Monitoring insect pollinators and flower visitation: The effectiveness and feasibility of different survey methods

Abstract

The status of pollinating insects is of international concern, but knowledge of the magnitude and extent of declines is limited by a lack of systematic monitoring. Standardized protocols are urgently needed, alongside a better understanding of how different methods and recorders (data collectors) influence estimates of pollinator abundance and diversity. We compared two common methods for sampling wild pollinating insects (solitary bees, bumblebees and hoverflies), pan traps and transects, in surveys of 1 km countryside squares (agricultural and semi‐natural habitats) and flowering crop fields across Great Britain, including the influence of local floral resources (nectar sugar availability or crop flower density) on the insects sampled. Further, we compared the performance of recorders with differing expertise (non‐specialist research staff, taxonomic experts and non‐expert volunteers) in applying methods. Pan traps and transects produced compositionally distinct samples of pollinator communities. In the wider countryside, pan traps sampled more species of solitary bee and hoverfly. In flowering crops, transects recorded a greater number of individual bumblebees, but fewer species. Across all taxonomic groups and countryside and crop samples, transects generally had lower rates of species accumulation per individual collected than pan traps. This demonstrates that differences between methods in estimating richness are not due to sampling effort alone. However, recorders possessing greater taxonomic expertise can produce species accumulation data from transects that are almost commensurate with pan trapping. The abundance and species richness of pollinators (except solitary bees) on transects in the wider countryside was positively related to the availability of estimated nectar sugar. In crops, pollinator abundance responses to flower densities were idiosyncratic according to crop type, but overall the response was positive and negative for transects and pan traps, respectively. Given these taxonomic and context‐specific differences in method performance, we assess their suitability for monitoring pollinating insect communities and pollination services. We discuss the relevance of these findings within the context of achieving standardized, large‐scale monitoring of pollinating insects.

Characterization of wild bee communities in apple and blueberry orchards

“…Here, a larger proportion of the local species pool could have been obtained with additional collection methods (e.g. bee visitation transects, blue vein traps) (Cane et al ., ; O'Connor et al ., ) and/or a higher sampling effort; but see Russo et al . ().…”

“…agricultural, landscapes might arise from local specifics in environmental factors, inducing climate, surrounding land use and habitat types, as well as biotic factors such as competition and parasitism (Tscharntke & Brandl, ; Diekötter et al ., ). This suggests that local surveys using a combination of active and passive approaches, including transect walks, sweep netting and traps (Rhoades et al ., ; O'Connor et al ., ) of crop‐associated wild bees, are invaluable because broad patterns and generalizations cannot be drawn across regions or agricultural systems.…”

“…Eleven sampling events were carried out per orchard, resulting in 108 pan trap samples (n = 55 in orchard 1 and n = 53 in orchard 2). Monitoring bees in this fashion may introduce some bias, with effects being positive -affected by flower density (Wood et al, 2015), negative -collecting fewer species (Cane et al, 2000;O'Connor et al, 2019) or neutral (Rhoades et al, 2017).…”

Crop pollination management needs flower‐visitor monitoring and target values

“…Crop pollination studies generally use one of two methods to assess the effect of flower visitors on crop yield: transect counts or visitation rates (Garibaldi et al, ; Vaissière, Freitas, & Gemmill‐Herren, ). Standardized transect counts can give a good idea of insect activity, and they are a commonly used method to compare insect densities across crop fields (O'Connor et al, ). However, transect counts might introduce noise because illegitimate flower visits and non‐visiting insects may also be recorded, and in some studies they do not register the number of flowers, nor do they quantify the between‐flower pollinator movement, which are essential for cross‐pollination.…”

“…For this method, an observer documents each legitimate visit (i.e. contacting reproductive structures) to a specified flower (or group of flowers) during a fixed observation period, which can vary depending on the crop, and repeats this observation several times for different flowers to ensure a representative average (Fijen & Kleijn, ; O'Connor et al, ). The advantage of this method is that it directly relates to scientific studies that estimate how many visits a single flower requires to fully fertilize its receptive ovules (i.e.…”