We get billions of dollars worth of free labor each year from the bees that pollinate the crops we eat, but these hard workers started vanishing a few years ago. The sudden and massive decline in the number of bee colonies, now called Colony Collapse Disorder, still has no clear cause. Many potential causes have been suggested -- everything from pests to parasites to pesticides to electromagnetic radiation -- but so far the evidence hasn't been strong enough to definitively link any of these to Colony Collapse Disorder.
There have been studies suggesting that commonly used neonicotinoid pesticides, found on the flowers upon which they feed, might lead to adverse effects in bees. But determining how real-world doses of these compounds affect real-world bee behavior has been difficult to study in the lab.
Bees leave their colonies to forage for food in the form of nectar and pollen found on flowering plants, venturing up to one to two kilometers from their colonies. Meanwhile in the colony, the worker bees pamper their queen so she can perform the important task of laying thousands of eggs to constantly replenish the population of the colony. This well-orchestrated life of the bee requires that foraging bees find flowers, collect food and then successfully find their way back to the colony. Life at the colony likewise depends on the queen, without which the colony doesn’t stand a chance.
Two recent studies published in the journal Science last week looked at whether behaviors important for bees foraging in the field were influenced by low doses of commonly encountered pesticides, and found that indeed they were. Both studies fed bees realistic doses of these pesticides and then tested how well these pesticide-intoxicated bees performed at everyday tasks necessary for survival.
One of the studies found that colonies of bumblebees fed low doses of a widely used pesticide called imidacloprid gained less weight while foraging in the field. The weight measured included both the weight of bees -- surviving adult bees and developing bees that haven’t yet matured -- along with their food stores, which are both important markers of a healthy colony. There was little difference between the pesticide-fed colonies and unexposed control colonies during the first two weeks of the experiment while they remained in the cage, but over the remaining six weeks in the field large differences became apparent. Also important for colony survival, the pesticide-fed bee colonies produced far fewer queen bees.
A study by a separate group of researchers stuck radio-frequency identification (RFID) tags on the thorax of honeybees to test their homing abilities. They found that bees fed a low dose of a neonicotinoid pesticide called thiamexoxam were significantly less likely to return to the colony after foraging in the field than control bees that were not exposed to the pesticide. The difference was even more dramatic when they were released into an unfamiliar field to forage, meaning that such pesticides likely lead to even greater defects in the real world than previously appreciated.
While these studies still don’t support a causal role of pesticides alone leading to Colony Collapse Disorder, they do give us something to think about when deciding just how safe a pesticide needs to be. The regulatory agencies might be listening too. The European Food Safety Authority is trying to incorporate risks to bees into their evaluations of pesticide safety, and the U.S. Environmental Protection Agency is planning to schedule a scientific advisory panel for similar reasons. (Read more about the new studies and what they mean here).