The contribution of semi-natural habitats to biological control is dependent on sentinel prey type.
It is widely recognized that landscape factors affect the biological control of weed seeds and insect pests in arable crops, but landscape effects have been found to be inconsistent between studies. Here, we compare six different types of sentinels (surrogate prey that was either live insects or seeds) to measure the effects of semi-natural habitats at field to landscape scales on levels of biological control in winter wheat in the UK. Sentinels were located in fields adjacent to three boundary types: grassy margin, hedgerows or woodland to study the local scale effects and in landscapes of varying heterogeneity in study areas of 1-km radius. Overall, mean levels of predation were higher for most insect prey (60.8%) located on the ground compared to the crop (12.2%) and was lower for seeds (5.8%). Predation of sentinels on the ground was attributed to generalist predators. Semi-natural habitats had both positive and negative effects at field and landscape scales, but the response varied with the sentinel type. Herbaceous linear semi-natural habitats had positive effects at local scales for Calliphora vomitoria and Sitobion avenae sentinels and provide evidence that farmers can introduce linear herbaceous features to benefit biological control. In contrast, our distance-weighted kernel models identified a positive relationship between woody habitats and the predation of C. vomitoria and Chenopodium album. Natural aphid infestations were lower in landscapes with more semi-natural habitat. Synthesis and applications. Sentinels may be sensitive enough to detect variation in levels of biological control influenced by semi-natural habitats, but this study confirms that landscape effects differ for different types of sentinel prey. This implies that it may not be possible to categorize landscapes as pest suppressive using a single sentinel type. Future studies should therefore consider using multiple sentinels to give a better perspective on predation intensity. The resulting recommendations for farm management include planting woodland adjacent wheat fields infested with seed predators and positioning herbaceous linear habitats adjacent wheat fields infested with Sitobion avenae, particularly if fields are bordered by woody liner habitats due to their association with decreased S. avenae predation.