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.

Data was collected using a combination of specimen collection with hand trapping, visual observations, and pan traps as described in "The very handy manual: how to catch and identify bees and manage a collection" (https://www.usgs.gov/media/files/how-catch-and-identify-bees-and-manage-a-collection). Floral VOCs were collected using a using a dynamic headspace method with nylon oven bags which had polytetrafluoroethylene (PTFE) ports that were fixed and connected to chemical traps consisting of 65 mm long and 3 mm internal diameter glass tubes packed with 20 mg of Super Q adsorbent (80/100 mesh size, DVB/ ethylvinylbenzene polymer, Alltech Associates Inc., Deerfield, IL, USA). VOC samples were then analyzed with a Thermo Trace 1310 GC coupled to a Thermo ISQ MS with electron ionization (EI) at 70.0 eV at 250 °C. Phenolic glycoside chemistry data was obtained from flash frozen floral and leaf tissue analyzed on UHPLC [Waters Acquity I-Class UPLC with a photodiode array detector (PDA) and a 3100 SQ mass spectrometer (MS), Milford, MA, USA] with commercially available standards of salicin (Sigma-Aldrich), and salicortin, tremuloidin, and tremulacin that had been previously isolated and purified from aspen foliage.

Data is ideal for looking at relationships between insect composition and chemistry composition using mantel tests, but total volatile output could be compared among subsequent years to determine how changing temperatures in early season may influence volatilization of catkin scent and overall insect activity on flowers.