Researchers’ Focus Returns To Pesticide Use In Identifying Cause Of Mystery Honeybee Disorder

By Nancy K. Crevier

Colony collapse disorder (CCD) has been a concern of beekeepers, gardeners, and entomologists since it was first identified in 2006. CCD is characterized by the sudden disappearance of bees from a hive, with some hives losing an entire squadron of worker bees in just a few days. Unlike other bee diseases, where keepers find dead bees littering the ground in front of the hive or evidence of infestation, with CCD bees seem to simply leave the hive and not return.

Researchers worldwide have spent the last six years investigating the honeybee die-off issue, testing numerous theories and hoping to identify the cause of CCD, which has decimated the honeybee population in both home and professional beekeeping sites.

 The disappearance of honeybees is of great concern. Honeybees are vital to crop pollination. If too many bees succumb to the disorder, agricultural yields would drop dramatically, creating a scarcity of many agricultural products. In Connecticut in 2007, said Kimberly Stoner, associate scientist in the entomology department with the Connecticut Agricultural Experiment Station (CAES) in New Haven, nearly 400 registered beekeepers with some 3,000 hives supplied honey to residents and provided pollination services for the many apple, pear, blueberry, and pumpkin crops grown in the state.

At the time, the CAES was investigating the role that pesticides may play in bee mortality. “Systemic insecticides travel inside the plant, and a systemic insecticide can be applied to the seed, travel through the plant as it grows, and appear in the nectar and pollen when the plant flowers,” Ms Stoner explained. Of particular interest five years ago was Imidacloprid, a neonictinoid. The insecticide is commonly used by homeowners on their lawns and ornamentals, as well as in agriculture.

Researches at CAES wondered in 2007 if changes in bee behavior and memory could be related to exposure to sublethal doses of pesticides. Because bees rely on detailed “dances” to convey to other bees the nectar sources and the directions to get to those sources and back to the hive again, an inability for a young bee to learn those directions or for adult bees to recall that information could explain the loss of bees.

Now, that theory has been raised again. In two reports, published March 29 in the online journal Science, a team of scientists from the United Kingdom and one from France look at the effects of neonictinoid insecticides. This family of insecticides was introduced in the early 1990s. According to the American Association for the Advancement of Science (AAAS) news archive dated March 30, “These compounds act on the insect’s central nervous system, and they spread to the nectar and pollen of flowering crops.”

Experiments by Penelope Whitehorn of the University of Stirling in Stirling, UK, and colleagues studied the effects of low levels of Imidacloprid on bumblebees, which are also suffering from a decline in population. “The treated colonies were on average 8 percent to 12 percent smaller than the control colonies at the end of the experiment. The treated colonies also produced about 85 percent fewer queens. This last finding is particularly important because queen production translates directly to the establishment of new nests following the winter die-off. Thus, 85 percent fewer queens could mean 85 percent fewer nests in the coming year.”

The experiments by the French scientists used another pesticide in the neonictinoid family, dosing honeybees with an amount of thiamethoxam equal to what might be experienced in normal foraging.

“Compared to control bees that were not exposed to the pesticide,” researchers report, “the treated bees were about two to three times more likely to die while away from their nests. These deaths probably occurred because the pesticide interfered with the bees’ homing systems.”

“Honey bee losses continue to be heavy on an annual basis in the US and in many other countries around the world, and there are also well-documented losses in bumblebee diversity in the US and in Europe,” said Ms Stoner, Tuesday, April 3. “There are now many more researchers looking at possible factors in those losses than there were in 2007. Our research at the Connecticut Agricultural Experiment Station focuses mainly on movement of the pesticides and possible routes of exposure. Other researchers, including those in the UK working on honeybees and bumblebees, focus on effects on behavior, colony health, and reproduction. Both lines of research lend support to the hypothesis that pesticides are among the factors affecting the health of honeybees and bumblebees, but there are other important factors, particularly pathogens causing bee diseases, that could be important.”

Fortunately for Connecticut beekeepers, a classic case of CCD has not yet been identified in the state, said Ms Stoner. However, she added, there are beekeepers reporting substantial losses of colonies.

Insecticide poisoning is not the only theory that has been put forth as to why beehives are suffering heavy losses each year. Scientists and beekeepers have considered the loss of foraging grounds, exposure to pathogens, viruses and parasites, single crop farming, contaminated feed supplements, or the intermingling of bees trucked by commercial beekeepers as factors contributing to CCD.

In October 2010, a year that was one of the worst since 2006 for commercial beekeepers, a study supported by the California Beekeepers Association, the Montana Agricultural Experiment Station, and the US Army Medical Research and Material Command indicated that co-infection with invertebrate iridescent virus (IIV) and Nosema, interacting with mites, might be the source of CCD.

According to that study, “Co-occurrence of these microbes consistently marked CCD in (1) bees from commercial apiaries sampled across the US in 2006-2007, (2) bees sequentially sampled as the disorder progressed in an observation hive colony in 2008, and (3) bees from a recurrence of CCD in Florida in 2009. The pathogen pairing was not observed in samples from colonies with no history of CCD…”

Area backyard beekeeper Dick Marron, who has intermittently suffered hive losses, noted in 2007 that beekeeping is as much an art as a science. He wondered if it was not a combination of many things, from weather conditions to verroa mite infestations, to pesticide exposure that were all working together to cause the disorder. Queen bee problems are always an issue for the home hobbyist beekeeper, as well, noted Mr Marron at the time.