In South Africa, the invasive harlequin ladybird may ultimately not be able to adapt to climate change, despite the fact that it has successfully invaded four continents over the last two decades.
This is the finding from a unique breeding experiment with over 400 harlequin ladybirds (Harmonia axyridis) undertaken by researchers from the Department of Botany and Zoology at Stellenbosch University (SU).
In this experiment, Drs Mike Logan and Ingrid Minnaar raised three generations of ladybeetles, creating more than 50 families with known parents and grandparents. This allowed them to create a family tree of 400 offspring. The goal of constructing such a large family tree was to disentangle the sources of variation of the traits measured. For example, the authors were able to determine if the variation of walking speed and temperature tolerance across beetles was explained mostly by genetic factors, maternal effects or the environment.
Dr Minnaar explains: “We were particularly interested to find out if specific performance traits of the beetle, such as the temperature at which peak performance occurs or the extreme temperatures at which performance ceases, have the potential to evolve or whether they are constrained. If we found evidence for the former, it would mean that the beetle has the capacity to adapt to climate change-related temperature shifts while the latter would suggest that the beetle may not perform well in warming scenarios."
The results of this study have recently been published in the journal Evolution, in an article entitled “The evolutionary potential of an insect invader under climate change".
The study found that the beetles have the potential to respond to shifts in temperature extremes, more so than to changes in mean temperature conditions. In addition, the highest and lowest temperatures tolerated by the beetles were found to not shift independently from each other – and results hint to what ecologists call a “specialist-generalist trade-off". This is when an increase in performance of one trait comes at the expense of another trait, and therefore, the peak performance for both traits cannot evolve simultaneously.
In other words, beetles that are able to withstand a heat wave are unlikely to also withstand a cold snap, limiting this species' capacity to adapt to climate change, explains Prof. Susana Clusella-Trullas, co-author on the paper and principal investigator of the Climate and Invasions: mechanisms in ectotherms lab (CL.I.M.E lab) at SU.
She says the harlequin beetle's lack of evolutionary potential could be due to the unique history of this species' introduction to South Africa; “It could be that the initial gene pool was small, or perhaps there was strong selection for specific traits in the first few generations after its introduction to South Africa. There are however other possibilities as well. For example, this species may have low potential to adapt to changes in temperature but high capacity to buffer this variation by taking refuge, or shift its behaviour temporarily."
In the meantime, research on this species is ongoing, and focuses on gaining a better understanding of how it uses its microhabitat, and the plasticity of its response to changes in temperature.
"The goal is to increase our knowledge of the population genetic structure of the species and of its potential impacts on native ladybirds and other insects in South Africa, as the species is now well established and broadly distributed across the country. Our study shows that the evolutionary potential of invasive species cannot be assumed and both native and invasive species' performance need to be assessed to determine winners and losers in a warming world," she concludes.
Photo: Ingrid Minnaar
Prof Susanna Clusella-Trullas