Natural Enemies of Russian Wheat Aphid (Homoptera: Aphididae) and Associated Cereal Aphid Species in Spring-Planted Wheat and Barley in Colorado

Colorado has experienced high levels of Russian wheat aphid, Diuraphis noxia (Kurdjumov), damage and is considered an important site for release of biocontrol agents. In June 1990, field surveys were initiated to provide baseline data on the identity and prevalence of existing aphid pathogens, parasites, and predators. Sampling was conducted in spring-planted grain in irrigated and dry-land fields near Fort Collins and Akron. D. noxia was the most abundant aphid; populations were high in both irrigated and dry-land fields. Parasite prevalence was low <5%) in all fields. D. noxia was the most frequently parasitized aphid, and the most common parasite was Diaeretiella rapae (M'Intosh). Syrphid fly larvae were the only predators consistently found feeding on aphids within the D. noxia –damaged (rolled) leaves. However, syrphid populations were low, <0.3 larvae per aphid-infested tiller. The ineffectiveness of the existing complex of parasites and predators in suppressing D. noxia populations underscores the need for introductions of more efficient biocontrol agents. Pathogenic fungi were active in nearly all fields; three species were found. In irrigated fields, Entomophthora chromaphidis Burger & Swain was the most common pathogen during the spring, with peak prevalence of 13% on 22 June. Pandora (= Erynia ) neoaphidis (Remaudière & Hennebert) was not detected until late June, but it rapidly reached epizootic levels (44% infection by 18 July). Highest prevalence (20%) of Conidiobolus obscurus (Hall & Dunn) coincided with that of P. neoaphidis . Prevalence of fungal pathogens in dry-land fields did not exceed 2.5%. The observation of only low levels of entomophthoralean fungal infection in the nonirrigated fields suggests that reliable use of these natural control agents against aphids in the semiarid West will require manipulation of environmental conditions through irrigation.