Where Have All The Bees Gone?

The pollination undertaken by bees provides roughly $200 billion dollars in commerce ever year. Though sudden dips in bee colony populations have been recorded periodically throughout history, Colony Collapse Disorder (CCD), a term first used to describe large honeybee losses in North America in 2006, is an increasing ecological and economic concern. Honeybee populations have been on the decline since the 1970s, but scientists began to notice a rapid acceleration in population loss over the last six years. This phenomenon is making commercial pollination services much more expensive, and is threatening wild bee species in the process.

Two recently published articles provide evidence that commonly-used agricultural pesticides are a prime contributor to Colony Collapse Disorder. In “Neonicotinoid Pesticide Reduces Bumble Bee Colony Growth,” authors Penelope R. Whitehorn, Stephanie O’Connor, Felix L. Wackers, and Dave Goulson ran an experiment to examine the effect of one neonicotinoid insecticide, Imidacloprid, on the bumblebee species Bombus Terrestris.

Increasingly, the crops and flowers that bees feed on are being treated with a number of pesticides to protect against insect-related damage. The most extensively used pesticide is Imidacloprid, which is used on major crops including cereals, oilseed rape, corn, cotton, sunflower, and sugar beets. Imidacloprid infuses itself in the nectar and pollen of flowering plants and is known to damage the functioning of specific neurotransmitter receptors within insects. Foraging bees are directly exposed, and in turn contaminate their entire colony via interchange of the poisoned material, a process that occurs over two to four week intervals during the flowering period.

To test the impacts of Imidacloprid on bees, the scientists assigned 75 Bombus Terrestris colonies to three treatment groups: control colonies received untreated pollen and nectar; ‘low’ treatment colonies received food with levels of Imidacloprid equal to those found in the field; the ‘high’ treatment colonies were fed double doses of the pesticide. Following the lab treatment, the colonies were placed in the wild and were monitored without interference for six weeks.

The authors found that Imidacloprid had a negative impact on colony populations. Low and high treatment colonies weighed between 8 and 12 percent less than control colonies. Moreover, relative to the control group, there were 19 and 33 percent more empty pupal cells (an indicator of Colony Collapse Disorder that occurs during larval development) in the low and high treatment groups, respectively.

Bumblebees live for a year and only queen bees survive past the winter to start new colonies, making the species’ survival highly dependent on the overall number and survival of queens. In control colonies, the average number of queens at the end of the experiment was 13.7, compared to just two in the low treatment colonies, and 1.4 in the high treatment groups. As the authors explain, “the drop in queen production is disproportionately large compared to the impact of Imidacloprid on colony growth.”

There is evidence that only the largest bee colonies are successful in producing queens, regardless of exposure to pesticides. Therefore, reductions in a colony’s population may bring it below the necessary threshold to produce queens, further deteriorating future species reproduction. The authors conclude that

[Even] trace levels of neonicotinoid pesticides can have strong negative consequences for queen production by bumblebee colonies under realistic field conditions, and this is likely to have a substantial population level impact.

A second article, entitled A Common Pesticide Decreases Foraging Success and Survival in Honey Bees, reviews a similar experiment conducted by authors Mickaël Henry, et al. The scientists exposed honeybees to a different neonicotinoid pesticide, the newly released Thiamethoxam, which is being approved for use in many countries.

The authors point to existing evidence which suggests that sub-lethal doses of pesticides–levels that are not high enough to directly kill honeybees–can still have damaging behavioral effects on memory and can lead to learning dysfunction, impaired navigational skills, and reduced olfactory (smell) memory. These effects are expressed through strange foraging behavior and degraded learning performance and orientation skills.

Their experiment involved measuring the honeybee death rate as a result of becoming lost (“mortality-induced homing failure,” or MHF) and how those rates affected overall colony performance. Six hundred fifty-three forager bees were tagged with RFID (radio frequency-identification) readers and separated into four treatment groups. Test groups received realistic sub-lethal doses of Thiamethoxam and were released away from their colony. RFID readers were also placed at hive entrances to detect the comings and goings of the test foragers. Mortality-induced homing failure was deduced from the number of non-returning treatment group foragers.

The experiment revealed, “substantial mortality due to post-exposure homing failure, MHF, with the proportion of treated foragers returning to the colony being significantly lower than that of control foragers.” 10.2 to 31.6 percent of contaminated honeybees failed to return to their colonies, roughly twice the natural death rate for uncontaminated foragers. Extrapolating from these findings, the authors demonstrate that colonies exposed to contaminated foragers could experience significant population declines under all tested scenarios. In the worst scenarios, MHF combined with the natural death rate for foragers could lead to colony collapse.

As concern over the economic and environmental impact of Colony Collapse Disorder grows, policymakers should reconsider the use and regulation of commercial pesticides. Together, these studies suggest that bee populations are much more sensitive to pesticide exposure than previously understood.