The controversial herbicide glyphosate not only kills plants, but also impairs the cognitive abilities of insects, a study with bumblebees confirms. Under the influence of glyphosate, the bumblebees’ ability to learn was drastically impaired and they were less able to perceive UV light. These pesticide effects could significantly reduce the chances of the bumblebees and their colonies surviving.
The broad-spectrum herbicide glyphosate is the most commonly used herbicide in the world. However, it not only kills unwanted plants, but also has negative effects on humans and animals. For example, it has already been shown that glyphosate disrupts the embryonic development of amphibians and impairs memory and vision in bees, among other things. Nevertheless, plant toxins have so far only been considered harmful to animals if they actually lead directly to their death instead of causing it indirectly.
Electric shocks in blue and green
“In view of the global insect mortality rate, which is progressing at an alarming rate, we need to examine the influence of the pesticides used in agriculture more closely and not just look at the mortality rates, as we have done up to now,” explains Morgane Nouvian from the University of Konstanz. Together with her colleagues, she researched how glyphosate affects bumblebees. It was previously known that the plant toxin impairs the temperature regulation of the fluffy nectar collectors, which means that they can no longer keep their brood sufficiently warm. Nouvian and her team have now investigated for the first time whether glyphosate also has an impact on the locomotion, UV light perception and learning ability of bumblebees.
To do this, they fed bumble bee workers, who are normally responsible for collecting nectar in the wild, a mixture of sugar water and glyphosate every day and then put them through various experiments. These took place in Y-shaped boxes, in which the bees had to decide whether to choose the left or right Y-arm on their way. In the experiment designed to test their learning abilities, one arm glowed green and the other blue. The highlight: one color was linked to electric shocks, the other safe. A healthy, adaptive bumblebee whose group blue stands for electric shocks should understand the connection between color and pain fairly quickly and opt for the green side instead. However, if the glyphosate affected their ability to learn,
“They don’t study at all anymore”
Indeed, those bumblebees exposed to glyphosate chose more or less randomly between blue and green, regardless of which side they were in danger of getting an electric shock on. Unlike the glyphosate-free control group, they had apparently not recognized the connection between color and pain even after several runs, according to the researchers. “As far as we can tell, they’re not learning at all anymore,” reports Nouvian. This is critical, because such a pronounced learning disability can quickly end fatally in nature. The researcher explains that only if a bumblebee can link certain signals with negative consequences is it also able to specifically avoid deadly predators, toxins or parasites. As a result of the learning impairment, workers could die more frequently while collecting nectar,
But Nouvian and her team saw another worrying consequence of glyphosate. In a variation of the Y-box experiment, the bees had to choose between green and UV light and blue and UV light. There was no threat of an electric shock anywhere. The researchers just wanted to see whether the bumblebees are most attracted to UV light, as they are in nature. This usually helps them find suitable buds and navigate in low light conditions. But apparently the glyphosate had also affected the bees’ UV perception, because unlike their glyphosate-free counterparts, their choice between ultraviolet and blue wasn’t quite as clear-cut as it should be.
The only ability that the glyphosate hadn’t affected was the bumblebee’s locomotion. Only at the beginning of the test series were they a little slower, but then they ran at a normal pace. Nouvian and her team recommend using their Y-shaped test setup for other insects and pesticides in the future. In this way, it would be possible to systematically study the non-lethal effects on insects of using different chemicals in agriculture.
Source: University of Konstanz; Technical article: Science of the Total Environment, d oi: 10.1016/j.scitotenv.2023.165527