Natural Control of Cereal Aphids (Homoptera: Aphididae) by Entomopathogenic Fungi (Zygomycetes: Entomophthorales) and Parasitoids (Hymenoptera: Braconidae and Encyrtidae) on Irrigated Spring Wheat in Southwestern Idaho

Abstract

Data on the natural control of cereal aphids, Diuraphis noxia (Mordvilko), Metopolophium dirhodum (Walker), and Sitobion avenae (F.), by entomophthoralean fungi and hymenopterous parasitoids on furrow-irrigated spring wheat in southwestern Idaho are presented. The important fungi involved were Pandora neoaphidis (Remaudire & Hennebert) Humber and Conidiobolus spp., including C. obscurus (Hall and Dunn) Remaudire, C. thromboides Drechsler, and C. coronatus (Constantin) Batko. During 1986–1989, M. dirhodum and S. avenae populations usually reached or exceeded economic levels. D. noxia did not enter the region until late June 1987. It was the first aphid found infesting the crop in 1988 and reached high densities that summer but was not found in 1989, perhaps because of mortality during an extremely cold winter. Fungal infection occurred ≍2,3, and 6 wk after colonization of the crop by M. dirhodum, S. avenae , and D. noxia, respectively. Epizootics occurred each summer but usually after the crop was damaged by large aphid populations. An exception occurred during 1987 when M. dirhodum and S. avenae populations were effectively suppressed by mycoses, apparently enhanced by unusually frequent rainfall during late May and June. M. dirhodum experienced much higher mortality from fungal infection than did S. avenae and D. noxia. P. neoaphidis was the dominant fungus infecting M. dirhodum and D. noxia, whereas Conidiobolus spp. were most important on S. avenae. Entomophthora chromaphidis Burger & Swain and two Zoophthora species sporadically infected cereal aphids. Parasitoids, mainly Aphidius ervi Haliday for S. avenae and M. dirhodum and Diaeretiella rapae (M'Intosh) for D. noxia, usually attacked cereal aphids earlier than the fungi but were less influential during the decline of host populations. Multiple regression and correlation analysis indicated that mycoses and parasitoids made significant contributions to the reduction of peak populations of each aphid species compared with the influence of crop maturation. Weak correlation between fungal infection and host density and climatic factors ( r 2 ranging from 0.35 to 0.53) was found. Development of fungal infection seemed to be better correlated with host density than with climatic factors. Precipitation was more associated with fungal infection of S. avenae, which inhabits upper portions of the host crop, but had little effect on that of M. dirhodum and D. noxia, which inhabit more humid microenvironments. No other climatic factors were significantly correlated with fungal infection. A hypothesis is proposed that the site on a wheat plant typically occupied by an aphid species influences the development of mycoses.