The reasons behind the global decline of bees and other insect pollinators have been as mysterious as they've been controversial, but now we have the first evidence to suggest that commercially available insecticides are impairing the brain activity of individual bumblebees, and the performance of entire colonies.

The culprit? Neonicotinoids - a relatively new class of insecticide, developed by Shell and Bayor around 20 to 30 years ago, that are now considered the most widely used class of insecticides in the world. According to Elizabeth Grossman at the Yale Environment 360 website, in the US, neonicotinoids are used on about 95 percent of corn and canola crops, and are the most commonly used insecticide on cotton, sorghum, and sugar beets, and are used on about half of all soybean crops. The majority of fruit and vegetable crops in the US, including apples, cherries, peaches, oranges, berries, leafy greens, tomatoes, and potatoes, are also treated with the stuff. 

But the widespread use of neonicotinoids, despite serious questions regarding their safety around bumblebees and other insect pollinators, has not gone unnoticed. In 2013, following the results of a year-long study concluding that continued use would have "high acute risks" for bees, the European Commission placed a two-year moratorium on the use of neonicotinoid chemicals on all plants that attract bees. And in Australia, a definitive report on neonicotinoids and bees, led by Robert Manning from the Western Australian Department of Agriculture and Food, will be released in the next few months, the results of which are expected to influence local policies regarding neonicotinoid use.

And now, a team from the Universities of St Andrews and Dundee in Scotland has published a study in the Journal of the Federation of American Societies for Experimental Biology that shows for the first time that even the small amounts of neonicotinoid insecticide found in nectar and pollen is enough to affect the brain activity of the bumblebees that consume it. 

This small amount, the team says, is about 2.5 parts per billion - so around 1 teaspoon in an Olympic swimming pool. There has been much debate over whether this amount could actually be having any effect on the bees at all, so the researchers fed it to a number of bumblebees to see what happened. And because the chemical is designed to target the brains of insects, they tested its accumulation in the bees' brains.

The researchers found that some of the neonicotinoid chemicals would very quickly shut down mitochondria in the brain cells of the bees, which affects the brain's ability to produce energy. The chemicals also messed with the ability of the different regions to communicate with each other, which affected the bees' ability to learn crucial skills, such as remembering how to get back to the colony, and connecting the scent of a flower to the anticipation of a food reward.

Having tested the amount in individual bees, they then tested whole colonies, contaminating their sugar water with neonicotinoids. The colonies ended up forming smaller, and less robust nests than they normally would, which also contained less bees. The team concluded that even super-low levels of neonicotinoids saw an estimated 55 percent reduction in live bee numbers, a 71 percent reduction in healthy brood cells, and a 57 percent reduction in the total bee mass of a nest.

"Our research demonstrates beyond doubt that the level of neonicotinoids generally accepted as the average level present in the wild causes brain dysfunction and colonies to perform poorly when consumed by bumblebees," one of the team, Chris Connolly from the Division of Neuroscience at Dundee's School of Medicine, said in a press release. "In fact, our research showed that the ability to perturb brain cells can be found at one-fifth to one-tenth of the levels that people think are present in the wild."

Connolly adds that the results are not so surprising - these insecticides are designed to affect the brains of insects, so of course they'd mess with non-pest species too - but neonicotinoid use has been allowed because the chemicals don't actually kill bees. "This is not proof that neonicotinoids are solely responsible for the decline in insect pollinators, but a clear linear relationship is now established. We can now be confident that at these levels, neonicotinoids disrupt brain function, bee learning and the ability to forage for food and so limit colony growth," he said.

One of the reasons neonicotinoid use has been allowed to continue for so long, despite serious concerns regarding its effects on non-pest pollinators, is the difficulty in transitioning the global agricultural industry from its most commonly used insecticide to another viable alternative. Neonicotinoid is very good at what it does, otherwise farmers wouldn't be using it, so they expect the scientific evidence as to why they shouldn't, to be rock-solid. And small sample sizes are one of the main gripes.

"Field studies of the effects of neonicotinoids on bees are plagued by small sample sizes and 'pseudo-replication', in which data are analysed assuming that each colony is independent, even though multiple colonies are housed within a single box, and so experience a common environment," one of the team, Steve Buckland, professor of statistics at the University of St Andrews, said in the press release.

Buckland explains:

"Small sample size in field trials has been used as an excuse to not carry out formal analysis, and to draw a conclusion that there is no observable effect of neonicotinoids from visual inspection of the data. Despite the limited true replication, we found very strong evidence that low levels of neonicotinoids have adverse effects on bumblebee colonies, with an estimated 55 percent reduction in live bee numbers, 71 percent reduction in healthy brood cells, and 57 percent reduction in the total bee mass of a nest."

According to Grossman from Yale Environment 360, over the past decade, beekeepers around the world - primarily in the US and in Europe - have been reporting a loss of at least 30 percent of their hives each year, and in 2013, that number hit 40 to 50 percent for many American beekeepers. And we're talking about the insects that make an estimated $215 billion contribution to the global economy, while also supporting a good deal of the world's crop production.

So, what's the solution? It may not be necessary to ban the use of neonicotinoids altogether, says Connolly, but instead we should be looking at stricter regulations regarding its use. "It may be possible to help bees if more food (bee-friendly plants) were available to bees in the countryside and in our gardens," he said. "We suggest that the neonicotinoids are no longer used on any bee-friendly garden plants, or on land that is, or will be, used by crops visited by bees or other insect pollinators."

Sources:, Yale Environment 360