The Aphidius Parasites of the Pea Aphid Acyrthosiphon pisum in the Eastern Half of the United States

During a 4-yr (1966-70) study of the Aphidius (Aphidiidae) parasites of the pea aphid, Acyrthosiphon pisum (Harris), in the eastern half of the United States (westward to South Dakota and Nebraska), 3 taxa were found: the Nearctic Aphidius ervi pulcher Baker, the Oriental Aphidius smithi Sharma and Subba Rao, and the Palearctic Aphidius ervi ervi Haliday. Crossmating tests using A. smithi from Asia and A. e. ervi from Europe confirmed the establishment of these exotic taxa in North America. Hybridization of A. smithi and A. ervi could not be induced, which confirmed their status as distinct species. By 1968, Aphidius smithi , introduced from India in 1958, had become the dominant parasite of pea aphid in the southern and midwestern states. By the end of 1970, however, A. e. ervi , introduced from France in 1959, had displaced A. smithi as far west as Iowa and as far north as central Wisconsin. In Mississippi and Georgia, A. smithi has remained the most abundant parasite of pea aphid. Although A. smithi could compete more successfully than A. e. ervi in the greenhouse at Moorestown, N. J., the opposite was true in nearby alfalfa fields. By the end of 1970, A. e. pulcher , which hybridizes to a limited extent with A. e. ervi , was rarely encountered in any of the areas surveyed. Therefore, any marked reduction of damage by the pea aphid since the introduction of A. smithi and A. ervi can probably be attributed to those 2 species.