What tangled webs they weave: evaluating potential prey items in hemlock canopies

We examined the spider community on eastern hemlock and their relationship with arthropod prey in light of the presence of an exotic invasive forest pest, the hemlock woolly adelgid, Adelges tsugae. Eastern hemlock is highly susceptible to the adelgid; feeding causes needle drop, twig die-back and rapid tree death. Eastern hemlock is a foundation species that plays key roles in ecosystem function by regulating nutrient and water cycling, air, soil, and water temperatures, and also serves as essential habitat for wildlife. Their loss would be devastating. The emphasis in adelgid management has been on classical biological control, while little attention has been given to the potential for endemic natural enemies. Using sticky traps to simulate spider webs, we evaluated what prey items are available by examining an adelgid-infested hemlock community and comparing it to an uninfested community. We found that the overwhelming majority (>70%) of alternative prey items available in hemlock canopies were Diptera, regardless of the presence of adelgids. Sticky trap orientation (vertical vs horizontal) and cardinal direction had little effect on prey capture (χ² ₁=0.35, P=0.55; χ² ₃=1.13, P=0.77 respectively), but some orientation × direction combinations caught a greater number of items than others, suggesting that spiders might locate their webs to optimize prey capture (χ² ₃=8.26, P=0.04). Understanding the response of spiders to potential prey availability is essential to understanding the trophic interactions involving these predators and their potential for influencing adelgid populations.