|Elliott, Norman - Norm|
Submitted to: European Journal of Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2008
Publication Date: 7/1/2008
Citation: Brewer, M.J., Noma, T., Elliott, N.C., Kravchenko, A.N., Hild, A.L. 2008. A landscape view of cereal aphid parasitoid dynamics reveals sensitivity to farm- and region-scale vegetation structure. European Journal of Entomology. 105:503-511. Interpretive Summary: The types of vegetation within an agricultural field and in the surrounding landscape may play an important role in determining how effective natural enemies will be in controlling crop pests. This is because the right kinds of surrounding vegetation can provide necessary food and shelter for natural enemies that would otherwise be absent. However, empirical evidence for this role is limited. We assessed effects of vegetative diversity in winter wheat farms and surrounding landscapes on aphid and aphid parasitoid populations in a temperate grassland environment. We found that vegetation within and surrounding fields affected the natural enemies in the wheat fields. Early in the growing season, when wheat was in the vegetative growth stage and sunflower had not yet emerged, a parasitoid, Aphelinus albipodus, was most abundant in wheat strips and in homogeneous landscapes. Late in the growing season, when wheat was mature or harvested and sunflower was available, two parasitoids, A. albipodus and Lysiphlebus testaceipes, were most abundant in sunflower strips and in landscapes with high plant diversity. Late in the growing season when hot and dry weather conditions were more common, both parasitoid species tended to be more abundant in cooler habitats regardless of vegetative diversity. The relevance of this study lies in establishing a body of evidence to assess whether vegetational diversity can be manipulated to improve biological control. The results add to an emerging picture, which suggests that potential exists to manipulate plant diversity at the within-field and landscape scales to influence natural enemies and their effectiveness in biological control.
Technical Abstract: We assessed the influence of a vegetation diversity gradient on aphid parasitoids in the temperate grasslands of the North American Great Plains. Regionally, the landscape was categorized into relatively heterogeneous and homogeneous vegetation classes. Farm-scale the landscape was classified into farms that followed diverse (wheat-sunflower-fallow) and simple (wheat-fallow) strip-crop rotations. In a 2 (regional-scale) by 2 (farm-scale) factorial design of 16 sites in a 14,000 km2 study region during two years, we commonly detected regional- and farm-scale diversity effects and seasonal sensitivity of the landscape scaling effects on prevalence of parasitoids attacking a key wheat pest. April through June, when wheat was at vegetative growth stage and sunflower was not emerged, the parasitoid Aphelinus albipodus was most frequent in wheat strips and farms with a simple crop rotation, but only when nested regionally in homogeneous vegetation. July through September, after wheat was harvested and sunflower became available, A. albipodus was more frequently detected in sunflower strips and in the diverse crop rotation than in the simple crop rotation, but only within the homogeneous vegetation class. In contrast, the parasitoid Lysiphlebus testaceipes was more frequently detected in the diverse crop rotation than in the simple rotation, and this was seen within both the heterogeneous and homogeneous vegetation classes in August. Not dismissing the species differences and seasonal sensitivities, farm-scale crop diversification appeared to be beneficial to conservation of these principal aphid parasitoids, especially in summer in regional homogeneous vegetation typical of the region.