Ecological intensification provides opportunity to increase agricultural productivity while minimizing negative environmental impacts, by supporting ecosystem services such as crop pollination and biological pest control. In place of creating larger areas of semi-natural habitat to support beneficial insects, more targeted management solutions that provide critical resources to organisms at the right time and place could be an option. We predicted that annual strips of flowering phacelia (Phacelia tanacetifolia) could support pollinators and natural enemies of seed eating weevils (Protapion sp.) in red clover (Trifolium pratense) seed fields, by providing early blooming flowers that provide nectar and pollen before the crop starts to flower. We also predicted that by supporting beneficial insects, the flower strips could be a tool to enhance the red clover seed yields. To explore these questions, we monitored insect pollinators, pests, natural enemies and seed yields in a total of 50 clover fields over two years and across two regions in southern Sweden, with about half of the fields with flower strips and the remaining fields without strips.
The clover fields were pollinated by 60% bumble bees (Bombus sp.) and 40% honey bees (Apis mellifera). The addition of a flower strip of the early blooming phacelia enhanced species richness of bumble bees in the clover fields, with the strongest influence in less intensively cropped landscapes. There were no detectable differences in densities of bumble bees or honey bees, seed eating weevils or weevil parasitation rates between crop fields with and without flower strips. However, clover seed yield increased with increasing size of the flower strip. This was probably a result of the combined positive relationship between flower strip area and short tongued pollinators and negative relationship for weevil density. The clover seed yield was positively associated with honey bee density and negatively associated with weevil density, but was unrelated to bumble bee species richness and density. We conclude that annual flower strips of early flower resources could be a management tool to support pollinating insect diversity and, if sufficiently large, also contribute to increased crop yields by attracting and supporting short-tongued pollinators and depressing pests.