How do bees and fungi work together?

Research into trophic levels has uncovered one of the world’s most important symbiotic relationships

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Common carder bumble bee (Bombus pascuorum) feeding on a flower in the Derbyshire countryside
Common carder bumble bee on a flower in the Derbyshire countryside Credit: Nature Picture Library

If you want a definition of a bee, “vegetarian wasp” is as good as any. Indeed “vegan wasp” is perhaps even better; wasps are all carnivorous, but their close relatives bees subsist entirely on nectar and pollen.

Or do they? Could it be that bees have been closet carnivores all along? Or at least, closet omnivores? To answer that (and please bear with me), it helps if we first consider the vegan attitude to mushrooms. Vegans advocate a plant-based diet, but many a vegan recipe contains mushrooms. In the vegan universe, mushrooms (and presumably fungi in general) seem to be honorary plants, even though they are more closely related to you and me than to cabbages and beans. This makes perfect sense; fungi may be unrelated to plants, but all the arguments for a plant-based diet apply equally to fungi. Not only that, if fungi were off the menu, avoiding the yeast in beer and wine could make life very dull.

Science takes a different view of your position in the food chain, simply counting how many steps you are away from primary producers such as plants, which are trophic level one. Herbivores are level two, carnivores are level three, and higher-level top predators are level four or five. So fungi (which “eat” plants, or plant remains) are trophic level two, which means that they are trophically equivalent to “meat” in the food chain, no different from caterpillars or sheep. Which means that eating fungi puts you at trophic level three. To complete the picture, there’s nothing to stop you occupying a fractional trophic level, so the mixed diet of the averagely omnivorous human comes out at around trophic level 2.5.

Bees incubate sealed cells that contain the pupae. We can also see empty cells and others filled with bee bread for raising the larvae Credit: Eric Tourneret / Photodisc

But what has any of this to do with bees? Well, researchers have been measuring the trophic position of bees, with an extremely clever technique that uses the way different isotopes of nitrogen are enriched or depleted with every step along the food chain (their results are published in the journal The American Naturalist). They found that bees come out at around trophic level 2.5, confirming that a large part of their diet is not plants, but consumers of plants. And what are these consumers? Microbes, mainly fungi but also a few bacteria.

What’s happening is that when bees collect pollen, they moisten it with nectar and regurgitated enzymes, digestive acids and microbes before storing it for later consumption by their larvae. In summer this mixture of sugar-rich nectar and protein-rich pollen is colonised and transformed by microbes. The fungi involved are far from random (they’re quite different from the fungi that live in the bee gut), and perform several beneficial functions: protecting the pollen from other, undesirable microbes, adding essential nutrients, and breaking down the tough “shell” of the pollen grains, allowing access to the protein-rich contents.

The result is that the raw pollen is converted into a microbe-rich larval food, known as “beebread”. The researchers measured the trophic position of the pollen mix fed to bee larvae and found it was about 1.5; in other words, about half the protein in the original pollen had been converted to fungal protein. This partnership between bees and fungi may be one of the world’s most important but least-appreciated symbioses. Microbes (not bees) may be the primary global consumers of pollen, and also the world’s most enthusiastic and ancient beekeepers.

Knowing all this, it’s hardly surprising that recent research has shown that fungicides have been linked to major colony declines in bumble bees. The recent emphasis has been on the bad things systemic insecticides may be doing to bees, but it’s clear that we also urgently need to know what fungicides are doing to the vital bee-microbe symbiosis.

  • Ken Thompson is a plant biologist with a keen interest in the science of gardening. His most recent book is Darwin’s Most Wonderful Plants. Visit books.telegraph.co.uk
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