Prevalences of fungal pathogens and other natural enemies of cereal aphids(Homoptera : Aphididae) in wheat under dryland and irrigated conditions inSouth Africa

Surveys were conducted in the summer and winter rainfall wheat producing re gions of South Africa in a first attempt to investigate the identity and im pact of entomopathogenic fungi within the cereal aphid complex. Wheat produ ced under dryland and irrigated conditions was surveyed during the 1996 and 1997 seasons. Six cereal aphid species were recorded of which the Russian wheat aphid, Diuraphis noxia, was the most abundant under dryland condition s in the summer rainfall region as opposed to the oat aphid, Rhopalosiphum padi, in the winter rainfall region. Rose grain aphid, Metopolophium dirhod um, was most prevalent under irrigated conditions in the summer rainfall re gion. Five species of entomopathogenic fungi were recorded including four e ntomophthorales and the hyphomycete, Beauveria bassiana. The Entomophthoral es included Pandora neoaphidis, Conidiobolus obscurus, C. thromboides, and Entomophthora planchoniana. Pandora neoaphidis was the most important etiol ogical agent recorded from D. noxia, with up to 50% mycosis recorded under dryland conditions in the Bethlehem summer rainfall region. Similarly, P. n eoaphidis was the most prevalent species within populations of M. dirhodum. under irrigated conditions in the Bergville/Winterton summer rainfall regi on (up to 77% mycosis). However, mycoses of R. padi did not exceed 1.7% in samples from these areas, suggesting that R. padi may be less susceptible t o P. neoaphidis than M. dirhodum. Epizootics in populations of D. noxia und er dryland conditions in both the winter and summer rainfall regions indica ted a high level of susceptibility to P. neoaphidis. Occurrences of hymenop terous parasitoids and predators in populations of D. noxia were low, altho ugh a parasitism level of 25% was recorded in one small sample of R. padi c ollected from an irrigated field in the summer rainfall region.