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[MANILA] A warming climate and occasional extreme high temperature events in tropical countries are likely to limit both the survival and distribution of the brown planthopper, a pest that has devastated rice crops in India, Indonesia, Thailand and Vietnam, according to UK researchers.

The scientists, based at the University of Birmingham, measured the upper thermal thresholds that could be survived by the brown planthopper, including the temperatures at which the insects became immobilised by heat stress, and those that were lethal.

Laboratory measurements were made on both planthopper nymphs and adult insects, using a population collected in Malaysia. The results obtained were compared with the normally experienced summer high temperatures in which infestation with brown planthopper occurs, and infrequent extreme high temperatures.

The researchers found that the average 'hot season' temperature ranges (measured in degrees Celsius) in tropical lowlands where brown planthopper outbreaks occur are: 20–31 in India; 20–33 in Vietnam; 21–24 in Korea; 22–27 in China; 22–32 in Bangladesh; 25–27 in Indonesia; 25–32 in the Philippines; 25–35 in Thailand; 26–36 in Myanmar; and 29.9–34.7 in Malaysia.

"In most countries, summer high temperatures were lower than those that would kill brown planthoppers, but high enough to limit mobility," Jeffrey Bale, professor of environmental biology in the university's School of Biosciences, told SciDev.Net.

Data from the study suggest that the brown planthopper is already living close to its upper thermal limit across parts of its distribution. Thus, a further increase in temperature would affect not only its mobility and annual migratory behaviour, but could also be lethal. The researchers believe that this may be true for other tropical insects as well.

The researchers are now studying whether insects such as the brown planthopper will be able to adapt and survive the additional heat stress, and whether they will still be able to reproduce, Bale said.

According to the International Rice Research Institute, based in the Philippines, between 2009 and 2011, more than 3 million hectares in Thailand were infested by brown planthoppers causing losses of more than 1.1 million tonnes of paddy rice, with an export potential of US$275 million.

Link to full paper in PLoS ONE

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