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Scientists have found that historical outbreaks of migratory locusts in China were associated with cold periods, suggesting that China's projected climate warming could decrease locust numbers.

The study was published in Proceedings of the National Academy of Sciences yesterday (17 September).

The researchers analysed the annual numbers of Locusta migratoria manilensis recorded by Chinese officials between 957 and 1956, and compared them with average temperatures over ten year periods.

Locust numbers were higher during cold periods than warm periods. Colder periods also tended to have more frequent droughts and floods.

Large locust plagues can cause severe damage to crops.

Historically, the main sources of Chinese locust plagues were marshland near river channels and lakes. Locust eggs depend on soil moisture for their development, and both low and high rainfall can benefit them, as receding water levels and formerly flooded areas provide moist soil.

"The relations between the decadal mean temperature and locust outbreak are rather indirect," Zhang Zhibin, corresponding author of the study, and director of Institute of Zoology of the Chinese Academy of Sciences, told SciDev.Net.

The article suggests that the warmer and wetter climate predicted for China might bring unfavourable breeding conditions for locusts, lessening their threat.

But earlier research has found the opposite. According to Zhang De'er, chief scientist of the China Meteorological Administration's National Climate Center, studies in the 1990s found that warm temperatures could favour locust larvae survival through winters, and enlarge the scale of locust plague in the following year.

But analysing annual and decadal effects of temperature on locust populations could yield totally different results, explains Zhibin.

"The contradiction shows that the ecological response of locusts to climate change can be related to the length of the cycle studied," he said, adding "In a decadal cycle, the favourable effect of warm temperature on locust growth may be covered by its unfavourable effect on the locust's habitats."

Zhang De'er believes that temperature was definitely not the sole factor determining the size of locust plagues. Varying rainfall could also play an important role, as the changes could alter water levels, thus affect the size of good locust habitat, she says.

Link to full paper in the Proceedings of the National Academy of Sciences*

*Only available to countries specified by PNAS 

Reference: Proceedings of the National Academy of Sciences doi 10.1073_pnas.0706813104 (2007)