Bees collect honeydew from bugs before spring blossoms arrive

Mould signals the sweet stuff

Paul Johnson, NPS

By Emily Benson

Need a sugar fix? When nectar is scarce, bees can tap into another source of sweet stuff: the droppings left behind by other insects.

This honeydew, a sugar-rich substance secreted by sap-sucking scale insects, may tide hungry bees over until spring flowers bloom.

Although we tend to think of bees as hive-living socialites, most bee species are solitary, with each female building a nest to protect her developing offspring. Adults emerge in the spring and live for just a few weeks, when they mate and gather pollen and nectar.

Fragrant, colourful flowers are like neon arrows pointing to those resources. But how wild bees survive if they mature before the blooms do was still largely a mystery, says Joan Meiners at the University of Florida in Gainesville.

Unlike colony-building honeybees, solitary bees don’t stockpile honey for times when blossoms are scarce. “There’s really not much that’s known about what bees do when there aren’t flowers,” Meiners says.

Mouldy clue

But fungus-covered bushes in central California’s Pinnacles National Park seemed to offer Meiners and her colleagues a clue.

Early in the spring, before flowers had started blooming, Meiners was surprised to see many solitary bees hovering around shrubs sporting sooty mould – a fungus that thrives on honeydew – while mostly ignoring mould-free plants.

To see if the bees were indeed seeking out the honeydew, her team sprayed non-mouldy shrubs with honeydew-mimicking sugar water or plain water.

There was one other possibility to check: the bees might have been going after the mould, perhaps as nest-building material. To test that idea, a quick-dissipating insecticide was applied to mouldy bushes to stop new honeydew production while leaving the fungus intact.

The team found that the sugar took the cake. More than 100 bees visited each group of sugar-sprayed shrubs — about ten times as many as stopped by plants misted with water alone — while only about 15 bees visited the shrubs treated with insecticide.

Crucial chow

The findings may indicate that honeydew is an important food for solitary bees, Meiners says, particularly as climate change begins to shift the timings of bee emergence and peak flower bloom.

But much remains uncertain. We still don’t know how the bees find the scentless and colourless honeydew. The team thinks that a bee may stumble across it when out looking for other stuff like water or minerals, recognise how useful it is and stay on the plant where it is found. Then other bees may notice that bee and investigate.

It is also unclear how much honeydew benefits bees, according to Jessica Forrest at the University of Ottawa in Canada. An interesting next step would be to see if the presence of honeydew translates into larger bee populations, she says.

And even where honeydew is plentiful, bees can’t do without flowers altogether, Forrest adds.

“They need pollen,” she says, as it is a crucial source of protein. “You can’t build a bee larva out of sugar water alone.”

Journal reference: bioRxiv, DOI: 10.1101/082271