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

Abstract

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 became infested with high levels of Rhopalosiphum padi, this aphid was studied as well. Densities of both aphid species were higher on caged plants than on uncaged plants. Diuraphis noxia density was higher at low plant density than at high plant density, and the difference was greater on caged plants than on uncaged plants. However, R. padi density did not vary with plant density. Parasitism by insect parasitoids was extremely low (approximately 1% overall) and thus they had little impact on aphid density. Incidence of infection of the aphids by Pandora neoaphidis reached high levels (40-50%). Infection 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 disease transmission may have been higher at high aphid density. The action of the pathogen cannot explain differences found in aphid densities between caged and uncaged plants because the pathogen was most common where aphids were most abundant. However, aphid density did tend to decline as incidence of infection increased over time. Predator densities were not measured, and mobile predators may have caused the difference in aphid density between caged and uncaged plants. The roles of plant density, predators and P. neoaphidis in limiting D. noxia abundance in Hungary are worth further investigation.