Research Focus: Metal contamination threatens bumble bee foraging efficiency in cities
Sarah Scott, Frances Sivakoff and Mary Gardiner
Post-industrial cities, including Cleveland, Ohio, have become focuses of urban greening initiatives due to their large holdings of vacant lands. These areas have a complex history of manufacturing, industrial activity, and deconstruction that has resulted in elevated metal contamination within the landscape. Urban bees are exposed to metals while foraging in contaminated landscapes at concentrations that cause elevated brood mortality. Sublethal exposure to certain metal pollutants challenge honey bee memory and recall abilities and reduce active foraging time on metal contaminated plants. However, the impact of sublethal metal consumption on bumble bee foraging ability was largely unknown.
The goal of our study was to determine if consuming concentrations of the metals arsenic, cadmium, chromium, lead, found in floral provisions within Cleveland influenced bumble bee forager behavior. We hypothesized that bumble bees fed metals will take fewer and shorter foraging trips outside of the colony due to challenged memory and learning.
We tested our hypothesis using novel RFID tracking systems. First, bees from twenty-eight naïve Bombus impatiens colonies were tagged with unique RFID tags (Figure 1), and each colony was randomly assigned to a heavy metal treatment or control. For three days, we fed each colony either a sucrose solution (control), or the same solution dosed with arsenic, cadmium, chromium, or lead. Next, we removed these food sources and placed the colonies into a RFID system to forage in the natural environment at Waterman Agriculture and Natural Resources Laboratory in Columbus, OH, USA for 14 days. The RFID system were constructed from modified 10-frame deep honey bee boxes and had drilled ventilation holes, a lattice bottom, and were lined with aluminum mesh to allow airflow but prevent unwanted pests and volunteer bee entry into the unit (Figure 2). Each RFID unit had two antenna readers connected in series separated by a 3 cm Delrin resin block. Bees access the outside by passing through a pair of RFID readers along a 0.5 m polyethylene semi-clear tube (1/2 in diameter) connected to the outside environment (Figure 2). As bees moved through the paired readers, the individual RFID transponder tag with a time stamp was recorded and the antenna ID to determine the direction of movement (exiting to forage versus returning to the colony).
We found that there was no difference in the number of tagged bees that left their colony or the number of foraging trips taken per bee between control and treatment colonies. However, chromium and cadmium fed bees performed shorter foraging bouts and were more likely to enter a non-natal colony at least one time compared to control bees. Our findings illustrate that sublethal chromium and cadmium consumption can effect bumble bee forage frequency, duration, and return navigation. Challenged navigation and memory performance has the potential to reduce the number of foragers returning with food resources and hinder colony success.