Epizootics caused by entomophthoralean fungi (Zygomycetes, Entomophthorales) in populations of cereal aphids (Hom., Aphididae) in irrigated small grains of southwestern Idaho, USA

Each summer from 1986 to 1989, cereal aphids, particularly Metopolophium dirhodum (Walker), Sitobion avenae (F.) and Diuraphis noxia (Mordvilko), were subject to entomophthoralean mycoses in spring wheat, winter wheat and spring barley grown under routine furrow irrigation in southwestern Idaho. In general, S. avenae was detected on the crops 1 or 2 weeks earlier than M. dirhodum, but in 1988 both species occurred 4 and 5 weeks, respectively, later than D. noxia. Diuraphis noxia was first found only on late‐sown spring wheat during late June 1987 and heavily infested the crops during the summer of 1988. It did not, however, establish consistently detectable populations in the summer of 1989 due to the unusually cold preceding winter. Because M. dirhodum multiplied more rapidly than S. avenae during the growing seasons, its populations became larger and reached their peak earlier or at about the same time each summer. Fungal infection of aphids, dominated by Pandora neoaphidis (Remaudière & Hennebert) Humber, was usually detected 2–3 weeks after detected infestation for M. dirhodum, 3–4 weeks for S. avenae and about 6 weeks for D. noxia (in 1988 only), indicating that the initial inoculum of P. neoaphidis was more closely associated with M. dirhodum than with the other species of aphids. The increase of fungal infection coincided with the period of exponential growth of aphid populations. The ratio of cadavers to live aphids (for each species or the whole aphid complex) increased over the growing season from a low but detectable level to 1:10, then 1:1, and eventually approached or passed 10:1. From these ratios, it was concluded that aphid epizootics occurred each summer, particularly on spring wheat, although usually too late to completely protect the crops from aphid damage. In 1987, however, earlier and higher levels of epizootics, apparently associated with more frequent and heavier rain during the first half of June, effectively suppressed aphid populations below economic injury levels at the milky stage of the crops. Epizootics caused by P. neoaphidis were more frequently observed in the populations of M. dirhodum (in most of the fields surveyed) than in those of S. avenae, which were more often subject to Conidiobolus‐caused enzootics. Cadaver density of each aphid species (number per tiller) generally correlated well to live aphid density observed during the preceding sampling week in Pearson's correlation analysis. However, based on maximum‐r2 multiple regression, aphid density was often found to negatively correlate to the increase of infection level (percent mortality), simply indicating an important role of epizootics in the decrease of aphid populations. One or two of other variables, including number of aphid days from initially observed infestation, number of rainy days, cumulative precipitation, sum of day degrees and a modified hydrothermal coefficient (which incorporates cumulative precipitation and sum of day degrees for both aphids and fungal pathogens), did significantly correlate to epizootic development in some cases. Thus, these other variables should be more useful in predicting the outcome of epizootics, at least in some crop‐aphid‐pathogen systems.