Treed field borders net-export over 82,000 more hoverflies per km every week into canola crops than herbaceous field borders, regardless of mass-flowering crop bloom

Hoverflies (Diptera: Syrphidae) provide dual ecosystem services, as the adults act as pollinators and the larvae can be predators of crop pests. Because bloom time is limited in mass-flowering crops, resources within crops for hoverfly adults can also be limited and change temporally. Therefore, hoverflies need to move between crops and their borders. It may be that some field border vegetation types support the provision of hoverflies to crops better than other vegetation types. We sought to determine how field border type (herbaceous vs. treed), canola bloom, and border vegetation structure and composition (border width, canopy cover, grass height, grass cover, plant cover, flower availability, and density of trees, shrubs, snags, stumps, and downed woody debris) affect hoverfly movement into and out of crop fields from field borders. We placed bi-directional Malaise traps in herbaceous and treed field borders at 10 fields seeded with canola, and sampled continuously from May 17 to August 20, 2021 in central Alberta, Canada. We found that field border type affected hoverfly movement such that, across the whole summer, net-export of hoverflies into crops was over 33-times higher from treed field borders (an estimated 84,699 hoverflies per km per week) than from herbaceous field borders (an estimated 2515 hoverflies per km per week). We did not find any single component of the vegetation within treed field borders that explained the difference in movement. We found more hoverfly activity in herbaceous field borders than in treed field borders during and after canola bloom, but that overall activity was equal between field border types prior to canola bloom. Treed borders had greater Hill-Shannon and Hill-Simpson diversity and evenness than herbaceous borders. Throughout the growing season, the community became dominated by Toxomerus marginatus, which drove all temporal trends. We conclude that treed field borders act as net exporters of hoverflies to canola fields and are therefore important features for optimizing the magnitude of the ecosystem services provided by hoverflies in agricultural systems.