USE OF LIFE-TABLES TO ASSESS HOST-PLANT RESISTANCE IN ALFALFA TO THERIOAPHIS-TRIFOLII-F-MACULATA (HOMOPTERA, APHIDIDAE) - HYPOTHESIS FOR MAINTENANCE OF RESISTANCE

The specific mechanisms of host plant resistance in alfalfa, Medicago sativa L., to the spotted alfalfa aphid, Therioaphis trifolii f. macul ata (Buckton), remain unknown despite three decades of research. Diffi culties with Painter's commonly used concept of host plant resistance were avoided when the Salt/Doutt paradigm of host selection by a paras ite was used instead. Mechanisms of host plant resistance proposed in the literature were reviewed with respect to this alternative concept. Published results on the effects of host plant resistance are not com parable across different studies because of arbitrary measurements use d. This problem is avoided with the use of laboratory age-specific lif e tables. In this study, four alfalfa cultivars were tested for host p lant resistance to the spotted alfalfa aphid at three temperatures, an d the statistical tools needed for the analysis are presented. The int rinsic rate of natural increase (r(m)) was Used as the measure of aphi d performance on each cultivar. At the same temperature, r(m) values o f the spotted alfalfa aphid on susceptible plants were significantly h igher than on resistant cultivars. A developmental threshold of 7.9 de grees C was estimated from the data, and the optimal temperature for g rowth and reproduction of this aphid was approximate to 21 degrees C. Life tables of two successive generations of the spotted alfalfa aphid on a resistant cultivar indicated that aphids do not transmit the cap acity to overcome resistance in a simple Mendelian fashion. A mechanis m involving the random transmission of internal symbionts required by this aphid to overcome alfalfa resistance is hypothesized.