The World's Most Popular Weed Killer Has a Previously Unknown Effect on Bumblebees

According to recent research, the world's most popular herbicide is making it more difficult for buff-tailed bumblebees to keep their hives warm enough to hatch larvae.

Due to habitat degradation and vast monocultures of agricultural crops, bumblebees (Bombus terrestris) are facing food shortages. They collect nectar from plants, just like honeybees, and store it in their nest. They also collect pollen and nectar to nourish their offspring.

Bumblebees are remarkable in that they can act as a collective 'thermostat,' keeping warm in places where other bees can't; they accomplish this by'shivering' to regulate their own body temperature and the colony's heat.

This makes them vital pollinators in colder climates, as well as necessary for growing larvae, who can only reach adulthood if their brood is maintained between 25 and 35 degrees Celsius.

When food is scarce, the colony cools down, and larval growth suffers as a result. However, according to a new research published in Science, resource depletion isn't the only factor affecting the bees' incubation.

Farmers and gardeners alike use glyphosate to control weeds and manage crops. Because the chemical inhibits an enzyme found only in plants, fungus, and a few bacteria, it was previously assumed to be safe for bees.

However, this is only the most recent of numerous recent research on glyphosate's nonlethal – but unquestionably detrimental – effects on bees.

The University of Konstanz housed 15 bumblebee colonies in a lab to acquire a good image of how this toxin affects them.

A wire mesh was used to split each colony into two halves, with equal numbers of worker bees on either side. Workers on one side were given regular sugar water and pollen. The sugar water on the opposite side was laced with 5 mg/L of glyphosate.

The workers could see and touch each other through the mesh, but cross-contamination was not an issue since bumblebees do not interchange liquid food as honeybees do.

To minimize bias, researchers feeding the bees were kept in the dark about which half of the colony was receiving the herbicide-laced syrup until all of the data was gathered.

To begin, they wanted to see if glyphosate exposure might alter individual bees.

They separated workers from opposite sides of each colony and gave each one a 'brood dummy,' which is a replica larva wrapped in wax from previous broods and cared for by the bees as if it were a genuine one.

The bees began tending to their dummy regardless of whether they had been fed glyphosate-laden or conventional sugar water, and while individual bees exposed to herbicide were marginally more slow in their incubation tasks, the findings of this experiment were statistically weak.

However, because bees are social animals, they must be watched as a colony to see the entire impact of any stressor. This is why the researchers investigated thermal ability "at the colony level," where they discovered considerable variances.

The researchers took temperature readings in two portions of a brood, one with pupae and the other with larvae, on each side of a colony.

The scientists limited their food supplies and began measuring variations in brood temperature on both sides of the nests thirty days after the colonies were separated and half of each fed on a diet of glyphosate-tainted sugar water.

"When colonies were undisturbed and well-fed,"the scientists reported, "no difference in mean nest temperature between the two sides of a colony was detected."

"However, when colonies experience resource limitation, effects of glyphosate exposure became evident." 

The nests that had not been treated to glyphosate cooled down when their food supply was decreased, but not below the optimal level for larval growth.

Temperatures decreased substantially quicker on the opposite side of the mesh, when the same resource constraint was combined with glyphosate exposure, finally plunging below the optimum range for producing young bumblebees.

This behavior in the wild might limit reproduction rates during times of food scarcity, contributing to the global loss of bumblebees.

Because bumblebees are vital pollinators – and are used as surrogates for how other wild bee species can be harmed in lab studies – the results of this study are both enlightening and frightening.

It's still unknown why glyphosate damaged the bumblebees that were seen, but based on past study, the scientists believe it's because of glyphosate's effects on the microbiota of the bees.

The study raises worries about the "subtle, nonlethal" consequences of a herbicide formerly assumed to be innocuous, regardless of the underlying chemical effects.