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The effectiveness of natural enemies to control pests can be enhanced through habitat manipulation. However, due to the differences in their ecology, generalist and specialist species may respond differently to the same manipulation. Moreover, interactions among natural enemies (i.e. cannibalism, intraguild predation, hyperparasitism) may complicate the assumption that a higher density of natural enemies would increase the level of biological control. We investigated the natural enemy guild composition and the predation rate along flower vs. grass margins at the edge of winter wheat (Triticum aestivum) fields in Denmark. Natural enemies were sampled by pitfall trapping and by suction sampling; predation intensity was measured using two different sentinel prey methods: artificial caterpillars made of plasticine, and sentinel aphid colonies. Specialist and generalist species responded differently to the two margin types: specialists (mostly parasitic wasps) were attracted by the flower margins, while generalists (ground beetles, rove beetles and spiders) were more active in grass margins. The number of artificial caterpillars attacked was significantly greater in grass margins (mean = 48.9%, SD = 24.3) than in flower margins (mean = 30.7%, SD = 17.4). We found a significant positive relationship between the number of artificial caterpillars attacked by chewing insects, and activity density for large (≥15 mm) ground beetles. Predation of sentinel aphids in wheat fields did not vary significantly in relation to margin type. Our results suggest that flowering margins may be beneficial for canopy‐active specialist natural enemies, but grassy margins are more useful for ground‐active generalist predators.

Increasing intrafield plant diversity has been shown to regulate pest populations in various agroecosystems. Among the suggested mechanisms for this bottom-up pest control, the disruptive crop hypothesis states that herbivores' abilities to locate and colonize their host plants are reduced by the presence of non-host plants. Under laboratory conditions, we evaluated how intercropping wheat and legumes modifies the behaviour of apterous cereal aphids, Sitobion avenae (Fabricius) (Hemiptera: Aphididae), in terms of host plant location and population growth. We compared two intercropping systemssoft winter wheat, Triticum aestivum L. (Poaceae), associated with winter pea, Pisum sativum L., or with white clover, Trifolium repens L. (both Fabaceae)and sole stands of soft winter wheat. Aphids needed more time to locate their wheat host plant and then spent less time on wheat when it was intercropped with clover. At the population level, and accounting for host plant biomass, only intercropping wheat with clover significantly reduced aphid densities on wheat, as this was particularly disruptive to S. avenae behaviour and population growth. Our laboratory study points out that the species used as non-host plants and their density are important parameters that should be taken into account in field studies on intercropping systems.

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