Common insecticides are causing brain damage in honey bees

Like intoxicated drivers battling to walk without stumbling, bees found to have been exposed to pesticides struggle to walk in a straight line.

Dr Tlou Masehela, in an article in South Africa, a heritage portal, says bees are important because they sustain populations of wild plants that support most biodiversity and ecosystem functions. Masehela is responsible for genetically modified organism (GMO) monitoring and research work at the SA National Biodiversity Institute. 

“Bees are the most important group of pollinators, visiting more than 90% of the leading 107 global crop types,” he said.

Research on the effects of pesticides on bees was published in Frontiers Insect Science, a peer-reviewed open-access scientific journal. It has shown that honeybee foragers exposed to the pesticides sulfoxaflor and imidacloprid have an impaired optomotor response.

Optomotor response is hereditary to insects and fish and the eyes are used to orientate the creature in a straight line while it moves.

This is according to a research article published on Wednesday titled Honeybee optomotor behaviour is impaired by chronic exposure to insecticides by researchers Rachel Parkinson from the University of Oxford, Caroline Fecher from Washington University and John R Gray from the University of Saskatchewan.

They found modern pesticides cause optomotor response impairment by damaging brain cells and dysregulating detoxification genes.

“These results add to the growing evidence that modern pesticides are highly damaging to beneficial insects like bees.”

The researchers used tests on bees, similar to the walk-and-turn test performed by police on suspected drunk drivers, to show for the first time that modern pesticides damage the nervous system of honeybees so it becomes difficult for them to walk in a straight line. 

Lead author Parkinson said: “Here we show that commonly used insecticides like sulfoxaflor and the neonicotinoid imidacloprid can profoundly impair the visually guided behaviour of honeybees.

“Our results are reason for concern because the ability of bees to respond appropriately to visual information is crucial for their flight and navigation, and thus their survival.”

The work adds to evidence provided by the UN Food and Agriculture Organization and the World Health Organization’s “rapidly growing body of evidence which strongly suggests the existing levels of environmental contamination [from neonicotinoid pesticides] are causing large-scale adverse effects on bees and other beneficial insects”.

According to the research paper, the optomotor response allows insects to orientate themselves back onto a straight trajectory if they are about to steer off course while walking or flying. 

The researchers challenged the optomotor response of walking honeybees to respond accurately and timely to videos of vertical bars that moved from left to right, or vice versa, across two screens in front of them.

“This ‘tricks’ the bee into assuming it has suddenly been blown off-course and needs to perform a corrective turn to return to a straight path. A healthy optomotor response will then instruct the bee’s motor system to orientate back to an illusory straight line midway between the optic flow from right and left.”

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. Each had been allowed to drink unlimited 1.5 molar sucrose solution over five days, either pure or contaminated with 50 ppb (parts per billion) imidacloprid, 50 ppb sulfoxaflor, or 25 ppb imidacloprid and 25 ppb sulfoxaflor simultaneously.

The findings were that all bees were worse at responding to the simulated optic flow when the bars were narrow or moving slowly, which makes them seem far away, than when they were wide or moving fast, which makes the bars seem close by.

“But for any width and speed, the bees who had ingested the pesticides performed poorly compared to control bees.

“For example, they turned quickly in one direction only and didn’t respond to changes in the direction of movement of the bars, or showed a lack of turning responses. The asymmetry between left and right turns was at least 2.4 times greater for pesticide-exposed bees than for control bees.”

They found bees exposed to pesticide tended to have elevated proportions of dead cells in parts of the brain’s optic lobes which are important for processing visual input.

“Likewise, key genes for detoxification were dysregulated after exposure.

“However these changes were relatively weak and highly variable across bees, and unlikely to be the sole explanation for the observed strong impairment of the optomotor response,” the research paper found.

Parkinson said neonicotinoid and sulfoximine insecticides “activate neurons in the insect brain and are not always recycled fast enough to prevent toxicity”.

She said the effects could be due to a type of rewiring in the brain to prevent neural damage by reducing the sensitivity of neurons to these compounds.

“To fully understand the risk of these insecticides to bees, we need to explore whether the effects we observed in walking bees occur in freely flying bees as well. The major concern is that if bees are unable to overcome any impairment while flying, there could be profound negative effects on their ability to forage, navigate, and pollinate wildflowers and crops.”

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