- Institutionen för ekologi och växtproduktionslära, Sveriges lantbruksuniversitet
Landscape and farm management influence generalist predators : effects on condition, abundance and biological control
Chemical pest control is currently the dominant form of pest management against insect pests in agroecosystems, but has negative side effects. Biological control by specialist predators has been successful in greenhouses, but has not been successfully applied in agroecosystems. Generalist predators have features constraining them as control agents, but have one advantage: they can survive when pests are absent by feeding on alternative prey. In this thesis I have studied the influence of landscape structures and farm management on generalist predators and their predation on an aphid pest on cereals, Rhopalosiphum padi. In accordance with theoretical predictions, predation on establishing aphids in spring was more important for the total aphid attack than predation during the aphid population growth phase. Predation on establishing aphids was higher on organic than conventional farms and highest in landscapes with abundant field margins and perennial crops. Predation during the population growth phase was, surprisingly, highest in simple landscapes, with no differences between organic and conventional farms. The presence of alternative prey can result in decreasing predation of generalist predators on pests, and I show that this effect is scale-dependent. On a field scale, generalist predators were aggregated at high prey densities, and predator abundance was more important for predation on aphids than alternative prey abundance. Thus, the effect of alternative prey was larger on the generalist predator abundance than on the predators' diet. However, higher alternative prey abundance in complex landscapes may still have resulted in the observed lower predation rates on aphids in these landscapes during aphid population growth. Condition of generalist predators, which often is food-limited, was better on organic farms and in landscapes with abundant field margins in a non-aphid year. Possibly because of an aphid outbreak, condition changed asynchronously between farms over three study years. The temporal and spatial environmental variation affected the condition of different species differently, which can be explained by the species’ diet and habitat selectivity. This thesis provides further evidence for earlier suggestions that more field margins and perennial crops in relation to annual crops increase alternative prey abundance and overwintering sites for the generalist predators and thereby their abundance, which can enhance biological control of cereal aphids.
bird cherry-oat aphid; Carabidae; Nabidae; fitness; natural enemies; patch choice; selectivity; spatial-temporal variation
Acta Universitatis Agriculturae Sueciae. Agraria
2002, nummer: 363
Utgivare: Swedish University of Agricultural Sciences
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