Some like it hot

Winners and losers of climate change. By Engela Duvenage.

There is increasing concern about how insect pests will respond to global warming. Will populations increase, or spread to other regions in South Africa? Will some species die out because of higher temperatures? “The verdict is still out on who will be the winners and the losers when it comes to climate change,” said Dr Minette Karsten of the Department of Conservation Ecology and Entomology at Stellenbosch University.

Karsten informed the audience that no real predictions have yet been made about the future distribution patterns or abundance of most insect species currently attacking South African deciduous fruit. That’s because so little is known about the thermal traits that are important to their growth, reproduction, and activity.

She said that even less information is available on insect pests’ ability to adapt to specific environmental conditions, such as seasonal changes.

The research group of which Karsten is part is increasingly focusing on insects’ ability to adapt to changing climates. Karsten and her colleagues are trying to understand which factors affect climatic stress resistance and the potential implications thereof.

“No basic physiological measurements or data needed to calculate thermal responses are available for many of our most important pest species,” she said. “There’s of course a very practical reason why so little information is known about the thermal traits of local pest species. Their small size makes them very difficult to measure.”

The most comprehensive information for insect pests of deciduous fruit is available for the Mediterranean fruit fly, the Oriental fruit fly, false codling moth and codling moth, followed by the obscure mealy bug, Cape fruit fly, green peach aphid, circular purple scale, grapevine mealy bug and Western flower thrip.

Insects in a warmer world

Karsten said it remains uncertain how rising temperatures will influence the distribution and population sizes of most agricultural pests, or how changes in abundance may alter herbivory or attack rates on deciduous crops. There is also uncertainty whether rising temperatures will allow pests to increase their range to areas which are currently unsuitable for them or to shift their distributions.

She explained that changing weather patterns could have a direct influence on insects’ behaviour and physiology. It could also influence them indirectly, for instance by changes in the life cycles of their host plants affecting timing of food availability.

Metrics based on thermal data compiled from the literature show that all the pest insects in the South African deciduous fruit industry for which we have reasonable data will be under increasing threat of overheating under future climate conditions if they do not compensate behaviourally or physiologically.

“Our results are similar to what was shown in a previous study by our group that investigated a number of global insect pests and also considered different life stages,” Karsten said.

She extracted mean annual temperatures from the WorldClim database, which contains global climate data for current conditions and future predictions. Using these data, it showed that up to at least 2050, most species will be able to withstand the temperature increases predicted for the Stellenbosch region.

“These pests are relatively safe when it comes to the impacts of increased future environmental temperature when we use mean annual temperatures, but this may not be the case if we use the mean for the warmest quarter,” she explained.

According to Karsten, much more research is needed into the thermal tolerance of specific insect species. Better microclimate data is also needed so that researchers are not only reliant on macroclimate data for their predictions. We currently know little about pest insect’s use of microclimate sites — such as under leaves in the shade — where they would be able to buffer themselves against environmental temperature.

Explained: Us versus them

People are endotherms, and capable of generating their own body heat. Insects, on the other hand, are ectotherms, and their body temperatures are therefore influenced by weather conditions and climate over the longer term. Abiotic factors such as moisture, temperature and solar radiation can help insects to thrive, to increase their range or disappear from specific regions.