Influence of Feeding Time, Stylet Penetration, and Developmental Instar on the Toxic Effect of the Spotted Alfalfa Aphid

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Abstract
Greenhouse experiments on the production of yellow vein-banding symptoms by the spotted alfalfa aphid, Therioaphis maculata (Buck ton) on seedling Caliverde alfalfa plants were conducted during the spring and summer of 1958. The percentage of plants affected with yellow vein-handing symptoms increased with an increase in the length of feeding time of individual aphids. The amount of time required for systemie symptom expression on 50% of the plants for third and fourth instars nymphs was estimated at approximately 10 hours. The symptom incidence was higher for late (third and fourth) instar than for early (first and second) instar nymphs. Most of the variability in symptom incidence was attributed to variable plant susceptibility and not inherent differences in aphid toxicity. T. maculata and Myzus persicae (Sulz.) each produced salivary sheaths when fed through a membrane into water. These sheaths were quite distinct between species in size and configuration. Experiments demonstrated that toxin injection, as measured by disease incidence, was a constant and somewhat additive function of feeding time, and was not dependent solely on the number of sheaths produced. Attempts to isolate the toxic principle were made; however, experiments to find a satisfactory bioassay method for it were negative.