Characterization of Salix nigra floral insect community and activity of three native Andrena bees

Salix nigra (black willow) is a widespread tree that hosts many species of
polylectic hymenopterans and oligolectic bees of the genus Andrena. The
early flowering of S. nigra makes it an important nutritive resource for
insects emerging from hibernation. However, since S. nigra is dioecious,
not all insect visits will lead to successful pollination. Using both
visual observation and pan-trapping we characterized the community of
insects that visited S. nigra flowers and assessed differences among male
and female trees as well as the chemical and visual drivers that
influenced community composition across three years. We found that male
trees consistently supported higher diversity of insects than female trees
and only three insect species, all Andrena spp., consistently visited both
sexes. Additionally, A. nigrae, which was the only insect that occurred
more on female than male flowers, correlated strongly to volatile cues.
This suggests that cross-pollinators cue into specific aspects of floral
scent, but diversity of floral visitors is driven strongly by visual cues
of yellow male pollen. Through time the floral activity of two Andrena
species remained stable, but A. nigrae visited less in 2017 when flowers
bloomed earlier than other years. When native bee emergence does not
synchronize with bloom, activity appears to be diminished which could
threaten species that subsist on a single host. Despite the community
diversity of S. nigra flowers, its productivity depends on a small
fraction of species that are not threatened by competition, but rather by
rapidly changing conditions that lead to host-insect asynchrony.