PLANT BIOCHEMISTRY AND APHID POPULATIONS - STUDIES ON THE SPOTTED ALFALFA APHID, THERIOAPHIS-MACULATA

Feeding by the spotted alfalfa aphid, Therioaphis maculata (Buckton), on susceptible alfalfa, Medicago sativa L., results in dramatic changes in plant biochemistry that in turn have profound effects on aphid physiology. These aphids select older leaves on the plant as feeding sites. One component of this selection process may be the amount and composition of plant epicuticular lipids, which vary with leaf age. Feeding aphids induce a senescence-like state in the leaf that is characterized by loss of chlorophyll, decreased levels of soluble protein and fatty acids, and increased production of ethylene. This process involves lipid peroxidation and, like senescence, is probably free-radical-mediated. Leaves of alfalfa having resistance to spotted alfalfa aphid contain higher activities of catalase than do susceptible leaves. This enzyme may function in concert with other antioxidant enzymes to quench aphid-induced free radical damage and thus impart resistance. Aphid fatty acid metabolism is altered by changes in plant metabolism and thus reflects the close relationship between aphid and plant biochemistry.