Do flowers use caffeine to attract more bees?

Many of us can’t imagine a day without a cup of coffee. It can brighten the mood and the caffeine it contains increases alertness and attention.

But coffee plants do not produce caffeine to help us get through the day. They and may other plants use its bitter taste to repel herbivores; and in high concentrations caffeine can even be toxic.

Surprisingly caffeine was also discovered in the nectar of many coffee plants, different citrus and linden trees and sainfoin. Doesn’t it deter pollinators? It appears not, and plants may actually use caffeine to induce bees to increase their services, possibly to the bees’ detriment.

Caffeine as a memory enhancer

Flower nectar contains much lower concentrations of caffeine than other plant parts. And it seems that at these low concentrations, caffeine is actually attractive to bees. Additionally, bees remember longer that a given scent is linked with sugar water if it also contains caffeine.

This should be profitable for plants – if bees can remember better that their scent (or any other characteristic) is associated with a reward, they will visit flowers of that species more often, increasing their reproductive success. Experiments with bumblebees showed that indeed artificial flowers containing nectar with caffeine in low concentrations received more artificial pollen.

But it seems that memory enhancement is only part of the story.

Tricked with caffeine

Another study suggested that flowers with caffeine might be perceived as more rewarding. Normally when honeybees find flowers with high sugar content, they visit these flowers more often. They also perform their waggle dance (which informs other colony members of the location of the good food source) more.

The study shows that adding a low dose of caffeine to sugar water had a similar activating effect on bees, even if the sugar concentration was the same as the other food source. This should lead to many more bees visiting caffeinated flowers. In addition, bees tend to return more persistently to the feeders that used to have caffeine, even once they contain no more sugar, and they were less likely to investigate other feeders. These effects of caffeine suggests that flowers containing it could potentially reduce their sugar content (and potentially save energy) while still receiving the same pollination services as more rewarding flowers.

Therefore, it seems that caffeine in nectar could lead bees to make suboptimal foraging decisions*: gathering less sugar, thereby decreasing honey production. It’s been even suggested that caffeine is a contributor to the massive deaths of bees (mainly bumblebees) under linden trees: bees may starve if they are attracted to flowers with lower sugar content and keep visiting them even after they stop producing nectar.

Caffeine as medicine

But it seems that it’s not all bad for the bees. Caffeine might actually be good for them. It seems to activate the immune system of honeybees. And it’s been shown recently in bumblebees to have both prophylactic and therapeutic effects against fungal infections. Honeybees also survive viral and fungal infections better when low doses of caffeine were added to their food.

Plants and pollinators benefit each other, but like in many collaborations one of the partners might try to cheat the other. Caffeine addition to nectar might be one of the many strategies plants use to attract bees against the insects best interest. More research is needed to see where the balance is in this relationship.

And there is still an open question: can bees get addicted to caffeine?

* It seems that some stingless bees are actually resistant to the effect of caffeine on foraging decisions.

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Photo: Barbara Feldmeyer.

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