Now, research from Imperial College London, UK, reveals that their effects extend to baby bumblebees (Bombus), through food brought back to the colony by the adults.
The results, published in the Proceedings of the Royal Society B, show that specific parts of the brain in bees who were exposed to pesticides grew abnormally during their larval stage, making them perform poorer at tasks later in life.
“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,” says lead researcher Richard Gill.
“Worryingly in this case, when young bees are fed on pesticide-contaminated food, this caused parts of the brain to grow less, leading to older adult bees possessing smaller and functionally impaired brains, an effect that appeared to be permanent and irreversible.”
The damage could impact their ability to forage for food.
For the experiment, the researchers provided one colony with a nectar substitute that was spiked with a class of widely used pesticides called neonicotinoids; the other colony was fed no pesticides.
Once the bees reached adulthood, the researchers tested whether each bee could learn to associate a smell with a food reward, scoring how many times out of ten each insect was successful.
The bees that were fed pesticides showed significantly impaired learning ability compared to those that weren’t.
Using detailed micro-CT scans, the researchers then scanned the brains of close to 100 bees and confirmed that those who had been exposed had a smaller volume of the mushroom body, an important part of the insect brain connected to learning ability.
The researchers note that this study highlights the need for exposure to be considered in pesticide usage guidelines.
“There has been growing evidence that pesticides can build up inside bee colonies,” says lead author Dylan Smith.
“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.
“Bees’ direct exposure to pesticides through residues on flowers should not be the only consideration when determining potential harm to the colony.