FIT for purpose? Sampling Bees in Flowering Canopies with Flight Interception Traps (FITs)

Depending on species and season, tree canopies can provide floral resources in abundance. However, researchers often do not consider these resources when sampling bee communities, because they are difficult to access. While observer-based methods, such as hand-netting along transects, are often used for collecting bees from near-ground resources like wildflower strips, flight interception traps (FITs), such as aerial Malaise traps (AMTs) and window interception traps (WITs), can be used for collecting bees from elevated resources like flowering canopies. We assessed the suitability of WITs and AMTs for sampling bees from flowering canopies. We sampled canopies of four different tree taxa (Salix, Malus, Robinia, Tilia) in Braunschweig, Germany, between March and June 2024. In total, we collected 395 bee individuals, including 247 honeybees. Sampled communities comprised ten genera and showed marked differences in the ratio of honeybee and wild bee individuals. In Salix trees, wild bees were more abundant than honeybees, whereas in Tilia trees and especially Robinia trees, honeybees outnumbered wild bees. The collectors above the interception surface of both WITs and AMTs collected no bees. This was surprising, because we expected bees to move upwards upon flight interception, as they do in ground-based Malaise traps. We conducted a systematic literature search in the WoS to compare our findings to previous studies using WITs and AMTs for sampling bees. To our knowledge, our study is the first to show that collectors above the interception surface are generally inefficient when sampling bees with WITs and AMTs. Our study provides methodological advice for future researchers seeking to sample pollinators in tree canopies.