Overwintering of pollen beetles and their predators in oilseed rape and semi-natural habitats

Semi-natural habitats (SNH) maintain high levels of biodiversity and ecosystem functioning within agroeco-systems. While management recommendations for SNH to promote biodiversity and optimize natural pest control, have mostly focused on improving food resources for predators, overwintering conditions have hitherto received less attention. The success of conservation biological control is often hampered by a lack of knowledge on how different habitats and their associated traits drive the overwintering of both, crop pests and their natural enemies. Moreover, there are concerns that SNH may act as reservoirs for crop pests. We investigated the overwintering of pollen beetles (Meligethes spp.) and their predators (predatory ground beetles) across 40 habitats of all major types of SNH as well as 8 winter oilseed rape (WOSR) fields, to identify the importance of type and key traits of habitats. Overwintering of pollen beetles was higher in WOSR fields (7 individuals per 0.25 m(2)) and forest edges (4) compared to flower strips (2) or forest interiors (1). Similarly, overwintering of predatory ground beetles was higher in forest edges and WOSR fields (11 and 10 individuals, respectively) than in flower strips, meadows or hedgerows (4, 4 and 2 individuals). The predator-prey ratio (number of predatory ground beetles / number of pollen beetles) was higher in forest edges than in flower strips, meadows or hedgerows, and intermediate in WOSR fields. Pollen beetle numbers were negatively associated with increasing litter cover and positively with increasing bare soil, while predatory ground beetles were positively associated with soil bulk density and bare soil cover. The proportion of SNH at the landscape level (1 km radius) did not affect overwintering pollen beetles, predatory ground beetles or predator-prey ratio. We conclude that WOSR fields, rather than SNH are the major overwintering habitat and source of spring colonisation of WOSR by pollen beetles. Forest edges promote predatory ground beetles rather than pollen beetle populations according to their disproportionally high value of overwintering individuals. Agri-environmental measures to promote predatory ground beetles rather than pollen beetles should promote overwintering habitats with compact soils and high litter cover.