Early spring orchard pollinators spill over from resource-rich adjacent forest patches

Pollinator communities are more abundant and diverse in agricultural
matrices with more natural habitats, although the reasons for these
correlations remain unclear. It is possible that forest fragments and
edges provide resources for pollinators in the important early weeks of
spring, after which time those insects can then “spill over” into crops
such as apple orchards during bloom. To explore how forest edges may feed
and therefore promote flower visitor communities in adjacent agricultural
habitats, we sampled springtime pollinators in nine orchards and their
adjacent forest edge canopies and understories. We identified pollen
consumed by pan-trapped bees and flower flies to assess if bees ate pollen
where they were caught and if their diets similarly “spill over” from
forest to orchard. We further explored sex differences in habitat usage.
Our spatially replicated sampling found that bee and flower fly abundance
peaks first in the forest understory, then in the forest canopy, and
finally in the orchard. Analysis of digestive tracts showed significant
usage of forest canopy pollen throughout the spring, especially before
apple bloom. Pollinators had often eaten pollen from a different habitat
than the one in which they were caught, suggesting frequent movement
between habitats. Digestive tract pollen is an underused but powerful
avenue for ecological insight. In Andrena, which are important orchard
pollinators and one of the most abundant wild bee taxa in this study, male
bees were primarily found in the woods but not the orchards where
conspecific females were later active. Synthesis and policy implications:
Forested areas, especially forest canopy trees, provide large amounts of
early spring resources that facilitate the build-up and spillover of wild
pollinator populations into apple orchards during bloom. Forests also
provide critical habitats for male bees, which were rarely found in
orchards. Despite their importance for bee reproduction, the needs of male
bees are usually not considered in conservation planning. Overall, our
data indicate that ensuring there is adequate forest habitat adjacent to
orchards can improve the long-term sustainability of pollinator
populations that provide essential crop pollination services.