CONTROL OF APHID DENSITY BY A COMPLEX OF PREDATORS

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Abstract
A series of field experiments using large field cages were used to evaluate the role of aphidophagous predators on the population dynamics of the pea aphid, Acyrthosiphon pisum (Harris), on alfalfa, Medicago sativa L. The pea aphid numbers increased rapidly in the cages to as much as 5 times the density in the adjacent fields. The difference between densities in the cage and field decreased as the season progressed. No significant differences in fecundity of the aphids were found either in clip cages in the field and cages, or in the laboratory using field-collected aphids that were allowed to larviposit for 24 h. We used an earlier simulation model of the aphid to show that if fecundity alone were responsible it would have to be 3 times the maximum rate we have ever observed. The return of the aphid density to the field value when one cage was removed and the aggregation of predators show the importance of predator movements in limiting prey numbers.