Field Ecology of the Primary Parasites and Hyperparasites of the Potato Aphid, Macrosiphum euphorbiae,1 in the East San Francisco Bay Area2

Abstract

This field study was divided into 2 phases: (1) a field sampling of the potato aphid, Macrosiphum cuphorbiae (Thomas) (Homoptera: Aphididae), to determine the seasonal fluctuations in the abundance of the aphid and the degree of parasitization by its primary parasites; and (2) laboratory rearings of field-collected mummies to identify the species of primary parasites and hyperparasites, their seasonal phenology, degree of hyperparasitization, and host affinities. The random field sampling over a 12-month period indicated that the average degree of parasitization (based on the ratio of mummified aphids to total aphids as an indicator) was 5.6%, with a range of 63.3% (August) to 0.2% (December). The most commonly encountered mummy, confirmed by later laboratory rearings, was that produced by Aphidius sp. ( nigripes complex; 50.8% of total), followed by Aphelinus howardii Della Torre (41.2%), and Ephedrus californicus Baker (8.0%). Laboratory rearings of field-collected mummies over an 18-month period showed that the average degree of hyperparasitization was quite high (66.5%). Host differences were also noted in that the hyperparasitization of Aphidius (76.4%) was higher than that of either Ephedrus (44.9%) or of Aphidius (57.4%). Among the 6 species of hyperparasites, Charips victrix (Westwood) showed the strongest preference for Aphidius , and was in turn the most common hyperparasite (43.4% of total). The endoparasitic or “indirect” hyperparasites (66.1%) were more abundant than the ectoparasitic or “direct” hyperparasites (33.9%). Ephedrus is reported as a new host record for Asaphes, Pachyneuron, Lygocerus , and Charips .