Studies show that glyphosate can severely affect honey bees. Now biologists have tested the effect of the herbicide on ground bumblebees.

The controversial herbicide glyphosate could endanger the breeding success of bumblebees.

According to a German study, the herbicide can make ground bumblebees less able to maintain temperature in the nest when food is scarce. Without sufficient heat, the brood is in danger and with it the survival of the entire wild bee colony, the team writes in the journal Science.

In recent years, studies have repeatedly provided evidence of how glyphosate affects honey bees (Apis mellifera) – for example on cognitive abilities or the immune system. But little is known about the effects of the herbicide on the nearly 20,000 wild bee species.

The team led by the biologist Anja Weidenmüller from the University of Konstanz has now examined dark bumblebees (Bombus terrestris), one of the largest and most common bumblebee species in Germany. They set up 15 colonies of bumblebees in the lab, each divided into two halves by a net: the feed box of one half contained pure sugar water, while the other half had glyphosate spiked in the sugar water.

Glyphosate exposure is not directly fatal

As the group observed, glyphosate exposure did not directly kill the insects. However, these colonies were worse at maintaining nest thermoregulation when food supply was restricted. For optimal development of the brood, the temperatures in the nest must be between 28 and 35 degrees Celsius.

“Bumblebee colonies are under very high pressure to grow as quickly as possible in a short period of time,” Weidenmüller is quoted as saying in a statement from her university. If they cannot maintain the necessary incubation temperature, the brood develops more slowly or not at all. This restricts the growth of the colony: “Only when they reach a certain colony size in the relatively short growth phase are they able to produce the sexually mature individuals of a colony, i.e. queens and drones.”

The insects generate heat by contracting their flight muscles. This costs a lot of energy, which is why this time in particular is closely linked to the food supply. If this was restricted in the experiment, the ability of the bumblebees to thermoregulate dropped by 25 percent. “They can no longer keep their brood warm for so long,” summarizes Weidenmüller.

Heat regulation has been neglected so far

For the biologist Vincent Doublet from the University of Ulm, this is a significant result, because research has so far neglected heat regulation. “The study shows that small effects at the individual level can have major consequences for the entire colony,” says Doublet, who was not involved in the work.

How glyphosate achieves this effect is still unclear. A study with honey bees has shown that the herbicide changes their intestinal flora and makes them more susceptible to certain pathogens. “It stands to reason that glyphosate also affects the microbiome of bumblebees and, for example, makes it harder for them to utilize nutrients and thus become weaker,” speculates the biologist.

Since the weed killer impairs the cognitive abilities of honey bees, similar effects are also conceivable for bumblebees: “They simply might not notice that the temperature in the nest is falling.” Ultimately, different mechanisms could also interact.

Research new pesticides more closely before approving them

The study shows for Doublet that herbicides do not necessarily have to be directly deadly for insects to have dramatic consequences. So far, the approval of such agents has often been based on experiments with well-fed honey bees living under the best conditions. Complex interactions of different stress factors such as food supply, weather and pathogens would not be recorded in this way.

Lead author Weidenmüller emphasizes: “The combination of resource scarcity in cleared agricultural landscapes and pesticides can pose a massive problem for the reproduction of bee colonies.” New pesticides would have to be examined more closely before they could be approved. So far, it has only been checked how many animals have died within 24 or 48 hours after being fed or coming into contact with a substance: “Sublethal effects, i.e. effects on organisms that are not fatal but differ, for example, in the physiology or behavior of the animal Animals that make them noticeable can have significant negative effects and should be taken into account in the future approval of plant protection products.”