Climate stress is changing the physical shape of UK bumblebees

Insect numbers have been found to be declining in almost every place we look.

As the climate crisis sets in and more land is intensively managed, the smallest of creatures are struggling to survive.

These changes to the environment can have direct impacts, such as the overuse of particular pesticides or extreme heat explicitly killing individuals. But in addition to these more immediate results, the insects can suffer from more subtle effects.

A new study has looked at four species of bumblebee from the UK in museum collections and found that, over the past century, the insects have been getting increasingly stressed. This could potentially have a knock-on impact on their health and their ability to adapt to the changing environment.

The research was able to use an approach that looked at the asymmetry of their wings to determine the amount of stress the individual insects were under when they were alive.

Dr Richard Gill at Imperial College London studies how human activities affect insect populations who was involved in the study.

'We're seeing that bees are declining in populations across the world,' explains Richard. 'What our work is starting to do is explain when, where and potentially how this stress is being placed on these bee species.'

'By doing this we get a better predictive framework for understanding and forecasting where and when some of these populations are going to be most at risk when we get further environmental change.'

'That's very important if we're going to start targeting conservation effort and start actually understanding how we can mitigate and safeguard these populations.' 

The results found that the asymmetry of bumblebee wings, a signal of their underlying stress, has increased over the past 100 years. By comparing these changes to data on the environmental conditions that were occurring when the bees were alive, they could determine that the bees were more stressed when the weather conditions were hotter and wetter.

With the climate crisis unfolding, and the frequency of these warmer, wetter years only predicted to increase, the results suggest that the bumblebees will be put under even more stress in the future.  

The results have been published in the Journal of Animal Ecology.

Pollinators under stress

While mortality is a (relatively) easy metric to measure, figuring out if an insect is stressed is far trickier.

Research in honeybees, for example, has previously looked at behavioural changes in the insects when exposed to adverse changes to their environments, such as the extension of the sting, or even the insects' response on a cellular level by seeing how the amount of different molecules or gene expression changes.

Many of these metrics, however, require living insects. This makes using museum collections to assess the stress that an individual insect has experienced in the past even more difficult to ascertain.

In a bid to solve this conundrum, the researchers turned to the insects' wings. Bumblebees have two pairs of symmetrical wings, a larger fore pair and a smaller hind pair. These are held together by a set of small comb-like teeth called hamuli.

The team found that, for four species of bumblebee, the symmetry (or lack thereof) in the wings is a proxy for the amount of stress that they experienced in life.

This allowed the team to look through historical collections of bees held in museums to see how this asymmetry in wing shape has changed over time, and then compare that to data on what the environmental conditions were like when the insects were alive.

'From the research we've done on these bees, and having a look at how their wing asymmetry, which is a proxy of stress, is expressed in these bees over the past century, we can predict that hotter and wetter years will put them under more stress,' explains Richard.

'There are a number of things these bees can do under hotter and wetter conditions: they can either adapt, evolve to survive, or potentially move and maybe migrate more northwards in the UK.'

The changes in the symmetry of the wings are thought to be due to alterations in the expression of the genes that regulate their shape, as a result of the differing environmental conditions.

While this particular study did not delve into the DNA of these museum specimens, a second new study has been doing just that.

How to build a bee

The field of ancient DNA has made rapid advances over the past decade.

Whilst it would once have been unthinkable to be able to extract pieces of genetic code from the bones of long-dead animals, today it is even possible to tell what was once living in a cave thousands of years ago simply by looking for bits of DNA preserved in floor sediment.

By turning this technology to museum specimens, researchers hope to be able to open up a whole new avenue to study the genetics of animals that were alive hundreds of years ago. 

While this has already been done for many larger animals, such as through sampling the hair of tiger skins or taking small pieces of ancient human bone, when it comes to insect specimens applying these techniques are a little harder.

This is partly because when many of these specimens were collected they were not preserved in ways that were conducive for DNA preservation. 

Dr Selina Brace is an ancient DNA expert at the Museum, who helped to extract the genomes of these bees for a new study published in Methods in Ecology & Evolution.

'This study is one of the first to look at the DNA of a large number of insect specimens across different museums,' explains Selina. 'It is really important to have been able to understand how that DNA is preserved.'

'What we see is that very quickly the DNA starts to break down, but we then noticed this degradation actually slows through time and you get a very constant pattern of DNA degradation and fragmentation.'

By understanding how the DNA breaks down over time, it allowed the scientists to figure out how to put it all back together again. And this is important to be able to peer into the past of these long-dead creatures.

This new ability to look at the genetics of insects held in museum collections, and specifically how the genetic diversity of species has changed over time, means that researchers will be able to start answering questions about how the changing environments have impacted them.

Combining both the genetic and physical changes seen in insects will help us better understand how these animals will respond to the multitude of threats currently bearing down on them.

But there are a few things that we can do to help bumblebees right now.   

'There is so much people can do to start protecting bees,' says Richard. 'From the smallest thing of putting out flowering plants, that will help. If you can, plant herbaceous plants that are fantastic for things like bumblebees, as well as butterflies, hoverflies and the whole diversity of insects that you should have in your garden.'

'Lavender is fantastic for bees, and it smells great! But some of the other herbaceous plants, like rosemary and thyme, can be used in cooking and will be attracting pollinators and supporting populations. It's a win-win.'