Many accessions of Lycopersicon hirsutum are highly resistant to insects. Trichomes and their secretions have been extensively indicated as factors of resistance. One mechanism of resistance mediated by secretions is repellency, a mechanism that is consistent with the observation that few insects visit plants of L. hirsutum. Trichome secretions from certain accessions of L. hirsutum f. typicum are repellent to spider mites. However, the composition of secretions from different accessions of f. typicum are chemically diverse. Sesquiterpene hydrocarbons are prevalent in secretions, but are structurally diverse. How structure may relate to repellency is of interest but difficult to address because isolation of pure sesquiterpene hydrocarbons from hydrocarbon mixtures is difficult. To begin examining relationships between structure and activity we determined how chain length of n-alkanes related to repellency of spider mites. n-Alkanes having chain lengths from 8 to 22 carbon atoms were assayed for repellency. The C16-C18 alkanes were most repellent. Smaller and larger hydrocarbons were less repellent. The EC50 for n-hexadecane was equal to that of the most repellent natural products we have isolated from trichome secretions of L. hirsutum.
John C. Snyder, George Antonious, and Richard Thacker
Matthew Byers, George Antonious, and Keenan Bishop
Using USLE standard plots, on 10% slope, on uniform Lowell silt loam soil, the influence of three soil treatments and two representative vegetable crops on soil runoff losses was determined. Yields (1990 and 1991) for each crop × soil treatment combination were also determined. Soil losses were determined by catching runoff, filtering, air drying and massing representative samples. With total volume per plot known, grams sediment per liter runoff were converted to kg/ha. Overall mean yields of pepper were 2.9, 4.7, and 3.7 and pumpkin were 47.3, 87.1 and 76.1 kg/7.3-m row, respectively. Mean sediment losses over five rainfall events in 1991 in peppers were 52, 1158, and 5362; and in pumpkins were 72, 3011, and 7271 kg/ha, for fescue, plastic and no-mulch treatments, respectively. Clearly, fescue in .6-m strips between rows, with comparable yields demonstrated (1991) and negligible sediment losses, was the best management practice (BMP).
George F. Antonious, Matthew E. Byers, John C. Snyder, and Douglas L. Dahlman
The development and deployment of crop varieties that resist or tolerate insect attack is one tactic of pest management that can eliminate one or more spray applications per season, a significant savings to the grower. Seven tomato, Lycopersicon esculentum Mill. cultivars (Marmand, Edkawy, VF-145, GS-27, Pakmore-B, Flordade, and UCX) were evaluated under greenhouse conditions for differences in mortality and feeding behavior (leaf-area ingested) of the 4th instar larvae of Spodoptera littoralis (Boisd) and the Colorado potato beetle, Leptinotarsa decemlineata (Say). The most resistant cultivars to S. littoralis during two summer seasons, 1990 and 1991, were Edkawy and UCX (37% mortality) and VF-145 (33% mortality). Mortality was least (20%) on the F1 hybrid GS-27, indicating that GS-27 was the most favorable cultivar for S. littoralis. L. decemlineata larvae reared on excised tomato leaflets of the same varieties indicated similar trends. Factors responsible for greater resistance of Edkawy and UCX to S. littoralis and L. decemlineata are under investigation.