Parasitoids (Hymenoptera: Aphidiidae and Aphelinidae) and their effect on aphid (Homoptera: Aphididae) populations in irrigated grain in southwestern Idaho

A survey of parasitoids of cereal aphids was conducted on irrigated wheat, Triticum aestivum L.; barley, Hordeum vulgare L.; and corn, Zea mays (L.) Moench., grown under irrigation in southwestern Idaho from 1986 to 1989. Six species of primary parasitoids (four species of Aphidiidae and two of Aphelinidae) and five species of hyperparasitoids (two species of Pteromalidae, one Megaspilidae, one Encyrtidae, and one Alloxystidae) were identified from 1,244 specimens obtained from the mummies of seven species of aphids that were found in the field or reared from 8,698 live, field-collected aphids. Aphidius ervi Haliday, most frequently parasitized Sitobion avenae (F.), Metopolophium dirhodum (Walker), and Macrosiphon euphorbiae (Thomas) (>91% of the primary parasitoids obtained), whereas Praon sp. (possibly gallicum Stary) attacked these aphids only occasionally. Multiple species of parasitoids were reared from Diuraphis noxia (Kurdjumov) (six species), Schizaphis graminum (Rondani) (four species), Rhopalosiphum padi (L.) (five species), and R. maidis (Fitch) (four species); however, only Diaeretiella rapae (M'intosh) and Aphelinus varipes (Foerster) were of importance for D. noxia or R. padi. Other parasitoids, including Lysiphlebus testaceipes (Cresson), A. ervi, Praon sp., and Aphelinus sp., were occasionally found parasitizing this host. Developmental time at room temperature from host mummification to emergence of parasitoid adults was 5.0-5.5 d for A. ervi, 6.5-6.9 d for D. rapae, approximately 8.5 d for Praon sp., and 9.8-12.1 d for A. varipes, with variation among the aphid hosts. Parasitism suppressed the populations of S. avenae more than those of M. dirhodum on small grains. The role of parasitoids in control of D. noxia was limited; parasitism exceeded 5% only during times of declining host populations. On corn, a high rate of parasitism of S. avenae, M. dirhodum, and M. euphorbiae, primarily by A. ervi, was observed from mid-June through July, while R. padi was subject to detectable mortality, mostly attributed to A. varipes, through early October.