Researchers at Imperial College London studied how specific parts of bumblebee brains grew abnormally when exposed to pesticides during their larval phase. They used detailed micro-CT scanning technology to gain insights into bee brain development under the effects of pesticide exposure. Results were just published in Proceedings of the Royal Society B.

Previous studies mostly tested the effect of pesticide on adult bees since they forage on pesticide-contaminated nectar and pollen. These food supplies are brought back to the colony to feed the young, so it stands to reason that baby bees might also feel the effects of contaminated food. This makes them poorer at performing tasks when they are older.

Here's a 1:40 minute long video about the amazing brains bumblebees usually have:

Dr. Richard Gill from the Department of Life Sciences at Imperial College, lead researcher on the project, said, “bee colonies act as superorganisms, so when any toxins enter the colony, these have the potential to cause problems with the development of the baby bees within it.”

The study indicates that when young bees are fed pesticide-contaminated food, parts of their brains grow less, leading to these bees, when older, having smaller and functionally impaired brains. This appears to be irreversible and permanent and is therefore very worrying.

As these young bees become adults that cannot forage for food properly, their colonies can suffer the impact from pesticides weeks after exposure. These findings reveal the need for guidelines on pesticide exposure.

A colony was provided with nectar substitute, spiked with neonicotinoid pesticide, which is widely used across the globe. After emerging as adults from their pupae, the learning ability of these bees were tested after 3 days and after 12 days. Some underwent micro-CT brain scans at the Natural History Museum.

These results were compared to the young from colonies where no pesticides were consumed, and also to colonies where pesticides were only fed to them once they had emerged as adults.

Bees that were fed pesticides during their development as larvae exhibited significant impairment in learning ability when compared to those that were not.

Bees were tested to see if they could learn to associate a smell with a food reward. Researchers scored them on how often out of ten times each bee successfully performed the task.

Researchers found that bees exposed to pesticides had a smaller volume of an important part of the insect brain known as the mushroom body, after scanning the brains of nearly 100 bees from the different colonies. The mushroom body is involved in insect learning ability. Poor performance on the learning task correlated with smaller mushroom body volume, so this supports the suggestion that smaller mushroom body volume associated with pesticide exposure causes the bees’ poor learning performance.

When bees were exposed to pesticides while in their larval development stage, but not as adults, they still showed mushroom body volume reduction and similar learning impairment when tested at 3 days and 12 days old as adults. This implies that at least during the unexposed 9 days when they were adults, the larval exposure effects could not be overcome, indicating a potentially permanent effect.

According to lead author of the study, Dr. Dylan Smith from Imperial’s Department of Life Sciences, growing evidence exists that pesticides build up inside bee colonies. This study reveals the risks to individual bees raised in a contaminated environment and exposes that the colony’s future workforce can be affected even weeks after they are first exposed.

In future, not only the direct exposure of bees to pesticides by way of flower residue should be considered when analyzing potential harm to the colony. Another important measure of colony health is the amount of pesticide residue present inside colonies after exposure.

This team of researchers pioneered the use of micro-CT to scan bee brains. This study shows how the technology can test ecologically applied questions requiring the measurement of tiny but important differences in shape and size.

You can ready the study here.