The relative performance of sampling methods for native bees: an empirical test and review of the literature

Abstract

Many bee species are declining globally, but to detect trends and monitor bee assemblages, robust sampling methods are required. Numerous sampling methods are used, but a critical review of their relative effectiveness is lacking. Moreover, evidence suggests the relative effectiveness of sampling methods depends on habitat, yet efficacy in urban areas has yet to be evaluated. This study compared the bee community documented using observational records, targeted netting, mobile gardens, pan traps (blue and yellow), vane traps (blue and yellow), and trap‐nests. The comparative surveys of native bees and honeybees were undertaken in an urbanized region of the southwest Australian biodiversity hot spot. The outcomes of the study were then compared to a synthesis based on a comprehensive literature review of studies where two or more bee sampling methods were conducted. Observational records far exceeded all other methods in terms of abundance of bees recorded, but were unable to distinguish finer taxonomic levels. Of methods that captured individuals, thereby permitting taxonomic identification, targeted sweep netting vastly outperformed the passive sampling methods, yielding a total of 1324 individuals, representing 131 taxonomic units—even when deployed over a shorter duration. The relative effectiveness of each method differed according to taxon. From the analysis of the literature, there was high variability in relative effectiveness of methods, but targeted sweep netting and blue vane traps tended to be most effective, in accordance with results from this study. However, results from the present study differed from most previous studies in the extremely low catch rates in pan traps. Species using trap‐nests represented only a subset of all potential cavity‐nesters, and their relative abundances in the trap‐nests differed from those in the field. Mobile gardens were relatively ineffective at attracting bees. For urbanized habitat within this biodiversity hot spot, targeted sweep netting is indispensable for obtaining a comprehensive indication of native bee assemblages; passive sampling methods alone recorded only a small fraction of the native bee community. Overall, a combination of methods should be used for sampling bee communities, as each has their own biases, and certain taxa were well represented in some methods, but poorly represented in others.

Livestock grazing is associated with seasonal reduction in pollinator biodiversity and functional dispersion but cheatgrass invasion is not: Variation in bee assemblages in a multi-use shortgrass prairie

“…First, it is possible that at cheatgrass-colonized sites vane traps were more visually apparent due to the relatively homogenous structure of the vegetation and thus more attractive to foraging bees. For example, some authors have suggested that passive traps tend to become increasingly attractive when floral displays are not abundant [ 29 , 32 , 51 ]. Similarly, bees captured in sites with high cheatgrass cover may be responding from nearby patches of foraging habitat or recruited from other distal locations.…”

“…Traps consisted of an ultra-violet reflective blue vane fixed to a yellow collection bucket (SpringStar, Woodinville, WA, USA). Although previous research suggests that bee sampling method may impact detection of habitat factors influencing bee communities [29], blue vane traps are well suited for collecting across large landscapes as they are easily deployed and are not biased to observer skill or abilities [30][31][32]. Traps were placed at the previously established central location at each site to sample bee assemblages over four separate periods (May, June, July, and August) that corresponded with the assessments of floral cover.…”

Honey bee hives decrease wild bee abundance, species richness, and fruit count on farms regardless of wildflower strips

“…And yet, supplemental floral resources provided by wildflower strips did not alleviate the negative effect of honey bee hive presence on wild bees in our study, as would be predicted under limited floral resources. However, wild bee populations did not measurably benefit from wildflower strips on farms and we cannot determine the extent to which they were utilized relative to control farm field margins, as trap counts cannot infer foraging behavior 41 , 42 . An additional possible mechanism is that honey bee foraging within an area can reduce wild bee floral visitation 51 ; thus, actively foraging honey bees on farms could have depressed wild bee activity.…”

“…For example, areas with relatively robust natural habitat, such as the Eastern Shore of the Mid-Atlantic, U.S., may demonstrate less of an effect from wildflower strips than areas with an intermediate proportion of natural habitat in the landscapes surrounding farms 39 , 40 . Additionally, while necessary in our study due to logistical constraints, pan and blue vane traps often under-sample bees relative to other methods such as netting at flowers or floral visitation observations 41 , 42 . For example, in the U.K., pan traps collected a lower proportion of pollinator populations where floral resources were abundant than where they were scarce due to competing sources of attractive stimuli 43 .…”

Pan Traps for Tracking Honey Bee Activity-Density: A Case Study in Soybeans

“…Furthermore, surrounding floral landscape variables may influence the activity-density of bees [ 13 ]. Consequently, it is suggested that researchers employ other methods, such as targeted sweep netting, to sample the bee community as an alternative or addition to pan traps [ 9 , 14 ]. Nonetheless, pan traps have a long history of use to estimate relative abundance and diversity of bees across treatments, landscape types, etc.…”