Sp. Wraight et al., NATURAL ENEMIES OF RUSSIAN WHEAT APHID (HOMOPTERA, APHIDIDAE) AND ASSOCIATED CEREAL APHID SPECIES IN SPRING-PLANTED WHEAT AND BARLEY IN COLORADO, Environmental entomology, 22(6), 1993, pp. 1383-1391
Colorado has experienced high levels of Russian wheat aphid, Diuraphis
noxia (Kurdjumov), damage and is considered an important site for rel
ease of biocontrol agents. In June 1990, field surveys were initiated
to provide baseline data on the identity and prevalence of existing ap
hid pathogens, parasites, and predators. Sampling was conducted in spr
ing-planted grain in irrigated and dry-land fields near Fort Collins a
nd 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). Syrphi
d fly larvae were the only predators consistently found feeding on aph
ids within the D. noxia-damaged (rolled) leaves. However, syrphid popu
lations were low, <0.3 larvae per aphid-infested tiller. The ineffecti
veness of the existing complex of parasites and predators in suppressi
ng D. noxia populations underscores the need for introductions of more
efficient biocontrol agents. Pathogenic fungi were active in nearly a
ll fields; three species were found. In irrigated fields, Entomophthor
a chromaphidis Burger & Swain was the most common pathogen during the
spring, with peak prevalence of 13% on 22 June. Pandora (= Erynia) neo
aphidis (Remaudiere & Hennebert) was not detected until late June, but
it rapidly reached epizootic levels (44% infection by 18 July). Highe
st prevalence (20%) of Conidiobolus obscurus (Hall & Dunn) coincided w
ith that of P. neoaphidis. Prevalence of fungal pathogens in dry-land
fields did not exceed 2.5%. The observation of only low levels of ento
mophthoralean fungal infection in the nonirrigated fields suggests tha
t reliable use of these natural control agents against aphids in the s
emiarid West will require manipulation of environmental conditions thr
ough irrigation.