- 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
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
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