There is not yet much research on the effects of heatwaves on bees. What little there is focuses on super extremes of weather that would kill an adult bee.

However, my new research with colleagues shows that UK populations of solitary bees may be much more sensitive than previously thought to the kinds of extreme weather we are now seeing regularly.

To find out what happens to bees during hot weather, my team recreated the three-day UK heatwave of July 2022. We subjected a group of developing larvae of red mason bees to three days where temperatures peaked daily at 40°C.

Red mason bees are common solitary bees found in UK gardens, and are important pollinators of apples and other fruits. At the same time, a control group experienced normal July temperatures for Hull, where the study was conducted, peaking daily at about 25°C.

After that, we treated both groups identically and allowed them to spin their cocoons and hibernate as normal. Nine months later, all the bees emerged fine, so it appeared initially that the heatwave had had no effect.

But this was before we dissected the bees to look at their reproductive health.

Staggeringly, in males from the heatwave group, sperm activity had dropped by half compared with the control group, and sperm counts by one third. In females, there was a 15% reduction in both the size and the number of developing eggs.

The heatwave had wrecked their fertility, especially in males.

Reduction of sperm motility in bees during heatwave

These numbers are shocking because they suggest solitary bee populations are much more sensitive to weather extremes than we thought, and that this should be factored into calculations of the broader effects of climate change. While bees did not die outright, their fertility was severely affected.

This means that a heatwave one year could lead to a drastic drop in the number of bees the following year, and therefore less efficient pollination for key crops like apples, cherries and oilseed rape.

This would leave commercial fruit growers even more reliant on temporarily renting honeybee hives, commonly called “hire-a-hive” schemes, to combat pollination deficits. This is at a time when research increasingly shows that wild bees, whose services come for free, are better pollinators than honeybees.

What else happens in heatwaves?

In honeybees and bumblebees, living together as a group is the key to withstanding weather extremes. With their social hives, honeybees can flexibly respond to periods of heavy rainfall and strong winds by rapidly reallocating the tasks that worker bees perform – switching from nest maintenance to foraging, for example.

Honeybees and bumblebees are also able to respond to temperature changes. They maintain their nests within strict temperature limits, with some workers switching to becoming living radiators when temperatures drop, buzzing their wing muscles to produce heat that keeps the brood at the ideal growing temperature.

Bumblebee nests begin with a single queen hibernating over winter and then working alone to build up her brood. New research is revealing secrets of their resilience: for example, hibernating bumblebee queens can survive underwater for up to a week when their nest is flooded.

However, honeybees and bumblebees are not most bees.

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Read more: Queen bumblebees can breathe underwater — for days. We discovered how

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Unlike honeybees and bumblebees, most bees are solitary, which means they don’t have social nestmates to help them when times get tough – they work entirely on their own. Nests of these solitary bees are at the mercy of the elements, so solitary bees are much more vulnerable to climate change than social bees.

Of course, heatwaves are not the only threats to bees. They have an array of other nightmares to cope with, including pesticides, diseases, nutritional stress and loss of habitat.

The priority now is to investigate how bees affected by heatwaves also cope with these other problems. Our lab heads up a government-funded study looking at how climate change affects the nutritional needs of growing wild bees, and how parent bees respond to these needs.

Excitingly, we are beginning to see patterns indicating that growing bees require different balances of nutrients when they are reared at different temperatures. We are now testing whether bee mothers are sensitive to these requirements, and can adjust the pollen they gather to compensate.

Extreme hot weather is becoming more prevalent, even in cooler countries. These studies show that severe weather, while not necessarily killing bees outright, has the ability to seriously damage the bee population – with long-term consequences for pollination as well as the human food chain.

James Gilbert currently receives funding from UKRI (BBSRC) and the study mentioned in this article, on which James is a coauthor, was funded by UKRI (NERC).

The importance of bees for pollinating wild plants and crops is well known. If we lose the bees, we lose our food. But this is only part of the picture. Bees also support a hidden network of other species, sometimes as mutual partners, sometimes as prey, sometimes as other unwilling victims.

Many organisms depend on bees for survival, and many of these interactions are not mutually supportive. Some predators focus on bees, for example bee wolves (Philanthus triangulum), capture bees to feed their young in their underground nests.

Crab spiders, also known as the white death spider, are often found camouflaged on the top of flowers. They wait for bees to sip on some nectar and then the spider consumes the bee, and afterwards vomits the corpse back up.

It’s not just insects, vertebrates depend on bees too. Birds such as bee‑eaters and great tits, as well as some species of bat consume bees as part of their diet, while badgers and foxes often raid nests for larvae and honey. And, of course, humans have been eating honey from before there were written records.

Playing host to unwelcome guests

Around 40% of animals are actually parasites and bees support a wide range of these species. The wingless fly Braula coeca, sometimes referred to as the bee louse, lives on honey bees, feeding on their secretions. Though small, these parasites are a constant presence in some colonies.

Another parasite, Sphaerularia bombi, the nematode (a type of worm-like creature), enters bumblebee queens during hibernation. Once inside they inflate, filling much of the queen’s body. When she emerges in the spring, this queen has been neutered by the parasite and is no longer able to find a new family. She instead just acts as a vehicle to spread the parasite to new sites.

Some bees need other bees to help them survive. Cuckoo bees infiltrate the nests of bumblebees. After they gain access they suppress the bumble bee queen and force her workers to raise their young.

Invading the lives of bees

Sometimes parasitic interactions go one step further and ultimately kill the bee by spending part of their lifecycle within their host. Strepsiptera are an unusual insect, which most people may not have heard of. Stylops are one genus of Strepsiptera which live in the abdomens of bees, visible only by a small protrusion in the abdomen. But when it is time for Stylops to mate they explode from the abdomen of their bee host, killing it.

Bee flies definitely deserves a mention, as they bear a striking resemblance to bees. In the UK, species such as Bombylius major dance around flowers with their fuzzy, bee‑like bodies. While the adults are harmless and actually serve a role as pollinators themselves, their larvae are parasitoids of solitary mining bees. Parasitoids are defined as those that live on (or in) their host eventually killing it, a subset of parasites. The females flick their eggs into the entrances of bee nests and when they hatch, the larva consumes bee eggs or young larvae before feeding on the pollen stores.

Using bees to hitch a ride

Some species just use bees for transport. Mites such as Chaetodactylus attach themselves to solitary bees in order to travel between nests. Their larvae however, are less benign. They greedily consume the pollen stores of nests, occasionally eating eggs.

Perhaps even weirder however are the trigulins (or larvae) of blister beetles. These often cluster around flowerheads. They wait for bees, only to then climb on board for a free ride – using them as a free taxi to a nest where they feed on its contents with a particular fondness for bee eggs.

Pseudoscorpions are a distant relative of scorpions. They bear a striking resemblance to true scorpions, but these instead of carrying a sting in their tail, use the bee for a free ride. Hanging on to the bees with their pincers they use the bees as a taxi, but in their case just as a way to save energy on long-distance travel.

In the end, bees – whether they are solitary bees, mining bees, honey bees or bumble bees – are far more than pollinators. They support a much wider ecosystem. Countless other organisms rely on bees as hosts, prey, transport, or providers of food and shelter every day. Without bees we would not only lose those plants they pollinate but also those animals that need the bees to feed them and help them reproduce.

Alex Dittrich does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.