Pesticides can Cause Brain Damage in Honeybees

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Modern pesticides cause brain damage in honeybees according to a new study published in the journal Frontiers in Insect Science .

Honeybees bees use something called “path integration” to find their way back to the hive. This is when an animal uses cues from its environment, like the sun to keep track of its location. By constantly updating this wide-field visual motion information, the bees are able to find their way back to the hive. A waggle dance then signals locations of rich food sources to the other bees.

Nowadays, countless insecticides and other agricultural chemicals claim to have concentration levels below the lethal dose for honeybees. There are numerous studies that report toxic effects of neurotoxic insecticides at sublethal quantities. However, this is the first study that examines the relationship between insecticide exposure and visually-guided behavior.

Sub-lethal Quantities of Modern Pesticides Cause Brain Damage in Honeybees

Researchers have discovered that even non-deadly quantities of seed treatment insecticides like neonicotinoids and sulfoxaflor have a detrimental effect on bees. Honeybees exposed to these pesticides experience a deficit in their navigating and homing ability. Researchers previously found that neonicotinoid insecticides disrupt visual motion detection in locusts, however this is the first time this behavior is proven in honeybees.

Two types of commonly used pesticides in particular, namely imidacloprid, a neonicotinoid and sulfoxaflor cause an impaired optomotor behavior in honeybees. Optomotor behavior is the instinctive movement of the eye, head or body that steadies the field of vision. The researchers believe that exposure to these insecticides causes brain damage and disrupts the honeybee’s ability to accurately encode wide-field visual motion. As a result the honeybees show a poor ability to manage responses or to keep them within an acceptable range of typical reactions while moving. In other words, they have problems to keep themselves on a straight path while moving. This impairment is accompanied by an abnormality or impairment in the regulation of detoxification genes.

How the Study was Conducted

For the purpose of testing the behavior of the honeybees, the researchers performed an examination similar to neurologists when checking patients for ataxia. Ataxia is a disorder where parts of the brain that coordinate movement are impaired. The simple test involves walking back and forth in a straight line, and is also used by police to test if drivers are intoxicated.
Insects can re-orientate themselves back onto a straight path when they are in danger to steer off-course while flying or walking. Parkinson and her colleagues divided honeybees into different groups of 22 to 28 bees per group. Over a period of 5 days, the bees were fed unlimited 1.5 molar sucrose solution. They received either pure or contaminated with 50 ppb (parts per billion) imidacloprid, 50 ppb sulfoxaflor, or a mixture of 25 ppb imidacloprid and 25 ppb sulfoxaflor.
Afterwards, the team tested the bees optomotor response. They tricked the bees to assume they had suddenly been blown off-course. The aim was for the bees to correct their turn and move back onto a straight path.


As a result, the bees who had ingested the pesticides performed poorly compared to control bees.
The leader of the study, Dr Rachael H Parkinson from Oxford University states,
“Here we show that commonly used insecticides like sulfoxaflor and the neonicotinoid imidacloprid can profoundly impair the visually guided behavior 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.”
According to the United Nations Food and Agriculture Organization and the World Health Organization the findings are part of a

 “rapidly growing body of evidence [which] strongly suggests that the existing levels of environmental contamination [from neonicotinoid pesticides] are causing large-scale adverse effects on bees and other beneficial insects”.

The researchers discovered that the bees exposed to the pesticides tended to have a higher count of dead cells in parts of the brain’s optic lobes. These lobes are vital for processing visual input. They also discovered an abnormality or impairment in key genes responsible for detoxification in the pesticide-exposed bees.

Parkinson explained concluding,

“Neonicotinoid and sulfoximine insecticides activate neurons in the insect brain and are not always recycled fast enough to prevent toxicity. The effects we observed 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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