Landscape composition and local floral resources influence foraging behavior but not the size of Bombus impatiens Cresson (Hymenoptera: Apidae) workers

Abstract

Bumble bee communities are strongly disrupted worldwide through the population decline of many species; a phenomenon that has been generally attributed to landscape modification, pesticide use, pathogens, and climate change. The mechanisms by which these causes act on bumble bee colonies are, however, likely to be complex and to involve many levels of organization spanning from the community down to the least understood individual level. Here, we assessed how the morphology, weight and foraging behavior of individual workers are affected by their surrounding landscape. We hypothesized that colonies established in landscapes showing high cover of intensive crops and low cover of flowering crops, as well as low amounts of local floral resources, would produce smaller workers, which would perform fewer foraging trips and collect pollen loads less constant in species composition. We tested these predictions with 80 colonies of commercially reared Bombus impatiens Cresson placed in 20 landscapes spanning a gradient of agricultural intensification in southern Qué bec, Canada. We estimated weekly rate at which workers entered and exited colonies and captured eight workers per colony over a period of 14 weeks during the spring and summer of 2016. Captured workers had their wing, thorax, head, tibia, and dry weight measured, as well as their pollen load extracted and identified to the lowest possible taxonomic level. We did not detect any effect of landscape habitat composition on worker morphology or body weight, but found that foraging activity decreased with intensive crops. Moreover, higher diversity of local floral resources led to lower pollen constancy in intensively cultivated landscapes. Finally, we found a negative correlation between the size of workers and the diversity of their pollen load. Our results provide additional evidence that conservation actions regarding pollinators in arable landscapes should be made at the landscape rather than at the farm level.

Novel pollen analogue technique shows bumblebees display low floral constancy and prefer sites with high floral diversity

“…In addition, in support of our Prediction for Question 1, our Model 1 results indicate that in more diverse sites pollinators visited more flower species resulting in higher pollen load diversity. Previous authors have also found that higher diversity of floral resources leads to lower floral constancy (Gervais et al 2020). Following on from our d' result, this low constancy in high diversity sites appears to be the result of 'active inconstancy' rather than passive inconstancy, as in the energetic costs and benefits hypothesis.…”

“…Comparisons between pollen load species composition and surrounding plant communities are most frequently used to estimate foraging range or to evaluate pollen selection at the colony level, but relationships between floral resource diversity and individual pollen load composition have rarely been investigated in the field (Gervais et al 2020). In addition, the behavioural drivers of the differences between pollen load composition and surrounding vegetation are rarely explored (Gervais et al 2020). The contribution of this study is to analyse pollen load composition at return to the nest in terms of floral resource availability at the sites visited in the field.…”

“…Thorax width was only significant as part of pairwise interactions with visitation to S and U, an indication of possible size-dependent foraging behaviour (e.g. Gervais et al 2020). The interaction between VS and TW is positive, suggesting that visitation to site S may have a positive effect on pollen load diversity depending on bee size.…”

“…Pollen from both corbiculae was removed using a pin and stored at 4°C. Because B. terrestris shows size polymorphism, and bumblebee size has been found to affect foraging behaviour (Spaethe and Weidenmüller 2002), and the diversity of pollen species in pollen loads (Gervais et al 2020), intertegular distance (ITD) of each bee was measured with a digital calliper and used as a proxy for individual size (Cane 1987). One researcher collected all of the ITD measurements.…”

Bees in the six: Determinants of bumblebee habitat quality in urban landscapes

“…We studied the Common Eastern Bumblebee Bombus impatiens (Williams et al, 2014) as it has many traits that may allow us to generalize our findings to other bumblebees and native bees (also see the Discussion section): (1) B. impatiens (Figure 1) is common F I G U R E 1 A common eastern bumblebee, Bombus impatiens, worker foraging on purple coneflower (Echinacea sp.). Photograph by Amro Zayed throughout the city, thereby allowing us to explore the relation between landscape features and bee habitat quality across a large swathe of Toronto; (2) B. impatiens is polylectic (i.e., pollen generalists) like the majority of bees found in cities (MacIvor et al, 2014;Matteson et al, 2008); (3) B. impatiens visits many plant species that are known to be attractive to a wide range of native bee species (Colla & Dumesh, 2010;Gervais et al, 2020;Vaudo et al, 2014); and…”