Bee learning ability slowed by pesticides, study finds

This image shows a bumble bee loaded with pollen. Image courtesy Wikimedia.

Bees exposed to pesticides might have a harder time learning how to pollinate flowers than those who are not exposed to the substances, a new study suggests.

The paper, which was published in the March 14 edition of Functional Ecology, is the first to show that neonicotinoids, a class of pesticides that is now commonly used to protect a variety of agricultural crops, may impact the ability of individual bees to forage for nectars and pollen. It examined the impact of a compound called thiamethoxam on bumblebees.

“What we found is that bees exposed to pesticide initially collected more pollen, but it took them more visits to flowers to properly ‘learn’ this behavior,” Dr. Dara Anne Stanley, the lead author of the paper and a researcher at the University of KwaZulu-Natal in South Africa, said. “Control bees collected less pollen, but learned the behavior properly after visits to less flowers. Therefore control bees may be investing more time in learning this behavior properly.”

That result is consistent with the findings of other recent studies. For example, a paper published last year showed that bumblebees exposed to pesticides suffer memory loss. Another paper published in 2015 demonstrated that bumblebee colonies that encounter neonicotinoids tend to forage for pollen less than those colonies that do not come into contact with the compounds.

Neonicotinoids appear to target the area of a bee’s brain that governs the insect’s behavior.

“Neonicotinoid pesticides in particular target a region of the insect brain called the nicotinic acetylcholinase receptors (nAChRs) where they target cholinergic signaling by inhibiting acetylcholinesterase,” Stanley said in an e-mail message. “As the nAChRs have been associated with behavior in insects, in particular learning and memory, this means these pesticides at sub-lethal doses have the potential to cause changes in insect behavior.”

The term cholinergic signaling refers to the movement of neurotransmitters in an organism’s nervous system. Those compounds carry information across synapses.

If the pesticide is used near a bee colony, then its impacts on the ability of individual bees to obtain pollen could negatively affect the whole colony.

“As bees in particular display a range of sophisticated behaviors involved in navigation, foraging etc, these pesticides have the potential to effect foraging success and ultimately colony health,” Stanley said.

That may be less of a worry for honeybees, which live in large colonies. Stanley explained that a recent study conducted in Sweden indicated that honeybees are affected by exposure to pesticide-treated crops less rapidly than bumble bees.

“Honeybee colonies are extremely large with thousands of workers, and therefore the effects of pesticides on individuals would have to be very strong to see any impacts on the whole colony, and if there are impacts it could take many years to become apparent,” she said. “However, bumblebee colonies are much smaller and therefore any impacts on individuals are likely to be much more influential on the colony as a whole. When it comes to solitary bees, each individual female builds her own nest, so they are likely to be even more susceptible to any behavioral effects on individuals.”

While sale of neonicotinoids are now worth about $2.6 billion per year to manufacturers, the substances are thought to be dangerous to many other organisms. A series of papers published in 2014 indicated that they “exhibit very high toxicity” to many invertebrates and that some bird and fish species are especially vulnerable to harm caused by the use of neonicotinoids.