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The Bees and the Manganese

Even at levels considered safe for humans, excess manganese from steel manufacturing could interfere with honey bee foraging, and, ultimately, our food supply.
Honey bee on a dandelion. (Photo: Orangeaurochs/Flickr)

Honey bee on a dandelion. (Photo: Orangeaurochs/Flickr)

Like many metals, manganese is both essential to life, but potentially bad for your health. Long-term exposure to high levels of manganese can lead to neurological problems and a Parkinson's disease-like syndrome called manganism. Now, researchers report manganese could have an indirect effect on human health and economic well-being through its effects on honey bees.

Concerns about honey bees aren't new. Scientists have grown increasingly concerned over the years about colony collapse disorder. While it's not particularly well-understood, the disorder threatens the pollination of billions of dollars of crops, along with wildflowers, fruit trees, and other flora. Environmental pollutants—such as manganese—are among the potential causes of colony collapse or the general decline in pollinators worldwide. While manganese is naturally present in low levels nearly everywhere, it doesn't help that it's also released by iron and steel factories and coke ovens, and can have significant negative effects on humans and other vertebrates.

Less is known about its effects on insects such as honey bees, a team led by Washington University post-doctoral fellow Eirik Søvik notes in Biology Letters. To begin exploring those effects, Søvik and his colleagues from Washington University in St. Louis and Macquarie University in Sydney, Australia, fed bees sugar water laced with different levels of manganese. Using radio-frequency identification tags—the same sort of device used in proximity keys and subway passes—the team monitored the bees' hunt for food over the course of four days. Afterwards, they measured octopamine, dopamine, and serotonin levels in bees as an indicator of the bees' brain function.

Søvik found that increased manganese in the bees' diet increased octopamine, dopamine, and serotonin levels, even at relatively low levels.

In contrast with recent results in mammals, Søvik and his team found that increased manganese in the bees' diet increased octopamine, dopamine, and serotonin levels, even at relatively low levels, suggesting exposure "at levels that are considered safe for humans can still affect insect behaviour," the team writes.

RFID data revealed some particularly worrying behavioral effects. For one, at the highest manganese concentrations—2.7 grams per liter of water, a level five to 10 times what's considered safe for humans—left the hive to forage for food about six days after they hatched, roughly two days earlier than bees exposed to lower levels of manganese. Those bees also went on fewer trips for food, and the trips themselves lasted about five minutes longer, suggesting that they were having trouble navigating or were in poor health.

"[Our] data indicate that in addition to the increased environmental pressures from parasites, pathogens, insecticides, and modern agricultural practices on the health of pollinators, it is important to consider other potential anthropogenic factors such as heavy metal pollution as possible risk factors," the team writes. Better understanding of those risks could lead to better management of bees and other pollinators essential to our food supply, they write.

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