Variation in the responses of spotted alfalfa aphids, Therioaphis maculata Buckton (Homoptera: Aphididae) and pea aphids, Acyrthosiphon pisum Harris (Homoptera: Aphididae) to drought conditions in alfalfa (Medicago sativa L., Fabaceae).

摘要

Spotted alfalfa aphids (Therioaphis maculata, Buckton) are a major pest of alfalfa in the arid western United States, yet are rarely encountered in Wisconsin. In Wisconsin, where severe drought conditions are rare, pea aphids (Acyrthosiphon pisum, Harris) are the predominant aphid herbivore in alfalfa. An unusually dry growing season in south-central Wisconsin in 2003 presented us with an opportunity to test two ideas. First, we conducted surveys to determine whether, due to the dry growing season, A. pisum density was lower than normally observed and the density of the more drought-tolerant T. maculata (Mittler and Sylvester, 1961) was greater in these dry areas. Second, we designed an experiment to determine whether these patterns were the result of environmentally mediated competition. We define this as competition between multiple species where the superior competitor is not fixed, but rather determined by the state of a variable environment (in this case, whether the water content of alfalfa determines which aphid species is the superior competitor). This type of competition is common in nature (Huston, 1979)..