A Comparison of the Injury to Alfalfa by the Aphids, Therioaphis maculata and Macrosiphum pisi

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Abstract
The fluid and nutrient drain imposed on alfalfa seedlings (Medicago sativa L. var. Caliverde) by the feeding activity of individual spotted alfalfa aphids (Therioaphis maculata (Buckton)) and pea aphids (Macrosiphum pisi (Harris)) was determined and related to the plant injury inflicted by these species. The fluid loss was estimated from the rates of excretory and non-excretory water loss of the two species throughout their nymphal development. The rate of excretory water loss, or rate of excretion, was determined from measurements of the frequency and volume with which honeydew droplets were excreted. The nonexcretory water loss was determined by measurements of loss in weight of starving aphids. The nutrient drain was estimated from determinations of the gain in live and dry weight of the aphids and the amount of sugar excreted by them during their development. The severity of the plant injury was estimated from the gains in weight of fresh green above-ground portions of infested and uninfested alfalfa seedlings. The results indicated that during nymphal development the average volume of honeydew excreted by M. pisi was some 40% greater than that excreted by T. maculata. Because M. pisi may also lose water by evaporation at a rate similar to that lost by excretion, the volume of sap ingested by M. pisi is probably twice as large as that injested by T. maculata. The relatively smaller rate of water loss by evaporation found for T. maculata may account for its greater resistance to severe desiccating conditions in the field. The increase in dry weight of M. pisi was approximately four times that of T. maculata, but the amount of sugar excreted by M. pisi was less than one-tenth of that excreted by T. maculata. Although the results obtained by previous investigators on the toxic effects of T. maculata on alfalfa plants were confirmed in part, the present study indicated that the rather considerable fluid and nutrient drain which T. maculata and M. pisi impose on alfalfa plants may largely be responsible for the severe injury caused by these aphids.