Published online 7 October 2009 | Nature | doi:10.1038/news.2009.978

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Global warming may worsen locust swarms

Ancient records link a hotter climate to more damaging infestations.

Locust swarmWarmer weather in China has been linked to worse locust outbreaks.Kazuyoshi Nomachi / Science Photo Library

Analysis of Chinese historical records stretching back for over a thousand years show that locust outbreaks are more likely to occur in warmer and drier weather, especially in the country's northern provinces, researchers say.

"The results are an alarm bell for yet another serious consequence of climate change," says Ge Quansheng, deputy director of the Beijing-based Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, who was not involved with the study.

The findings, by climate researcher Yu Ge and her colleagues at the Nanjing Institute of Geography and Limnology, part of the Chinese Academy of Sciences in Jiangsu province, are published in the Journal of Geophysical Research1.

In population ecology, researchers have been debating what controls the size of species populations over long time periods. Some think that climate has a dominant role, whereas others hold that internal biological mechanisms, such as competition and predation, are more important.

To determine which model is correct, long-term data on changes in species populations are crucial. This has led researchers to turn to historical records of locust outbreaks. Such swarms can ravish crops, causing famine and consequent social unrest, so for more than 2,000 years, officials in China have recorded details of the outbreaks — such as their frequency and severity, the affected areas and the number of people who died of famine following infestations — with the aim of predicting and controlling them.

Locust focus

Around 50 years ago, Ma Shijun, a entomologist who worked at Peking University in Beijing, used these records to rank the severity of locust outbreaks — mainly in eastern China where agriculture is concentrated — over the past 1,000 years on a scale of one to ten2.

Ma's data were used by Yu and co-authors in the latest study, but their findings contradict an earlier study3 by University of Oslo ecologist Nils Stenseth and his colleagues. That work was based on the same data but showed that locust infestations are more likely to follow periods of cooler and wetter weather (see 'Cooler weather favours Chinese locusts'). So why did the groups come to drastically different conclusions?

"The results from Stenseth's group were rather curious," says Yu. They contradict several earlier studies, including Ma's, on the correlation between climate and locust outbreaks, and could not be explained by the biology of locust development, she adds.

“The results are an alarm bell for yet another serious consequence of climate change.”

Ge Quansheng
Institute of Geographical Sciences and Natural Resources Research, CAS

Digging a bit deeper, Yu and her colleagues noted that Stenseth's team used decadal temperature proxies for the whole of China to correlate records of locust outbreaks in eastern China. "China is a vast country," says Yu. "There are large variations in climate and [locust] plague problems at all corners of its territory." So northern and southern parts of China have very different climates, which can have a different impact on insect infestations.

Furthermore, locust eggs that have survived over winter, develop into adults in and breed in spring resulting in huge numbers of adults that migrate and swarm in summer. Each stage of their development is controlled by different climatic factors, so using decadal mean temperatures and precipitation might not be appropriate for the analyses of locust outbreaks, Yu adds. "Only regional and seasonal analyses could resolve the issues," she says.

Warm for swarms

Yu and her colleagues therefore analysed the relationship between climate and locust outbreaks on three different time scales. They first correlated the monthly temperatures and precipitation — as recorded by regional weather stations in the past 100 years — with locust outbreaks in northern and southern China, respectively.

Next, they determined if decadal temperature and precipitation for summer and winter — derived from other studies — were associated with locust outbreaks in the two regions for both the past 300 years and the past 1,000 years.

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The results show that a warmer climate was more likely to cause locust outbreaks in both regions. The team discerned an association between drier summer months and more frequent and severe swarms in the north but not in the south. "This might be because southern China is wetter [than northern China] in the first place and doesn't get as dry in the summer," says Yu.

In addition, the frequency of locust outbreaks in both areas went through cycles that were linked to regional temperatures for about 60% of the time over the past 1,000 years.

"The new study points to the importance of delineating regional and seasonal variations in climate-association studies," Ge says.

Stenseth agrees that the contradictory conclusions of the two studies are "probably explainable through the different spatial scales of the temperature proxies used".

Yu's study is an important contribution to our understanding of climate impact on locust plagues, he adds, and will help to devise mitigation strategies in response to global warming. 

  • References

    1. Yu, G., Shen, H. & Liu, J. J. Geophys. Res. 114, D18104, doi:10.1029/2009JD011833 (2009).
    2. Ma, S. C. Acta Ent. Sinica 8, 1-40 (1958).
    3. Stige, L. C., Chan, K.-S., Zhang, Z., Frank, D. & Stenseth, N. C. Proc. Natl Acad. Sci. USA 104, 16188-16193 (2007). | Article
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