20180208A weclipart.com, clipart-library.com & naturalawakeningsli.com.jpg
Some flowers turn on the heat to attract bees. Source: weclipart.com, clipart-tlibrary.com & naturalawakeningsli.com

Many flowers go out of their way to attract pollinators, providing abundant nectar to feed them, offering lurid colors and exotic perfumes to draw them in, modifying their shape so they can easily land and get cosy and even adding nectar guides to direct them more precisely to their food source. All of this, to ensure pollination. These attractants even include colors our eyes can’t see, but some insects can (read What Bees See for further information). And it works! Study after study with different pollinators confirms that insects quickly learn to home in on the most generous flowers and pollinate them in priority, abandoning those that aren’t as giving.

But now we’re learning flowers have a lesser-known trick: heat.

Some Flowers are Hot, Others are Not!
It’s long been understood that flowers are often warmer than the air around them. Many flowers have specialized cells that capture and concentrate solar radiation and still others practice thermogenesis: they actually produce their own heat. Until recently, though, it was assumed the whole flower heated up equally.

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Thermal photography shows parts of the flower—those nearest the sexual organs or leading towards them—are hotter than the rest. Note that the flower at the top left (Papaver rhoeas) shows no directional heating: it’s among the 45% of flowers tested that do not produce heat to attract and direct pollinators. Source: University of Bristol

According to a study in eLife, though, many flowers (55% of the 118 species studied) display temperature variations greater than 2º C (3.6º F) within a single flower. The 2º C difference is an important detail, because it was already known that bees (in this case, bumblebees were studied) are able to detect temperature variations of 2º C and greater. The study found flowers using directional temperatures show on average a 5º C (9º F) degree difference between heated and unheated tissues; one even displayed an astounding 11º C (20º F) difference! These warmer cells are only found on part of the flower, directing the bees towards its nectar supply, their ultimate goal in visiting the flower. And bees can detect them thanks to special heat sensors in their legs and antennae.

The path they take as they follow the heat source towards the center of the flower leads the bees to brush past the flower’s stigma, leaving any pollen they may carry and picking up fresh pollen. That done, the flower rewards them with nectar when they reach the “hot spot.”

20180208C Viola x wittrockiana 'Mystique Blue Halo' www.thompson-morgan.com.jpg
The lines in this pansy (Viola × wittrockiana ‘Mystique Blue Halo’ ) are nectar guides, directing pollinators to the flower’s nectar source. Other flowers do similar things, but with heat. Source: www.thompson-morgan.com

These patterns were discovered using thermal photography and often show similar patterns to the nectar guides sometimes visible to human eyes.

In the study, the searchers used artificial flowers with different heated patterns, some of which offered nectar and others, plain water. The bumblebees quickly learned to recognize the patterns of those flowers offering nectar and to focus on them exclusively.

Offering heat to visiting insects might also have another use. Bees (and insects in general) need to maintain a certain body temperature to be able to fly in cool weather, so visiting “warming” flowers could be an additional incentive, especially in colder climates and in inclement weather.

Bees and Beyond

Does this thermal attraction apply to other insects beyond bumblebees, such as other bees, hoverflies, butterflies, wasps, etc.? That remains to be seen (I’m sure there are many more studies to come!), but it certainly looks like flowers have more up their sleeve when it comes to attracting pollinators than simply flower color and perfume.20180208A weclipart.com, clipart-library.com & naturalawakeningsli.com

Garden writer and blogger, author of 65 gardening books, lecturer and communicator, the Laidback Gardener, Larry Hodgson, passed away in October 2022. Known for his great generosity, his thoroughness and his sense of humor, he reached several generations of amateur and professional gardeners over his 40-year career. Thanks to his son, Mathieu Hodgson, and a team of contributors, laidbackgardener.blog will continue its mission of demystifying gardening and making it more accessible to all.

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