Colony collapse: are the bees getting lost?

As honeybee colonies in the United States and Europe continue to suffer from a mysterious syndrome called colony collapse disorder (CCD), scientists are scrambling for answers. Another answer arrived this week, with a publication¹ that implicates a widely used insecticide.

CCD endangers many crops, but none more than almonds, which are pollinated by bees in more than a million hives trucked to California during the flowering season. Trucking stresses the bees, and stress is one of several likely contributors to the collapse syndrome.

Indeed, CCD could be several conditions lumped under one name, but here’s the trademark: The bees die away from the hive, obscuring the cause or causes of the collapse.

In the new study, scientists in France glued radio frequency identification tags to bees. Half were fed non-lethal doses of thiamethoxam, a common insecticide, then all the bees were released 1 kilometer from the hive. At the hive, the scientists used a radio-frequency gizmo to count how many flew home.

The tags did not affect the results, says Mickaël Henry, a researcher at the French National Institute for Agricultural Research, in Avignon. “Previous studies have shown that they do not impair movement or behavior of bees, or their time budgets for foraging activity.”

In any case, the control bees also sported tags.

What’s wrong?

How did the insecticide reduce the return rate so significantly? Most likely by causing difficulties with orientation, or locomotor activity, or both, Henry says.

When the experiment was repeated over a distance of just 70 meters, 92 percent of exposed and 98 percent of control bees returned, so both sets of bees were able to fly. The major impairment of exposed bees on the unfamiliar, longer route suggests that the insecticide was most damaging to the ability to learn a new route.

The neonicotinoid insecticides, the category that includes thiamethoxam, trigger nicotinic acetylcholine receptors, which are normally excited by a signal from a neurotransmitter. According to the new study, “Effects of sublethal neonicotinoid exposures in honey bees may include abnormal foraging activity, reduced olfactory memory and learning performance, and possibly impaired orientation.”

These insecticides make bees stupid, in other words.

The experiment was designed to count how many bees failed to return rather than pinpoint the reasons for that failure, Henry stresses. “The next step is to go into deeper detail about the behavior, with time-activity budgets, and looking at their foraging.”

Not the whole story

“This is a nice study, and it does clarify something that a lot of people have pointed to in the disappearance of bees,” says Phil Pellitteri, a faculty associate in entomology at the University of Wisconsin-Madison. “Insecticides have been known to cause bees to get lost, that’s one symptom of collapse. But colony collapse is a complex thing, and you can’t hang it all on one factor.”

Honeybees have long been attacked by viruses, protozoans and mites, Pellitteri says, and pesticides may decrease immunity, thus increasing susceptibility to pathogens. These, combined with the stress of long-distance travel and the scarcity of natural foraging grounds “are the general direction a lot of CCD research is pointing to. It’s a number of things, and their interactions.”

Henry and colleagues fed their data on return rates into a mathematical model, which predicted a perilous slide in colony populations. “The disappearances we observed may cause the colony to reach a population size low enough to be sensitive to other stressors,” he says. “Most bees are exposed to pesticides, and this confirms that exposure can put the colony at risk of collapse; this is the take-home message.”