Influence of Pea Aphids1 and Spotted Alfalfa Aphids1 on the Stand, Yield of Dry Matter, and Chemical Composition of Resistant and Susceptible Varieties of Alfalfa2

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Abstract
When pea aphids, Acyrthosiphon pisum (Harris), and spotted alfalfa aphids, Therioaphis maculata (Buckton), were introduced into field cages containing aphid-re-sistant (Dawson or KS-IO) and aphid-susceptible (‘Ranger’ or ‘Vernal’) varieties of alfalfa, both species increased until harvest on all varieties, but at harvest, fewer were present on resistant than on susceptible varieties. Also the contents of protein, carotene, and digestible dry matter (dry matter disappearance) in the dry matter of the susceptible varieties were generally reduced more by both aphids than that of the resistant varieties. Over 2 years, pea aphids reduced the yield of dry matter from resistant and susceptible varieties by an average 23 and 48%, respectively; spotted alfalfa aphids reduced yields of the same varieties by an average of 19 and 38%. The 2-year average reduction in yield of chemical constituents caused by pea aphids was 26% for the resistant varieties and 57% for the susceptible varieties; that caused by spotted alfalfa aphids was 24 and 48%, respectively. Stand losses were low the spring following the artificial infestation the preceding year. However, pea aphids reduced stands of resistant and susceptible varieties by an average 2 and 6%, respectively; spotted alfalfa aphids reduced stands of the same varieties by an average 0 and 6%.

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