Impact of plant density and natural enemy exclosure on abundance of Diuraphis noxia (Kurdjumov) and Rhopalosiphum padi (L.) (Hom., Aphididae) in Hungary

A field experiment was carried out in central Hungary to test the impact of plant density and natural enemy exclusion on density of Diuraphis noxia on artificially infested barley plants. As the experimental plants also becam e infested with high levels of Rhopalosiphum padi, this aphid was studied a s well. Densities of both aphid species were higher on caged plants than on uncaged plants. Diuraphis noxia density was higher at low plant density th an at high plant density, and the difference was greater on caged plants th an on uncaged plants. However, R. padi density did not vary with plant dens ity. Parasitism by insect parasitoids was extremely low (approximately 1% o verall) and thus they had little impact on aphid density. Incidence of infe ction of the aphids by Pandora neoaphidis reached high levels (40-50%). Inf ection incidence was higher on caged than on uncaged plants for both aphids , and it was higher at high plant density than at low plant density for R. padi. Higher infection incidence at higher aphid density suggests that dise ase transmission may have been higher at high aphid density. The action of the pathogen cannot explain differences found in aphid densities between ca ged and uncaged plants because the pathogen was most common where aphids we re most abundant. However, aphid density did tend to decline as incidence o f infection increased over time. Predator densities were not measured, and mobile predators may have caused the difference in aphid density between ca ged and uncaged plants. The roles of plant density, predators and P. neoaph idis in limiting D. noxia abundance in Hungary are worth further investigat ion.