|TWOSPOTTED SPIDER MITES|
|AN ABSTRACT OF THE THESIS
Katherine Cole Larson
for the degree of Master of Science in Entomology
presented on August 12 1982,
Oregon State University
Title: INFLUENCE OF THE SECONDARY METABOLITES OF MENTHA
PIPERITA L. ON TETRANYCHUS URTICAE KOCH
A laboratory experiment was conducted to determine the response of twospotted spider mites (Tetranychus urticae Koch) to varying amounts of monoterpenes and phenolic compounds found in peppermint (Mentha piperita L.) leaves of different age and growth form. The reproductive success of twospotted spider mites was analyzed on four different abed leaves per plant by confining mites in sticky TackTrap cages, and measuring the number of eggs laid per leaf, the development time from larvae to adult, and the percentage of mites caught in the TackTrap cages. The number of mites caught in the TackTrap cages was used as a measure of leaf repellency (Rodriguez 1969), Secondary metabolite content of leaves was estimated by analyzing the opposite leaf pair of each infested leaf for phenolic content and monoterpene composition. Another experiment was conducted to analyze the effects of individual monoterpene vapors on twospotted spider mites feeding on bean leaf disks.
Twospotted spider mite reproductive success was greatest on upper, mature main stem leaves, and lowest on the expanding leaves of lower lateral branches. Lateral leaves had nearly twice the amount of total phenolic compounds (mg quercitin per leaf disk) and monoterpenes (ug/gm/Fr. wt.) as the main stem leaves.
The monoterpene content of leaves did not significantly affect spider mite reproductive success, although pulegone vapor, and to a lesser extent, menthol vapor was toxic to mites on bean leaf disks. Twospotted spider mites feeding on peppermint leaves probably do not encounter significant amounts of monoterpenes, which are sequestered in glandular trichomes on the leaf surface. Phenolic content of leaves significantly affected spider mite reproductive success; as the phenolic content of leaves increased, the number of eggs laid on each leaf decreased, the time required to complete development increased, and the percentage of immature mites caught in the TackTrap enclosures increased. Phenolic compounds probably act as feeding deterrents.