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Are lime trees killing our bees?

Lime trees are said to entice bees and leave them feeling dopey - www.alamy.com
Lime trees are said to entice bees and leave them feeling dopey - www.alamy.com

Is the nectar of lime-tree flowers toxic for bees? Plenty of people seem to think so. In his super book, A Sting in the Tale, Dave Goulson says: “Buff-tailed and white tailed bumblebees love the flowers of lime trees, although there is something in the nectar which seems to make them dopey and even sometimes kill them”.

Sometimes pollen gets the blame; in his book Scent in Your Garden, Stephen Lacey says: “Limes have extremely powerfully scented flowers, and they would be planted more often if… their pollen did not contain a narcotic element that stupefies bees”.

The lime most often claimed to have toxic nectar is the silver lime, Tilia tomentosa, about which the RHS website says: “Clusters of highly scented yellowish flowers are narcotic to bees”. Although several limes are on the RHS “perfect for pollinators” list, the silver lime is not, presumably on account of its alleged toxicity.

The belief that lime trees can harm bees has been around since at least the 16th century, so a couple of Kew botanists decided that it was time to review the evidence

The belief that lime trees can harm bees has been around since at least the 16th century, so a couple of Kew botanists decided that it was time to review the evidence. Their conclusions were published in the journal Biology Letters.

More than 50 years ago it was suggested that the sugar mannose, which is toxic to bees, was present in lime nectar, and this is still frequently cited as the problem. But recent analyses show that lime nectar does not contain mannose. In fact, the whole idea that there might be anything toxic in lime nectar is hard to sustain; if comatose bees found under lime trees are fed lime nectar, they quickly recover and fly away. So lime trees are not like, for example, Rhododendron ponticum, whose nectar really is toxic.

What are the other possibilities? Very occasionally, bees may be killed by a lime tree that has been sprayed with insecticides to control aphids. Predation is another possibility; in his book Goulson reports dead bees under lime trees that had been killed by great tits. But most of the time the dead or dying bees found under lime trees are uninjured, so there must be some other cause. Which really only leaves us with starvation.

Towards the end of the flowering season, lime flowers secrete little nectar, but bees continue to visit. Bees have been recorded returning “empty” from visiting lime trees, and dying bees found below lime trees have very low energy reserves. The starvation hypothesis would also explain why the dead bees found under lime trees are usually bumblebees rather than honeybees, since the latter have access to honey stores back at the hive.

Researchers are investigating the long-held idea that the flowers contain a narcotic element or harm certain pollinator species - Credit: Alamy
Researchers are investigating the long-held idea that the flowers contain a narcotic element or harm certain pollinator species Credit: Alamy

But why do bees continue to visit lime flowers when there is little or no nectar? Lime flowers emit the same volatile compounds that bumblebees use to recruit workers to a good food source. But these chemicals are produced by many other flowering plants, so although they may have evolved by plants to attract pollinators, this is far from unique to tilia. But lime nectar also contains caffeine, and recent research has shown that caffeine-laced nectar persuades bees to return to the same flowers, even if they offer little reward.

The relationship between bees and flowers may look like one of benign cooperation, but in reality each side is striving to get more than its fair share from the bargain. Using caffeine to trick bees into continuing to visit empty flowers may just be one of the weapons in this long-running struggle.

Ken Thompson is a plant biologist with a keen interest in the science of gardening. He writes and lectures extensively and has written five gardening books, including The Sceptical Gardener, a collection of his columns for The Telegraph. Visit books.telegraph.co.uk