Metallic cocktails are giving honeybees a hangover - and they may not recover from it

By: Chris MacLeod

Who doesn’t love a good cocktail? Delicious mixtures of flavours and ingredients, working together to compose a tasty concoction. Unfortunately for honeybees, a different kind of cocktail may ultimately lead to their downfall. For decades, metal pollutants - traces of metals like copper and arsenic - have contaminated our environment from agricultural practices. While pesticides are often in the hotseat for hurting wildlife, metals have slowly, but surely, begun to demonstrate their ability to harm wildlife and humans. Studies have shown that exposure to high quantities of these metals impact brain functions like memory and learning in many animals, including humans and mice. Many of these studies have looked at the effects of single metals; unfortunately metals frequently work together, forming a destructive mixture that increases the overall impact on an animal. Looking at the combined effects of these metals paints a more environmentally-accurate picture of what these animals face, however few studies have shown their effects on the honeybee, an important pollinator and honey-maker. With little knowledge on how these metallic cocktails impact our buzzing little friends’ brains, a team of 5 scientists led by Coline Monchanin teamed up in 2021 to investigate just how lousy a bee hangover can be.

Monchanin and her team first investigated how exposure to metals impacted the learning ability of honeybees. They collected 381 bees and divided them into 9 groups: 1 that was exposed to no metals, and 8 that were exposed to copper, arsenic, lead, or combinations of them. After introducing the bees to metals, the researchers introduced them to a smell they would likely encounter in the wild, while simultaneously feeding them. Afterwards, they exposed the bees to the same smell, but offered no food. This was repeated 4 times, and any bees that responded to the smell without food were considered “learners”, as they had learned to associate food with the smell. The team discovered that “hungover bees” (those exposed to metals) learned the smell less frequently than those not exposed to metals. In fact, the fewest number of learners was observed in the group exposed to a combination of all 3 metals. This lack of learning in the lab, they reasoned, reflects the inability of honeybees to learn the best flowers for food in areas where metal pollutants are abundant.

Any bee that proved itself as a learner was then subjected to a memory test. Similar to the learning test, they gave bees food while exposed to the smell, and then offered no food when exposed to the smell a second time. In this test though, the team waited 24 hours after the smell’s introduction to hit the bees with it again. The team also introduced 2 other smells after the 24 hours had passed, in a random order, to discover if bees could distinguish between different scents. Again, hungover bees did not respond to smells as frequently as those not exposed to metals. When they did respond, it was often to the incorrect scent, indicating a lack in the ability to distinguish between smells. Thus, exposure to metals may hinder the ability of bees to remember a smell; honeybees use smell to locate food, so memory loss could easily translate into losing a meal, and potentially even death.

This study sets a foundation for further research and conservation strategies for different pollinators, while providing insight into the current issue. The findings demonstrate losses in memory and learning capabilities in honeybees exposed to environmentally realistic amounts of metals, particularly a cocktail of metals, which is critical in light of honeybees using these abilities to find food. Reduction in brain function increases the chance of honeybee populations dying off; fewer honeybees mean less pollination, and also less honey! So, the next time you’re sipping a cocktail, don’t forget: humans are responsible for the metallic concoctions hurting honeybees. If they’re going to survive, they’ll need our help too.

Monchanin, C., Drujont, E., Devaud, J. M., Lihoreau, M., Barron, A. B. (2021) Metal pollutants have additive negative effects on honey bee cognition. Journal of Experimental Biology. 224, jeb241869.