Implications of pea aphid host-plant specialization for the potential colonization of vegetables following post-harvest emigration from forage crops

The pea aphid, Acyrthosiphon pisum (Harris), attacks a wide range of legume s including many important forage and vegetable crops. Although economic in jury levels for pea aphids are seldom exceeded in forage crops, aphid densi ties can reach extremely high levels during a single harvest cycle. After h arvest of forage crops, large numbers of pea aphids may disperse into proxi mate vegetable fields. The objective of this study. was to determine the po tential impact of immigrating pea aphids oil vegetable crops adjoining fora ge crops in the agricultural landscape. pea aphids collected from alfalfa a nd clover were evaluated for their ability to survive on and their propensi ty to feed on vegetable crops. The survival of pea aphids collected from cl over and alfalfa was significantly higher on these forage crops than oil pe as, green beans and lima beans in laboratory transplant experiments. In add itional laboratory experiments, alfalfa-adapted aphids produced significant ly fewer aphid offspring on lima Leans than on alfalfa. and the adults and offspring had significantly higher mortality on lima bean leaves than on al falfa leaves. Analysis of aphid feeding with an electronic monitor confirme d 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 immigr ating from alfalfa and other forage crops during harvest would not establis h persistent, damage-causing populations in nearby vegetable crops. Our hel d data on pea aphid populations in lima beans were consistent with this hyp othesis. High densities of pea aphids were found in lima beans immediately after harvests of nearly alfalfa fields, but high aphid densities did not p ersist >3 d. Our study. suggests that although aphids can emigrate from for age 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 t reat aphids in lima beans and other vegetables when aphids have immigrated from harvested forage crops. These findings suggest that pest management de cision-making ill vegetables can be improved by considering the source of i mmigrating pests such as pea aphids.