Both woody and herbaceous semi-natural habitats are essential for spider overwintering in European farmland

Semi-natural habitats (SNH) support biodiversity and associated ecosystem services on farmland, thereby contributing to sustainable agriculture. However, little is known about the specific suitability of different types of SNH as overwintering habitat for predatory arthropods, despite the crucial role of such knowledge for conservation biological control. We used ground photoeclectors to sample spiders in 65 habitats comprising crop fields and four major SNH types (herbaceous areal, herbaceous linear, woody areal, woody linear) across two study regions in Switzerland and Germany to identify the most important overwintering habitats for spiders. Spider emergence started in February and over half of the individuals of the most common spiders had already emerged by late April. SNH supported on average 146% higher species richness and 194% higher densities of spiders than crop fields. Woody and herbaceous SNH supported distinct spider communities, with individual species exhibiting marked preferences for either woody or herbaceous SNH. Habitat shape (areal or linear) had no significant effects on spider overwintering. Woody SNH hosted more species, whereas herbaceous SNH had higher densities of agrobiont species (i.e. species known to reach high densities in crops in the growing season). Given the higher number of overwintering agrobionts in herbaceous SNH, this habitat type has potential to promote pest control by spill-over of spiders into nearby crops early in the season. In contrast, woody SNH seem to be more important for the conservation of diverse spider communities that are highly distinct from those inhabiting open land. Hence, creating and preserving a variety of SNH is key to provide suitable overwintering sites for species in agricultural landscapes.