Crop-damaging pests are moving towards the poles at a rate of more than 25km a decade, aided by global warming and human transport, posing a potential threat to world food security, a study showed on Sunday.
The spread of beetles, moths, bacteria, worms, funghi and other pests in a warming world may be quicker than for many types of wild animals and plants, perhaps because people are accidentally moving them with harvests, it said.
Scientists based in Britain studied more than 600 types of pests around the world and found that their ranges shifted on average towards the poles by 26.6km per decade since the 1960s, occupying vast new areas.
"We believe the spread is driven to a large degree by global warming," lead author Daniel Bebber of Exeter University told Reuters of the findings in the journal Nature Climate Change. They wrote it was the first study to estimate how pests are moving because of a changing climate.
The spread of pests is "a growing threat to global food security", the study said. Between 10 and 16 per cent of crop production is lost to pests, with similar losses after harvest, they wrote.
The rate of spread, away from the equator and towards the north and south poles, is slightly faster than the 17.6km found in a study in 2011 for wild animals and plants that was in turn quicker than 6.1km for wildlife estimated in a 2003 study.
The rate, however, is virtually identical to a theoretical prediction in 2011 that rising temperatures would allow a poleward shift of wildlife of 27.3km a decade, the experts wrote. Many crops are growing nearer the poles due to warming.
Researchers say crop pests are moving into new areas at a quicker rate than their predators, meaning they can do more damage to crops.
Wild species may find it harder to move because their habitats are getting fragmented by deforestation, farms, roads or cities. "Pest species are constantly being shifted around the world by trade...We are giving them a helping hand," Bebber said.
"I'm not surprised," by the faster rate than for wild animals and plants, said Gary Yohe, a professor at Wesleyan University in the United States who was co-author of the 2003 study that put the average pole wards shift at 6.1km.
A tiny pest is more likely than the average animal or plant to be carried inadvertently be taken on an train, truck or airplane to a new area, he noted. And he said the 2003 study was conservative.
Another possibility is that the rate of movement by wildlife "has really speeded up" in recent decades, said Michael Singer, a professor who works at both Plymouth University in England and the University of Texas.
And some insects pests may be getting more mobile since they are often forced to move by humans. "They have to be mobile because humans are constantly ploughing or otherwise modifying their habitats," Singer said.
Sunday's study said that there were many problems in determining how far climate was driving the pests' movements.
"New crop varieties and agricultural technologies have extended the agricultural margin northward in the United States and deforestation has increased production in the tropics, thus providing new opportunities for pest invasions at high and low latitudes," it said.
The scientists urged governments to think more about where to plant crops and monitor trade more closely to limit losses.