Bombus (Hymenoptera: Apidae) Microcolonies as a Tool for Biological Understanding and Pesticide Risk Assessment

Abstract

Bumble bees provide valuable pollination services to many wild and agricultural plants. Populations of some bumble bee species are in decline, prompting the need to better understand bumble bee biology and to develop methodologies for assessing the effects of environmental stressors on these bees. Use of bumble bee microcolonies as an experimental tool is steadily increasing. This review closely examines the microcolony model using peer-reviewed published literature identified by searching three databases through November 2018. Microcolonies have been successfully used for investigating a range of endpoints including behavior, the gut microbiome, nutrition, development, pathogens, chemical biology, and pesticides/xenobiotics. Methods for the initiation and monitoring of microcolonies, as well as the recorded variables were catalogued and described. From this information, we identified a series of recommendations for standardizing core elements of microcolony studies. Standardization is critical to establishing the foundation needed to support use of this model for biological response investigations and particularly for supporting use in pesticide risk assessment.

Keywords: bumble bee; hazard assessment; methodology; pesticide.

Figure 1.. Microcolony development timeline.

A) Microcolonies were initiated with five callow Bombus impatiens workers and provisioned with a pollen patty (2 g) and syrup to promote nest building. After 7 days, microcolonies were fed fresh pollen paste and syrup every Monday, Wednesday, and Friday. B) Starting from nest initiation, photos show microcolony progression from uncapped egg chambers to pupal cells.

Figure 2.. Number of microcolony publications per year.

Google Scholar, Web of Science and the Pro Quest Agricultural and Environmental Science Database were searched through November 2018 using a common search string. All results were compiled and organized by year of publication. Given that our search was completed before all publications for 2018 were available, the year 2018 is captured with a grey bar.

Figure 3.. Microcolony design parameters.

Microcolony design parameters were extracted from research articles published through November 2018 (Table S1) and plotted to show the relationships between the number of workers, microcolony container volume and experimental duration. Only studies reporting data for all three parameters were included (i.e., 52 of the 75 published articles). When a study included microcolonies composed of different numbers of workers, each configuration was counted individually. If study duration indicated a specific number of days after egg laying, we assigned a value of 6 days for egg initiation.

Figure 4.. Example microcolony chambers.

A) “Queen box” (17.8 × 15.2 × 10.1 cm) used by commercial bumble bee vendors (i.e., Koppert, Biobest) with syringe feeder (USDA-ARS Pollinating Insect Research Unit; North Logan, UT). B) Disposable “deli cup” (16 oz, 11.4 cm tall × 7.6 cm diameter) modified with mesh to improve ventilation and to accommodate a syringe feeder (USDA-ARS Pollinating Insect Research Unit; North Logan, UT). C) Food pan (1/4 size, 12.7 × 11.4 × 10.1 cm) modified to include a raised perforated stainless-steel mesh floor to separate bee waste from the nest area, see-through red plexiglass lid, syringe feeder and holes for additional ventilation (US EPA, RTP, NC). D) Stainless steel geology sieve (4 cm tall × 12.7 cm diameter) with pass-through floor, ventilated baseplate, see-through lid and syringe feeder (US EPA, RTP, NC; design adopted from Bayer CropSciences). All container designs are shown with petri dish containing a pollen ball for nest initiation and a separate dish for pollen feeding.

Figure 5.. Relationship between microcolony endpoints.

Factors (in bold) known to affect individual bumble bee development (A) and microcolony performance (B) with some examples shown below. Symbols indicate microcolony assessment endpoints (C) impacted by these factors.