(ORDO NEWS) — A globally used herbicide makes it harder for bumble bees living in hives to retain enough heat to incubate larvae, according to new research.
Bumblebees (Bombus terrestris) are facing food shortages due to habitat loss and ubiquitous crop monocultures. Like honey bees, they feed on nectar collected from plants and accumulate it in the nest. They also collect nectar and pollen to feed their chicks.
Bumblebees are unique in their ability to maintain a collective “thermostat” to keep warm where other bees cannot; they do this by regulating their own body temperature and the warmth of the colony with “shivering”.
This makes them important pollinators in cooler areas, and also necessary for the development of larvae, which can only reach adulthood if their brood is kept between 25 and 35°C.
When food is scarce, the colony cools off and larval development may suffer. But a new study published in the journal Science has found that resource depletion isn’t the only thing disrupting bee incubation.
Glyphosate is used by farmers and gardeners to kill weeds and regulate crops. This chemical inhibits an enzyme found only in plants, fungi, and some bacteria, so it was long thought to be harmless to bees.
However, this study is the latest of many recent reports of non-lethal – but undoubtedly harmful – effects of glyphosate on bees.
To get a clear picture of how this chemical affects bumblebees, researchers at the University of Konstanz kept 15 bumblebee colonies in the lab.
Each colony was divided into two parts by wire mesh, with an equal number of worker bees on each side. On one side, workers were given regular sugar water and pollen. The other side was fed the same, only 5 mg/l of glyphosate was added to sugar water.
Workers could see and touch each other through the mesh, but because bumblebees don’t exchange liquid food like honey bees do, cross-contamination was not a concern.
To avoid bias, the researchers feeding the bees were not told which side of the colony the herbicide liquid was given until all the data had been collected.
First of all, they wanted to find out whether exposure to glyphosate would affect individual bees.
They isolated workers on both sides of each colony and gave each bee a “brood doll”: an imitation larva covered in wax from the previous brood, which the bees care for like a real one.
Regardless of whether the bees were fed glyphosate or regular sugar water, they looked after their doll, and although individual bees exposed to the herbicide were slightly slower in their incubation duties, the results of this experiment were statistically weak.
But of course, as social animals, bees need to observe the colony to see the full effects of any stress. Therefore, the scientists continued to study the thermal capacity “at the colony level” – this is where they found significant differences.
On each side of the colony, the researchers recorded temperature data in two brood sections: one with pupae and one with larvae.
Thirty days after the colonies were separated and half of each was fed a diet of glyphosate-tinted sugar water, the scientists limited food resources and began measuring changes in brood temperature on both sides of the nests.
“When colonies were undisturbed and well fed,” the authors write, “no difference was found in mean nest temperature between the two sides of the colony.”
“However, when the colonies experienced resource constraints, the effects of glyphosate exposure became apparent.”
When food supplies were reduced, non-glyphosate-exposed nests cooled, but not below the optimal range for larval development.
But on the other side of the grid, where the same resource constraint was associated with exposure to glyphosate, temperatures dropped much faster, eventually dropping below the optimum range for growing young bumblebees.
In the wild, this phenomenon may reduce breeding rates during periods of food shortage and contribute to further declines in bumblebee populations worldwide.
Because bumblebees are important pollinators and are considered surrogates in laboratory studies for how other wild bee species might suffer, the results of this study are both enlightening and alarming.
It is not yet clear why exactly glyphosate affected bumblebees, but based on previous research, scientists suggest that it may be due to the effects of glyphosate on the bee microbiome.
Regardless of what underlies the chemical attack, the study raises concerns about the “subtle, non-lethal” effects of a herbicide previously thought to be harmless.
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