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202312 Fresh Quarterly Issue 23 08 Insect Pests Web
Issue 23December 2023

Insect pests in a warming world

What can science tell us about the likely impact of climate change on deciduous-fruit production? By Minette Karsten.

Climate change has shifted the average weather conditions globally, as demonstrated by regular reports of extreme weather events such as heatwaves, freezing spells, flooding, and droughts. South African deciduous-fruit production areas have already been impacted by many of these.

“Data show that temperatures will continue to increase in all major production areas,” says Prof. Stephanie Midgley, Specialist Scientist for Climate Change at the Western Cape Department of Agriculture.

According to Midgley, temperatures will rise by 2 °C on average, so some production areas will experience greater increases.

“We can expect to see the projected levels of warming and rainfall change manifest by the mid-2040s to 2050s,” she says. “It is possible that areas already facing challenges with rising temperatures and drought will experience even more extreme weather events in the future.”

Climate change is also likely to affect the occurrence and distribution of insect pests and diseases.

How temperature influences pest biology

Prof. John Terblanche heads the Applied Physiological Ecology research group within the Department of Conservation Ecology and Entomology at Stellenbosch University. The group explores how insect pests respond to altered environmental conditions.

Insect pests are particularly sensitive to temperature changes because they are ectotherms — their body temperatures and biological functions all depend on environmental temperatures. Their biological processes are optimal at a specific temperature and can only occur within a certain temperature range. Knowing these temperatures can help predict the impact of climate change on the development, survival, reproduction, and range expansion of insect pests.

Terblanche’s group uses thermal performance curves to evaluate the effect of temperatures on insect activities — called performance traits — such as walking and flying speed, and feeding rate. Thermal performance curves allow researchers to determine the lowest and highest temperatures for performance traits and the temperature at which the trait performance is maximised or optimal.

Thermal performance curves for different traits can look very different. For example, the optimal temperatures for development and flight may not be the same. Understanding thermal performance curves allows researchers to estimate how an increase in average temperature might impact a trait and how much pest performance might change under future climate scenarios.

The anticipated warming climate is expected to boost pest populations due to faster insect development at higher temperatures. This will lead to more generations per year and likely range expansions. More generations could mean increased levels of damage, which will raise the cost of control.

Pests in time and space

Changes in when and where pest insects are in the orchard could wreak havoc with control measures, including biological control agents.

For example, Aphelinus mali, a parasitoid of woolly apple aphids present in many apple-growing areas in South Africa, seems to already lag its host in some areas, probably due to differences in the temperature requirements of the host and its parasitoid for reproduction and development. This reduces the success of the parasitoid as a natural enemy.

In addition, some minor pests may even change their status and become major pests, with changes in the timing of peaks in population numbers. “How different insect pest species will respond to a rise in temperature is not entirely clear,” explains Terblanche, “and research shows conflicting views based on diverse outcomes.”

Terblanche collaborated with Prof. Philipp Lehmann from the Department of Zoology at Stockholm University in Sweden on an analysis of studies on climate change’s effect on agricultural and forest pests. They focused on temperature, but other variables, such as precipitation and solar radiation, are also important for insect responses to climate change.

The team observed a predicted increase in pest damage in 41% of cases but found mixed responses in 55% and a reduction in pest damage in 4% of cases. These varied results are due to differing responses and observed impacts even in the same species, probably due to factors such as where the studies were done.

The outlook for orchard pests

“Crop loss due to insect pests and the increased cost in management have been determined for field crops such as maize, rice, and wheat globally,” says Terblanche, “but no such estimate is available for deciduous fruit.”

Research on field crops predicts yield losses of 10%–15% and will likely correspond to increased future crop-protection costs.

In a recent Hortgro-funded project, Midgley and colleagues modelled the life cycle of codling moths under future climate projections based on their upper temperature thresholds and degree-day requirements in major pome-production areas in South Africa.

The model predicted that the number of codling-moth generations per year may increase by 0.5–1.5 by the middle of the 2040s. This is concerning since codling-moth control is based on adult flights, and adjustments may be necessary to the codling-moth degree-day phenology model to determine the timing and frequency of control measures.

Besides altering the behaviour of existing pests, climate change may increase the risk of introduced invasive pest insects establishing here, so biosecurity at ports of entry should be emphasised. Orchard floor ecology and interactions with pests and diseases could also be affected.

Although researchers are still determining what a warmer future holds for pests and diseases, they all agree that constant monitoring, planning, and risk mitigation are critical for a climate-resilient deciduous-fruit industry.

About the author

Dr Minette Karsten is an applied entomology researcher with Hortgro seconded to the Department of Conservation Ecology and Entomology at Stellenbosch University.

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