Observations of Feeding Habits of the Spotted Alfalfa Aphid on Resistant and Susceptible Alfalfa Plants

Abstract

Spotted alfalfa aphids (Therioaphis maculata (Buckton)) placed on both resistant and susceptible alfalfa plants soon assumed a feeding position. On resistant plants, they became restless in 1 to 4 hours and eventually died or left the plants. Little or no honeydew was produced on resistant plants whereas profuse honeydew production occurred on susceptible plants. Survival of aphids placed first on resistant plants and later transferred to susceptible plants was as good as the survival of aphids transferred from susceptible plants, indicating no residual toxic effect from the resistant plants. Aphids confined to highly resistant plants were found to die at nearly the same rate as aphids confined with no source of food. Microscopic examination of leaf sections indicated that phloem tissue is the principal feeding site. However, it was observed that the setal tubes entered the phloem less frequently in resistant than in susceptible plants, suggesting that the aphid's mechanism for locating the phloem was interfered with in resistant plants. Results of these studies indicate that mortality of the spotted alfalfa aphid on highly resistant plants is caused by starvation or desiccation resulting from a failure to ingest a sufficient quantity of plant sap.