The herbicide glyphosate is controversial. For humans it should be carcinogenic, some say, others disagree. A new study shows how the herbicide affects insects and fuels the extinction of species. This also endangers the tomatoes on your plate.

Glyphosate kills weeds. It is the best-selling drug of its kind in the world. Since an investigation by the international cancer research agency, the herbicide has been criticized and classified as “probably carcinogenic”. Follow-up studies contradict a cancer risk for humans. However, a new study now shows that the use of glyphosate could be a previously unknown factor in the decline of important, social pollinating insects.

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The subject of the study, which was published in the journal Science on Thursday, was the widespread bumblebee ( Bombus terrestris ). Commercial bumblebee breeding is mainly used in greenhouse cultivation to pollinate tomatoes.

The research team led by Anja Weidenmüller from the University of Konstanz examined 15 bumblebee colonies. Each colony was divided, the number of workers balanced, and the respective queen moved between the two groups on a daily basis.

During the study period, part of the colony received pure sugar water, the other part received sugar water mixed with glyphosate at a concentration of five milligrams per liter.

The research team found that the bumblebees were significantly less able to maintain their temperature in their nest if they consumed sugar water laced with glyphosate and at the same time did not find enough food.

This thermoregulation of bumblebees is important for their survival. The brood of the bumblebee colonies can only develop optimally in a certain temperature range. In addition, bumblebees live in small colonies and these are only annuals, since only the queen overwinters. As a result, even the smallest changes for the growing brood have a massive impact on the colony’s chances of survival.

“Bumblebees only build up small reserves and are therefore dependent on a continuous supply of nectar and pollen,” explains Ulrich Ernst, research associate at the State Institute for Apiology at the University of Hohenheim. If glyphosate application first leads to reduced thermoregulation in a bee colony and then to a reduction in food supply, the bee colony would be doubly affected.

During the course of the study, it was found that the bumblebees from the glyphosate-treated groups tended to invest less time in incubation. If the colonies had enough food available, no difference in the average nest temperature could be seen. In the case of food restriction, on the other hand, there were clear differences: the colony sides fed with glyphosate-containing sugar water were only able to keep the mean nest temperature above 28 degrees Celsius three quarters of the time.

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“Glyphosate is demonstrably taken up by pollinating insects, preferably via flowering plants that grow on the fields before or after harvest, and via the flora in the vicinity of the treated areas,” says Randolf Menzel from the Institute of Biology at Freie Universität Berlin. Glyphosate is not only so dangerous for pollinating insects because it destroys important food resources, but also because it massively impairs their development and resistance to stress.

The effect of the herbicide is therefore not limited to the target organisms, i.e. weeds. The food webs and with them entire ecosystems are also changing through direct and indirect effects on other living beings. For honey bees, for example, it has already been demonstrated that glyphosate, despite its low acute toxicity, has negative effects through such indirect effects.

Experts emphasize that the high-quality study can be transferred to real agricultural conditions. Most agree that the results make an important contribution to the debate about the use of glyphosate.

Teja Tscharntke, head of the Department of Agroecology at the Georg-August-Universität Göttingen, classifies the results as further evidence “that not only insecticides, but also herbicides and fungicides can have a negative impact on the vitality, lifespan and reproduction of insects”.