The impact of pan trap diameter on sampled bee communities and bycatch weight

Although pan traps are an established method for sampling bees across a wide range of habitats and geographical regions, uncertainty persists as to how pan-trap characteristics influence sampling results. We investigated the effect of pan-trap diameter (23 cm versus 12 cm), interacting with trap colour, on sampled bee communities and bycatch in wildflower strips within agricultural landscapes in Germany, using fluorescent blue, white and yellow pan traps. Based on 1147 collected bee individuals identified on a species level (69 species), we observed interacting effects of pan trap colour and size on the number of sampled bee individuals and species. Large pan traps collected significantly more bee individuals (white and yellow pan traps) and species (all colours) than small pan traps. Large pan traps also collected significantly more bycatch biomass than small pan traps, irrespectively of pan trap colour. The estimated number of sampled species based on the same number of sampled individuals (individual-based rarefaction) was higher for larger pan traps than for smaller pan traps at all sampling sites. Implications for insect conservation: using larger pan traps for sampling bees increases trap efficacy (absolute numbers of sampled individuals and species) and efficiency (sampled species per sampled individuals), but larger traps also capture more bycatch biomass. Particularly in long-term monitoring schemes, we encourage a limitation of this potential impact on the non-target insect fauna by employing methodological refinements. The option to use more small traps per site versus fewer large traps remains to be explored.