Up to 40% of invertebrate pollinators, particularly bees and butterflies, are facing extinction, according to UN estimates. The list of threats is large but mainly includes climate change, habitat decline and the use of pesticides in agriculture.

Now, researchers have found a new way through which pesticides are affecting bees: by hurting the brains of baby bees.

In the new study, researchers at the Imperial College of London explain that pesticides can also disturb the brain of baby bees, which suffer the effects of food contaminated with pesticides brought by worker bees in the colony.

“Bee colonies act like superorganisms, so when toxins enter the colony, they have the potential to cause problems with the development of bees,” Richard Gill, author of the study, told CNN. “When young bees feed on food contaminated with pesticides it leads to less growth of parts of the brain, a permanent and irreversible effect.

To do the experiment, the researchers enriched the nectar obtained by bees with a class of pesticides called neonicotinoids in a concentration similar to that found in wildflowers. Then, they used that contaminated nectar in a bee colony that was set up in a laboratory.

The team waited for the bees to become adults and then tested their learning skills, first after three days and then after 12 days. Then, they compared the results with bees from colonies that hadn’t been fed with pesticides and that were fed with pesticides.

The learning ability of the bees that had been fed with nectar enriched with pesticides was significantly impaired, the results showed. The researchers did tests to see if the bees could associate a smell with a food reward, looking at the number of times out of 10 each did the task correctly.

“There has been growing evidence that pesticides can build up inside bee colonies. Our study reveals the risks to individuals being reared in such an environment, and that a colony’s future workforce can be affected weeks after they are first exposed,” Dylan Smith, also with the Department of Life Sciences at Imperial, told CNN.

The study also involved scanning the brains of up to 100 bees from different colonies, using a micro-CT scanning technology. The results showed that the bees exposed to the pesticides had a smaller volume of an important part of the insect brain, known as the mushroom body — a structure in insects brain known to be associated with olfactory learning and memory, among others.

The neonicotinoids used in the study are a type of pesticide very common in agriculture but with a clear negative effect on the health of bees, causing the death of whole swarms. For years, beekeepers have been warning over their effect, pushing for stronger regulations.

Fewer bees in the world can lead to the loss of biodiversity and even affect our food supply, as bees pollinate a large number of plants. More than 75% of the world’s food crops are estimated to depend to some extent on pollination by bees.

A European Union moratorium has restricted the application of three neonicotinoids to crops since 2013. Nevertheless, a study published last year showed residues of these insecticides can still be detected in rape nectar from 48% of the plots of studied fields in the EU.

Meanwhile, in the US the Environmental Protection Agency (EPA) banned last year 12 products containing neonicotinoid, leaving 47 neonicotinoid-based products on the market. In 2017, beekeepers in the US reported losing about 40% of their hives, a trend that has continued since then.

The study was published in Proceedings of the Royal Society.