Influence of Plant Age on the Survival of Alfalfa Varieties Differing in Resistance to the Spotted Alfalfa Aphid

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Abstract
The reaction of four alfalfa varieties―Lahontan, Moapa, African, and Caliverde―of three ages of the plants was examined during a prolonged, severe, induced attack of the spotted alfalfa aphid (Therioaphis maculata (Buckton)). Increased plant age significantly decreased mortality of resistant varieties Lahontan and Moapa, but had no effect on that of Caliverde except at the shorter exposure period of 9 days. Older plants of African, a more tolerant but susceptible variety, survived slightly longer than the younger seedlings. About half the seedlings of Lahontan and Moapa infested in the cotyledon stage and about one-fourth of those in the second or third trifoliolate leaf stage were killed after 21 days’ exposure to the increasing aphid population. Very low mortalities (0% and 3%) were recorded for plants 41 days old when infested. African at all age groups was more tolerant than Caliverde, which was killed in 21 days. The higher survival of Lahontan and Moapa, despite high aphid populations, indicated that tolerance as well as antibiosis contributed to their over-all resistance.

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