Mechanisms of hop resistance to the twospotted spider mite (Tetranychus urticae Koch) Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/j96023606

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  • Hop varieties were compared under field and greenhouse conditions to determine their relative susceptibility to the twospotted spider mite, Tetranychus urticac Koch. A fertility schedule was prepared for mites on resistant and susceptible varieties, and tests were conducted to compare sex ratio, oviposition rate, survival rate, and developmental rate of mites on resistant and susceptible varieties. Hop leaves were examined for traits that could affect mite life histories. Moisture content of leaves, leaf area, and density of ventral glands and hairs were related to oviposition, sex ratio, developmental times, survival, and female migration, using linear regression. Thin layer chromatography (TLC), column chromatography, and gas-liquid chromatography (GLC) were used to analyze farnesol content of hop foliage and oil. Farnesol, nerolidol, geraniol, nerol, and an ether extract of twospotted spider mites were topically applied to female mites to determine effects on oviposition rates. In the greenhouse, mite densities were significantly greater (P < 0.05) on Comet and Fuggle than on L-8, L-1, and Cascade, using artificially infested plants. In the field, natural infestations were significantly lower (P < 0.005) on Fuggle and Cascade than on Talisman or Comet, and L-16 had the highest density. Mites on Cascade had a higher intrinsic rate of natural increase than on Fuggle, Comet, Talisman, or L-16, and mites on Cascade had the shortest generation time. The net reproductive rate was higher on Fuggle than on Cascade, Comet, Talisman, or L-16. No consistent differences were found in oviposition rates, sex ratio, or survival of mites reared on resistant and susceptible varieties. Highly significant differences (P < 0.005) were found in developmental rates, and the developmental rate of mites on L- 16 was consistently slowed. Immature mites on L-16 and Talisman developed slowly, and differed in coloration from mites on Cascade, Fuggle, and Comet. Leaves from Cascade, Comet, L- 16, and Talisman differed significantly (P < 0.005) in moisture content and density of ventral hairs and glands. Statistically significant regressions were obtained relating oviposition to leaf area, sex ratio to ventral hairs, and development time to leaf moisture content and ventral hairs. Migrating females preferred Comet leaves to leaves from Talisman, Cascade, Fuggle, and L- 16; preference was not significantly related to leaf area, ventral hair density, ventral gland density, or Leaf moisture content. TLC and GLC methods were developed for detecting farnesol in hop foliage. Farnesol was detected in hop oil obtained from Fuggle cones. No farnesol was detected in petroleum ether extracts of hop foliage, using detection levels ranging from 6-200 farnesol/g of dry foliage. Topical applications of farnesol, nerolidol, geraniol, nerol, and mite extract produced no consistent effect on mite oviposition. Farnesol does not appear to be related to the resistance of hops to the twos potted spider mite, and it does not appear to act as a gonadotropin.
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