Dear Readers, there has been quite a lot of research on the subject of bumblebees and climate change. After all, one look at these large, ‘furry’ insects is enough for me to worry about them buzzing about wearing the equivalent of a mink coat. They are, in fact, adapted for tundra conditions – their large size relative to pollinators like honeybees means that they lose heat more slowly than smaller insects (remember that surface area to volume ratio thing that many of us learned in O-Level biology). The ‘fur’ (actually outgrowths of the abdomen and thorax called setae) helps to insulate the bee against cold temperatures, and the bees also have very large flight muscles, which they can warm up by vibrating them – bumblebees are often the only pollinators in flight on a cold morning, and queens can sometimes be seen foraging as early as January in the UK. They also, unusually for insects, can generate their own body heat – they use enzymes that can break down the sugars found in nectar to help them to warm up. The queen in the photo below was spotted on my birthday, 20th January, earlier this year.

Bumblebees generally have one generation per year. In the spring, a queen bumblebee comes out of hibernation, and sets off to find a spot for her nest – you will often see large bumblebees flying low over the grass as if looking for something. A mousehole, dense ivy or a gap under a shed are all desirable spots. Once in her nest, the queen lays her first few eggs, and feeds them all by herself until they hatch as workers and are able to forage themselves. Eventually, there are enough workers for the queen to concentrate on laying her eggs. The colony will comprise less than 1000 individuals, and towards the end of summer the queen will start to lay eggs that will develop into fertile queens, and males. These will leave the nest and mate. The queens, already pregnant, will find a space to hibernate, and the males, the original queen and the rest of the workers will die.

So, bumblebees are basically big and hairy in order to survive in cold climates. How, then, are they doing in the heat?

It’s true that there are bumblebees in tropical and sub-tropical habitats, but these are mostly found at high altitude, where it’s colder, and  they tend to be smaller, meaning that they can lose heat more easily. However, the bumblebee species found in southern Europe and the southern USA are thought to be pretty much at the edge of their range, and are expected to disappear as the summers get hotter and drier, and the winters milder.

For bumblebees in the UK the effects of climate change will depend on the rapidity of change, and intensity of individual events. At over 37 degrees Centigrade, bumblebees forage much less effectively. Queens have something called heat stress proteins, which can protect them against extremes of temperature while they’re hibernating, but workers and males do not have these proteins, as far as we can tell. Bumblebees are also prone to water-stress (desiccation) – one thing that I did during the extreme heat of the past few days/weeks was put out a dish almost full of gravel, and then topped up with water so that insects could land safely and drink.

Bumblebees are tough, adaptable creatures ( I often think of them as the Einsteins of the insect world) and they have a number of behaviours that help during extreme weather conditions. Bumblebee nests are tiny compared to those of honeybees (under a thousand individuals as opposed to tens of thousands), but the workers will club together to  vibrate their wings to try to keep the temperature between 28 and 32 degrees Centigrade. They will also change their foraging behaviour during hot spells, going out in the early morning and later in the day to avoid the heat of midday. Bumblebees also seem to be choosing more isolated, cooler spots in order to make their nests.

However, finding those cooler, more isolated spots is clearly becoming more problematic due to a combination of our habit of paving over our gardens,  using fake grass ( a particular bugbear of mine), agricultural intensification, deforestation and urbanisation. Some bees are migrating to try to find better habitat, and others are seeking spots at higher altitudes, as are many other animals and plants that are adapted to colder conditions. One important  question is ‘what happens when the temperature in mountainous habitats also rises?’

So, how are the bees coping so far? One interesting adaptation seems to be that some buff-tailed bumblebee colonies are surviving through the winter in the UK – presumably people are planting enough winter-flowering honeysuckle and mahonia to enable the colony to survive on iron rations until the spring comes. This may be preferable in some circumstances to a situation where a hibernating queen keeps waking up during the winter and hence burns through the fat reserves that would normally be used to produce her first few eggs. However, a non-hibernating queen will also produce fewer workers, so it may be a trade-off against different conditions.

Climate change is also affecting the delicate balance between the different stages of the bumblebee life cycle and the emergence of the flowers that they rely on – it’s clear that plants are flowering earlier than they used to, which may favour colonies that keep going through the winter, and fortunately many species of bumblebee in the UK can feed on a variety of different kinds of plants, so they will not be as affected as some of the bee species which are more specific in their requirements.

The bottom line is that bumblebees probably can adapt to the higher temperatures of climate change, but only up to a point, and only if there is enough genetic diversity for those adaptive factors to be present in the community of bees. Like animals trapped on islands, bumblebees are increasingly isolated from one another, trapped in small areas of botanical plenty while surrounded by deserts of concrete and agricultural monoculture. The idea of ‘pollinator pathways’, whereby bees of all kinds can forage more widely and can meet and mate with bees that they wouldn’t otherwise meet, is an important one, and it depends on everything from the planting of bee-friendly street trees to putting a pot of lavender or rosemary on the balcony. Will it be enough? Only time will tell.

Common carder bumblebee on bittersweet

This piece is largely taken from ‘Bumblebee resilience to climate change, through plastic and adaptive responses’ by Maebe et al, published in June 2021. The abstract is available here though it looks as if you might need access to a university library (or $15) to read the whole thing. Shout if there’s anything that you want me to look at in more detail – this is a meaty piece of work, and I have tried to precis it. 

I also found this article interesting, especially on the subject of  bee body size and mountainous habitats.