Stellenbosch University – PhD
It looks innocent with its black spots and big eyes, but the harlequin ladybird, an invasive species native to Asia, has taken over the globe, leaving a trail of destruction in its wake. It outcompetes native ladybirds for resources and cannibalises young and adult alike. During its global conquest, it has successfully colonised many new regions because it has the ability to rapidly adapt to the environmental conditions of new habitats.
The harlequin ladybird is a taste of what is to come. If climate change favours invaders, it will speed up the decline of native species, and ultimately lead to less biodiversity. Different animal and plant species play key roles – such as increasing water quality and soil fertility, staving off diseases, and pollinating crops – in our ecosystems. That is why biodiversity is so closely linked to a healthy ecosystem. With fewer species, ecosystems will have a smaller arsenal to combat the effects of climate change. So, we need to know how native and invasive species will respond to climate change, such as increased environmental temperatures, to predict how ecosystems will be shaped in the future.
Harlequin ladybirds have probably already caused great changes to ecosystems by reducing the population sizes of native ladybird species. They are also a great nuisance to people through allergic reactions and by staining surfaces with a toxic yellow substance they produce when provoked. In the USA they have caused the loss of millions of dollars by tainting wine with their toxic and bitter secretions, and by damaging fruit crops. Climate change may make things worse, so we need to understand how these pesky beetles are going to fare in future climate scenarios.
The harlequin is a master of adaptation, with many outfits. If you have a large gathering of harlequins, it is hard to believe that the mosaic of colours clumped together are all one species. They come in all shades of yellow, orange and red; sometimes have no spots at all, and sometimes have so many spots that they appear black.
In cold environments, such as Russia, it is an advantage to have many black spots: more black spots means they can absorb more heat from the sun. In warmer areas, such as South Africa, they have fewer black spots. One possible reason for this observation is that when harlequin ladybirds first arrived in South Africa, those with many black spots absorbed too much heat from the sun, overheated and died. Those that had fewer black spots coped better with the warmer conditions and lived long enough to make baby harlequins. Having fewer spots was now preferable and, over time, the genes for fewer spots got passed on more often. If this is the case, individuals we come across today will have, on average, fewer black spots than previous generations. This change will be reflected in their DNA since it is passed on from parent to offspring.
Evolution is one way in which creatures adapt to changing climates. Unfortunately, climate change is happening faster than most traits can change, which is giving certain species – such as invasives – an edge over others. For example, the harlequin can change its spots, while native ladybird species often lack this ability. With the rising temperatures predicted by climate change, natives could be in more danger of overheating and drastically declining in numbers.
We know theoretically that this can happen, but we need evidence to validate our predictions, and to uncover if there is perhaps a faster way to respond to climate change without the need for evolution to take place. To this end, my colleagues at Stellenbosch University and I conducted experiments to see how the invasive harlequin ladybird would respond to increasing temperatures. We divided ladybird eggs from the same mother and father equally and placed them into cold (need-a-jersey-weather) and warm (need-an-aircon-weather) environments. We let them grow in these environments and, when they matured into adults, we recorded the number and size of the black spots on their wings.
Ladybirds that matured in the colder environment had more and bigger black spots than those that developed in the warmer environment. Even though these ladybird siblings had the same DNA code for colour pattern (because they shared the same parents), they displayed different colour patterns because they were placed in different environments. The different environments caused the DNA to be expressed differently, but did not change the underlying code. This can happen because different chemicals were produced in the different environments. These chemicals attached to the DNA and affected its expression. This process is similar to making doughnuts and sweetened bread: you’re using the same dough, but the difference lies in whether you fry the dough or bake it. The end product changes, even though the ingredients stay the same. In our experiments, because the DNA itself did not change, evolution did not take place.
This flexibility in genetic expression is not limited to young developing ladybirds, but can also occur when organisms are fully grown. For example, we tested the highest temperature that ladybirds could tolerate when they were placed in either the cold or warm environment. It wasn’t a surprise that those in beach-lounging weather were able to withstand higher temperatures. Again, there was no genetic change, but rather a change in the expression of the genetics brought about by a change in environmental conditions, in this case temperature. Since this flexibility takes place within the lifetime of an individual, it happens at a much faster rate than evolution (which can only occur when DNA changes are inherited from one generation to the next).
Heat tolerance is an important indication of how animals will cope with rising temperatures, but how rising temperatures could affect their ability to reproduce is equally important: it is a direct measure of fitness. To this end, we also counted the number of eggs ladybirds laid at different temperatures. In the warm environment, the invasive harlequin lay on average three times more eggs that the native lunate ladybird. This means that even though both ladybirds were able to increase their temperature tolerance, the warm environment greatly hindered the reproduction ability or fitness of the native ladybird.
The harlequin ladybird is a formidable invader. Its great trait flexibility, and broad tolerance of environmental change, could be the main reason for its invasive success in so many different habitats across the globe. Climate change may truly favour this invader.
At the end of this century, if not sooner, it seems likely that we will be living in a world that is dominated by the invasive harlequin, and so it is very important that we develop strategies to control this pest now. The data we collect here at the CL·I·M·E lab at the Centre for Invasion Biology at Stellenbosch University is giving us important information that policy makers and programme managers can use to eradicate this invader. We are currently working on a project that will determine if a fungus can be used as a biocontrol agent to stop the harlequin in its tracks.