Natural Enemy Impact on the Abundance of Diuraphis noxia (Homoptera: Aphididae) in Wheat in Southern France

Abstract

Natural enemy impact on Diuraphis noxia (Mordvilko) abundance was measured with field exclosure experiments in wheat near Montpellier, France, during three springs. Plants were artificially infested with D. noxia; predators, parasitoids, and aphid–home pathogens were excluded from some plants but not others; and subsequent changes in aphid densities were compared by destructive plant sampling. Each year, D. noxia density in all treatments increased and then declined as plant quality deteriorated, but peak densities were 10– to 18–fold higher on closed–caged plants than on open–caged or uncaged plants. Differences in microclimate, emigration, and aphid competition were eliminated as explanations for differences in D. noxia densities. The mortality required to explain differences in dynamics between closed–caged and open–caged/uncaged D. noxia populations was calculated. Natural enemies were sampled in the surrounding wheat to assess their capacity to inflict this required mortality. The predatory coccinellid Coccinellaseptempunctata L. reached densities of 4–16 per square meter and the predatory syrphids Episyrphus balteatus De Geer, Metasyrphus corollae F., and SphaerophOria scripta L. together reached one per square meter. The parasitoids Aphellinus asychis walker, Aphelinus varipes (Foerster), Diaeretiella rapae (McIntosh), and Aphidius matricariae were also abundant. Given predator search capacities and observed parasitism rates, mortality from natural enemies could easily account for the differences in D. noxia densities between closed–caged and open–caged/uncaged plants.