The sight of dead or dying bees under some flowering lime (linden) trees, particularly under Silver Lime, Tilia tomentosa, has been reported for more than a hundred years.

Some studies have shown caffeine to be present in nectar, and while in itself it may not be poisoning the bees, Kew scientists think it may be affecting the bees’ judgement.

Two earlier studies have shown that caffeine can enhance the memory of bees for floral traits such as odours that are associated with food. Bees may be tricked into overvaluing a caffeinated food source, and will keep returning to it even if it is inferior to other food sources, or completely depleted.

Lime trees are noted for their far reaching, sweet scent. The teams at Kew think that it may be this odour together with caffeine in the flowers that results in a fatal attraction of the bees to the point where they run out of energy and starve.

The earliest proposed explanations for the significant death rate around lime trees were nectar toxins. However, more recent research suggests there is nothing in the nectar that is poisoning the bees.

Nectar and pollen are the main sources of protein, sugars and fats for bees, but these rewards that plants offer in return for the bee’s pollination service may contain other plant chemicals, some of which may be bioactive or toxic. Some of these substances may harm bees while others may be beneficial. Toxins in nectar can help plants to filter out bees and other pollinators that do not do a good job in pollinating their flowers and restrict visits to genuine pollinators.

For example, the nectar of monkshood (Aconitum) contains toxic alkaloids. Kew has shown that these toxins deter the buff-tailed bumblebee from robbing nectar from the flowers without pollinating, but the garden bumblebee, a legitimate pollinator, can tolerate higher concentrations of these substances and continues to visit and pollinate. Similarly, toxins in the nectar of Rhododendron species can deter or even kill honeybees, while bumblebees, who may be the preferred pollinators of rhododendrons, are not affected.