopersicon pennellii (Corr.) D'Arcy LA716, mediate the resistance of this accession to silverleaf whitefly, Bemisia argentifolii Bellows & Perring, n. sp. Reduction in the settling of the adult silverleaf whiteflies correlates with the concomitant increase in applied acylsugars. Oviposition of B. argentifolii is also affected by acylsugars, resulting in a reduction in the number of eggs and nymphs found; however, acylsugars do not affect hatching of nymphs. The threshold amount of acylsugars required for deterring settling and oviposition is under the amount of acylsugars (50 to 70 μg·cm–1) required for control of other insects.