Bumblebees (apparently fish, if you’re in California) are at risk from the world’s most widely used herbicide. The weedkiller glyphosate, it turns out, can impede a colony’s ability to maintain the right temperature, making it harder for the bees to incubate their larvae.

A new study, published in the journal Science, is the first to identify the risks of the pesticide in wild bees – vital pollinators on which food systems and habitats rely.

Glyphosate was once thought to be harmless to bees. While it may not be directly lethal, studies are now showing it can have other damaging effects. In honeybees – for example, it disrupts their gut microbiome, making them more vulnerable to infection.

For bumblebees (Bombus terrestris), it seems to result in a failure to regulate the temperature in colonies running low on food. It’s far more serious than just a couple of frozen bees – issues regulating temperature can have dire consequences for populations.

A temperature drop can impact bumblebee larvae incubation, for which there is a very tight temperature window. Larvae develop best at 28-35 °C (82-95 °F). At 25 °C (77 °F), survival is massively reduced. Any lower than this, and there won’t be any baby bees. To maintain these temperatures, bumblebees collectively “shiver” (vibrate their wing muscles) to warm things up, and fan their wings to cool things down. 

This thermoregulation is negatively affected by glyphosate herbicides, according to the new study. On an individual level the effects were weak, but for whole colonies, the ability to maintain high enough temperatures while food was limited was reduced by more than 25 percent.

To come to this conclusion, the researchers studied 15 bumblebee colonies in a lab. Half were fed sugar water, while the other half also received 5 mg/liter of glyphosate.

Individually, bees fed glyphosate were a bit less invested in incubating larvae, but the results weren’t statistically significant. On a colony level, however, it was a different story. When scientists limited their food resources, temperature regulation began to suffer.

“When colonies were undisturbed and well-fed, no difference in mean nest temperature between the two sides of a colony was detected,” the authors write.

“However, when colonies experienced resource limitation, effects of glyphosate exposure became evident.”

Temperatures dropped much more quickly in colonies exposed to glyphosate, and the majority of the time they dropped to 28 °C (82.4 °F) before the control colonies did. On average, the glyphosate treated colonies maintained temperatures above this for 26 percent less time than the controls.

“That can have massive effects on colony growth,” study lead Dr Anja Weidenmüller at the University of Konstanz, Germany, told The Guardian.

Bad news for bumblebees, who already face declining populations due to habitat loss, disease, and the effects of climate change – and also bad news for the rest of us.

“Bumblebees are a vitally important group of pollinators [and] the new findings are especially important given the widespread global use of glyphosate,” Prof James Crall, at the University of Wisconsin–Madison, US, also told The Guardian.

More than a third of the world’s food crops rely on animal pollinators – including, and especially, bees. Without them, or just with fewer of them, the global food system will suffer. So perhaps it might be time to rethink our herbicide, or at least glyphosate, usage and regulation.