The challenge to let people walk back and forth in a straight line isn't just used by police to test if drivers are drunk: it's also used by neurologists to diagnose neurological disorders. Researchers used an insect version of the challenge to show that pesticides damage honeybees' nervous system and make it hard for them to walk in a straight line. The results are in a journal.
We show that honeybees can be adversely affected by the use of commonly used pesticides. Dr. Rachel H Parkinson is a scientist at the University of Oxford.
The results add to what the Food and Agriculture Organization of the United Nations and the World Health Organization have called the "rapidly growing body of evidence."
The motor response keeps insects together.
When insects threaten to steer off-course, they have an "optomotor response" which allows them to orient themselves back onto a straight path. Parkinson and colleagues challenged the opto motor response of walking honeybees to respond accurately and timely to videos of vertical bars that moved from left to right across two screens in front of them. The bee assumes that she has been blown off-course and needs to perform a corrective turn to get back to her normal course. The bee's motor system will be instructed to orient back to an illusory straight line after a healthy opto motor response.
The researchers compared the efficiency of the optomotor response between four groups of wild-caught forager honeybees, with between 22 and 28 bees tested per group.
Exposure to pesticides results in a worse motor response.
When the bars were narrow or moving slowly, the bees were less good at responding to the simulation. The bees who had taken the pesticides did not perform as well as the control bees. They turned in one direction only and didn't respond to changes in the direction of the bars. The asymmetry between left and right turns was more pronounced for pesticide exposed bees.
There is a small amount of brain damage.
The researchers found that bees exposed to pesticides had higher proportions of dead cells in parts of the brain that process visual input. Key genes for detoxification were altered after being exposed. The changes were weak and variable, and unlikely to be the sole explanation for the strong impairment of the opto motor response.
It's not always possible to recycle neonicotinoid and sulfoximine quickly enough to prevent toxicity. Parkinson said that the effects could be due to a type of rewiring in the brain.
Parkinson says that to fully understand the risk of these insecticides to bees, we need to explore whether the effects we observed in walking bees also occur in flying bees. If bees are unable to overcome any impairment while flying there could be negative effects on their ability to pollinate flowers and crops.
More information: Rachel H. Parkinson et al, Honeybee optomotor behaviour is impaired by chronic exposure to insecticides, Frontiers in Insect Science (2022). DOI: 10.3389/finsc.2022.936826
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