|TWOSPOTTED SPIDER MITES|
|AN ABSTRACT OF THE THESIS
Jack Douglas DeAngelis
for the degree of Doctor of Philosophy in Entomology
presented on August 7 1981,
Oregon State University
Title: EFFECTS OF TETRANYCHUS URTICAE KOCH FEEDING INJURY ON PHYSIOLOGICAL PROCESS IN MENTHA PIPERITA L.
Studies were conducted to examine the effects of feeding injury by the twospotted spider mite (Tetranychus urticae Koch) on physiological processes of its host plant, peppermint (Mentha piperita L.). Aspects of miteinduced host plant physiological stress that were studied included: 1) effects of injury on plantwater relations; 2) photosynthesis and leaf chlorophyll content; and 3) soluble leaf carbohydrates and starch. The effects of feeding injury are discussed in terms of underlying physiological mechanisms. In addition, new methods for sequential extraction and analysis of peppermint tissues are presented. Development of this methodology was necessary in order to satisfy the special requirements of these studies.
The most detrimental effect of feeding injury was damage to leaf epidermis and cuticle and the consequent alteration of plantwater relations. Injured leaves were found to transpire more water at night than did uninjured leaves, resulting in symptoms of plant water stress the following day. These symptoms included reduced leaf water potential (psychrometrically determined) and stomatal closure. Levels of soluble peppermint leaf carbohydrates (mainly sucrose, raffinose, and stachyose) were higher on a per leaf basis in miteinjured leaves than in uninjured leaves. It is suggested that an osmotic adjustment mechanism, utilizing soluble carbohydrate as osmoticum, may be operating to maintain turgor in mite injuryinduced waterstressed leaves. Stressinduced stomatal closure inhibited photosynthesis, presumably by restricting carbon dioxide exchange. In addition, mite feeding removed significant amounts of leaf chlorophyll, resulting in localized nonphotosynthetic, necrotic patches.
An injury index based on the number of feeding adult female mites, duration of feeding, and leaf area is presented. This index was used throughout the studies to estimate the relative degree of injury of infested leaves.