Regression Sampling Plan for
Twospotted Spider Mite
(Acari: Tetranychidae) in Oregon Peppermint¹
CRAIG S. HOLLINGSWORTH² AND RALPH E. BERRY
Department of Entomology, Oregon State University, Corvallis, Oregon 97331
© Copyright 1978 Entomological Society of America
J. Econ. Entomol. 75: 497 - 500 (1982)
A sampling plan for detecting aggregations of Tetranychus urticae Koch was
developed to aid pest management decisions in peppermint, Mentha piperita L. Ten
to 14 sites were sampled for every 74 ha. At each site, 45 leaves (15 top, 15 middle, 15
bottom) were evaluated for the presence of five or more T. urticae. The sample
mean was estimated from the equation: log (x ± 1) = 0.018 ± 0.59 log (INF5 ± 1), which
was based on the relationship between the mean density of mites (x) and the number of
leaves per sample with five or more mites present (INF5). The coefficient of determination
for this relationship is 0.92. The equation provides an accurate estimate of mean mite
densities over widely separated regions, during different growing seasons, and for fields
with different mite densities.
AN ABSTRACT OF THE THESIS OF
Craig Stephen Hollingsworth
for the degree of Doctor of Philosophy in Entomology
presented on December 19 1980,
Oregon State University
Title: TWOSPOTTED SPIDER MITE, TETRANYCHUS URTICAE KOCH, IN OREGON PEPPERMINT, MENTHA
PIPERITA L.: SAMPLING, POPULATION DYNAMICS AND ECONOMIC INJURY
A pest management program for Tetranychus urticae Koch in Oregon peppermint was
developed based on studies of sampling, plant phenology, population dynamics and damage
A sample size of 45 leaves provided a confidence
level of 90% and a confidence interval of 50%. Estimation of the sample mean was based on
the relationship between the mean density of mites (x) and the number of leaves infested
with 5 or more mites (INF5) in a 45 leaf sample: Log (x + 1.0) = 0.18 + 0.59(INF5 + 1.0).
The coefficient of determination for the relationship was 0.92. This equation was
consistent over widely separated regions and during different growing seasons. Sampling
with this method was less tedious and three times faster than direct counting techniques.
Plant development in commercial mint fields in the Willamette Valley and central Oregon
was assessed by weekly observations of primary nodes and leaves, lateral branches, stem
length and flower bud formation. Stems were longer and more primary leaves were Present in
Willamette Valley fields, but the number of nodes and lateral branches were not
significantly different between regions. Flower buds were present earlier in central
Oregon than in the Willamette Valley. The number of primary leaves showed significant
variation between fields in central Oregon, reflecting the greater range of stress present
among fields in central Oregon.
Studies of T. urticae populations in commercial peppermint showed wide variation
in population trends. Population dispersion were examined relative to vertical strata of
the plant canopy. T. urticae populations were highly aggregated in all strata at
the beginning of the season, but as the population increased, dispersion became more
random. Mites on the older, middle and bottom leaves became more aggregated as populations
declined, but mites on the newly formed top leaves remained randomly distributed.
Eighteen species of predators were identified from peppermint samples, but none were
sufficiently abundant to regulate Mite populations in commercial peppermint plots.
Observations of a T. urticae population regulated by Amblyseius fallacis
(Acari: Phytoseiidae), showed that biocontrol of T. urticae in peppermint was
feasible, but probably only in less disturbed sites.
T. urticae reproduction increased on mint
plants subjected water stress but not on plants stressed by Verticillium. Mite populations
were reduced by fall flaming in western Oregon, and fall plowing delayed the onset of
economic levels of T. urticae in central Oregon.
The yield of field grown peppermint was not reduced by mite populations reaching 50 mites
per leaf at mid season. Mite density did not affect the total concentration of terpenes
from individual leaves, but changes of concentration of some terpenes indicated increased
maturation of the oil. Neomenthol was higher in leaves from mite infested plots, and the
concentration of its precursor, menthone, was lower.
An action threshold for T. urticae on
peppermint of 10 mites per leaf was established based on the tendency for mites to undergo
exponential increase once the density reaches 10 mites/leaf.