Manganese turns honeybees into bumbling foragers

A heavy metal pollutant muddles honeybee behavior. Just a small dose of manganese boosts certain brain chemicals and makes bees inept foragers, researchers report online March 25 in Biology Letters.

Scientists knew that the metal is toxic in high amounts. The results show that even low levels considered safe for people impact the pollinators, says coauthor Yehuda Ben-Shahar, a geneticist at Washington University in St. Louis.

Manganese is used in making steel, matchsticks and batteries. Pollution from these industries may allow manganese to accumulate in the flowers that bees pollinate. As the bees carry more and more nectar back to the colony, the trace metal can build up in the hive.

To see how the mineral may impact honeybees (Apis mellifera), Ben-Shahar and his colleagues fed the insects manganese over four days. The team tested the bees’ brains for dopamine, serotonin and octopamine — messenger molecules released by nerve cells that help control behavior.

Previous studies in flies, humans and other mammals found that high levels of manganese kill nerve cells, leading to fewer messenger molecules. But the low doses in the new study actually increased messenger molecules.

“The same metal can have two opposite effects,” says Ben-Shahar.

Manganese also sped up the bees’ transition from colony-bound workers to foragers. Getting a head start made the insects less competent foragers:  Tagged bees took longer food-gathering expeditions and made fewer trips than bees that weren’t fed the metal.

It is not yet clear why exposure to manganese makes honeybees poor foragers. The bees may be weaker than their peers, less capable navigators or impaired in some other way.