Abundance and effects of predators and parasitoids on the Russian wheat aphid (Homoptera : Aphididae) under organic farming conditions in Colorado

Diuraphis noxia (Mordvilko) is an exotic, economically important pest in No rth American cereal crops. A survey of insect natural enemies of D. noxia w as made from 1991 to 1994 on a farm using organic farming methods. Crested wheatgrass, Agropyron cristatum L., had fewer D. noxia and natural enemies than barley. Hordeum vulgare L., or wheat, Triticum aestivum L., but on all plants D. noxia ws the most abundant aphid. We observed 41 species of natu ral enemies: 15 carabids, 12 coccinellids, six spiders, five syrphids, two nabids, and two chrysopids. The most consistently abundant were the coccine llids and nabids. Hippodamia convergens (Geurin) and Nabis alternatus Parsh were the most common species in each family, respectively. Diaeretiella ra pae M'Intosh was the only primary parasitoid found in the 4-yr study, and D . noxia parasitism rates were generally <3%. Four hyperparasitoids, found i n association with D. rapae, combined to make a 29% average hyperparasitism rate. Predator and parasitoid exclusion studies using cages showed aphid p opulations to be between 2.6 and 11.2 times higher in cages compared with w heat plants exposed to natural enemies. Four predatory species were release d-Eupeodes nuda (F.), Hippodamia variegata (Goeze), Leucopis ninae Tanasijt shuk, and Propylea quatuordecimpunctata (L.). Of these, only a small colony of eight L. ninae and only one P. quatuordecimpunctata were later observed . Four parasitoids species were released-Aphelinus asychis Walker, Aphelinu s varipes Forester, Aphelinus matricariae Haliday, and Diaerietilla rapae. Only A. asychis and D. rapae were observed after the release date, although D. rapae were already present.