Pesticides may cause abnormal brain growth in bumblebees

A foraging bumblebee
A foraging bumblebee.  Tara Cox / Imperial College London.

Pesticide contamination may be responsible for triggering abnormal brain development in baby bumblebees, according to the results of a new scientific paper. Researchers scanned the brains of 92 worker bees using micro-CT technology, and discovered that a region of the insect brain linked to learning had not grown correctly, potentially undermining a bee’s ability to serve the colony as an adult.

Bees are known to be an important contributor to the health of countless ecosystems in their role as a pollinator – helping plants breed, grow, and produce food. Sadly, many species are in decline, partly due to human actions, such as the spraying of crops with pesticides.

“Bee colonies act as super-organisms, so when any toxins enter the colony, these have the potential to cause problems with the development of the baby bees within it,” comments Dr. Richard Gill of the Department of Life Sciences at Imperial College London, and lead researcher of the new study. “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 researchers behind the study gave a number of bumblebee colonies a pesticide-contaminated nectar substitute, which was then fed on by the bee larvae. Three days after the insects emerged from their pupae, the researchers tested their learning ability by examining whether the young bees could learn to link a certain smell with the reward of food.

Their ability to learn was tested again 12 days after they had emerged, when they were considered to be adult bees. At this point the bees had been free of the pesticide contamination for nine days.

The team then used micro-computed tomography technology to scan the brains of 92 worker bees. Some of the bees were selected because they had been exposed to pesticides from a larval stage, while others were fed pesticides after they were considered adults. The final group was not exposed to the harmful substance at all.

Bees that were exposed to the pesticides were found to have an abnormal brain structure. A region of the insect brain called the “mushroom body” – which is linked with an individual’s capacity to learn – had a reduced volume compared to the other specimens. As with the behavioral testing, the CT scans were carried out three and 12 days after the bees left their pupae.

Those that were exposed to pesticides from the larval stage exhibited an impaired ability to learn in the two testing phases compared to the other bees. Furthermore, the larval group’s brains did not improve when they were scanned for the second time – when they had been free from the substance for a number of days. This suggests that the negative effects of the pesticides on brain formation may be permanent.

According to the researchers, the pesticide impairment could make them poor task performers as adult bees, potentially heightening the risk of colony collapse.

The authors of the study say that their work highlights the importance of imposing new guidelines on the use of pesticide.

The paper has been published in the journal Proceedings of the Royal Society B.

Source: Imperial College London

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