Bumblebee and honeybee populations have been declining quickly over the past couple of decades. This event has been dubbed the Colony Collapse Disorder, or CCD. Beekeepers and researchers have been baffled, unable to determine what causes this sudden declination.

But just last week, researchers at the University of Stirling, UK and the French National Institute for Agricultural Research (INRA) conducted two separate studies that point to the culprit: commonly used pesticides, which have been proven to damage the bees’ central nervous systems and homing abilities (learning and memory skills for remember the paths to the plants) and affects the birth of queens for colony growth.

The decline is believed to have been caused specifically by neonitinoid insecticides (which is a class of chemicals), which have been used since the early 90s. The bees pick up the pesticide along with the pollen and transfer it to other plants and bring it home to their hives, making other bees susceptible to the poison.

“Some bumblebee species have declined hugely,” Dave Goulson of the University of Stirling in Stirling, U.K. says in the  AAAS news release. “For example in North America, several bumblebee species which used to be common have more or less disappeared from the entire continent. In the U.K., three species have gone extinct.”

The study at Stirling University worked with bumblebees, a couple colonies of which were exposed to low levels (low enough to not be fatal) of the chemical imidacloprid – which is found in many brands such as Gaucho, Conguard, and Xytect – while a couple more were not. The researchers weighed all of the hives separately to determine how much each would grow by the end of the experiment.

They placed the bumblebees in an enclosed space outdoors for six weeks so they could go about their business in natural conditions. It turned out the exposed colonies gained less weight. The hives were 8-12% smaller. The population decreased as wells the amount of honey/nectar. Also, these hives contained 85% fewer queens (2 queens compared to 14 produced by unexposed colonies) in comparison to the unexposed hives.

The researchers in Avignon, France worked with honeybees. They gave one colony nonlethal doses of thiamethoxam, another chemical found in pesticides. Next, they tagged honeybees with radio-frequency identification microchips to track them. Following the experiment, the researchers found out that the colonies with the infected bees were 2 to 3 times more likely to die than nonexposed colonies. The pesticides were believed to have greatly damaged the bees’ homing systems, reducing the amount of nectar brought the colonies.

The INRA researchers conducted a second study in which they produced a mathematical model of the dynamics of honeybee populations. When the failure of the homing abilities was incorporated, the model predicted honeybee colonies would be impossible to recover at a certain point.

In addition to the honeybees and bumblebees, all other kinds of bees pollinate major vegetable and fruit crops as well as flowers. Because of their waning populations, farms would face dilemmas for the lack of pollination, which would instigate a decrease in the development of crops.

“There are obviously big question marks as to whether the safety testing that was done on these was really adequate,” Goulson adds.

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