The pea aphid, Acyrthosiphon pisum (Harris), attacks a wide range of legumes including many important forage and vegetable crops. Although economic injury levels for pea aphids are seldom exceeded in forage crops, aphid densities can reach extremely high levels during a single harvest cycle. After harvest of forage crops, large numbers of pea aphids may disperse into proximate vegetable fields. The objective of this study was to determine the potential impact of immigrating pea aphids on vegetable crops adjoining forage crops in the agricultural landscape. Pea aphids collected from alfalfa and clover were evaluated for their ability to survive on and their propensity to feed on vegetable crops. The survival of pea aphids collected from clover and alfalfa was significantly higher on these forage crops than on peas, green beans and lima beans in laboratory transplant experiments. In additional laboratory experiments, alfalfa-adapted aphids produced significantly fewer aphid offspring on lima beans than on alfalfa, and the adults and offspring had significantly higher mortality on lima bean leaves than on alfalfa leaves. Analysis of aphid feeding with an electronic monitor confirmed that alfalfa-adapted pea aphids feed on phloem sap from alfalfa but never feed on lima beans. These results led us to predict that pea aphids immigrating from alfalfa and other forage crops during harvest would not establish persistent, damage-causing populations in nearby vegetable crops. Our field data on pea aphid populations in lima beans were consistent with this hypothesis. High densities of pea aphids were found in lima beans immediately after harvests of nearby alfalfa fields, but high aphid densities did not persist >3 d. Our study suggests that although aphids can emigrate from forage crops to vegetable crops at densities above published action thresholds, the amount of damage actually caused by these forage-crop adapted aphids in higher-value vegetable crops will be minimal. It may be unnecessary to treat aphids in lima beans and other vegetables when aphids have immigrated from harvested forage crops. These findings suggest that pest management decision-making in vegetables can be improved by considering the source of immigrating pests such as pea aphids.
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