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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 thresholds.

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.