More frequent or more severe pest infestations may cut crop yields and drive up the price of corn, used for food and animal feed and to produce renewable fuels.

"Our projections showed all of the species studied spreading into agricultural areas where they currently are not endemic," said Noah Diffenbaugh, a Purdue University associate professor of earth and atmospheric sciences who led the study.

"The greatest potential range expansion was seen with the corn earworm, which is known to infest other high-value crops such as sweet corn and tomatoes. Warming could allow populations to survive the winter in the upper Midwest, the key region for corn production, as well as areas of the West where other high-value crops are grown," he said.

Researchers used climate model simulations that suggest winters will be milder more often later in the 21st century while summer growing seasons will be longer and warmer more often than they are now.

They compared the climate models to the temperature survival thresholds of four common corn pests found in the United States, the world's top corn producer and exporter.

"Basically, we examined both the number of days warm enough for the pests to grow and the number of days cold enough to kill the pests," said Purdue entomologist Christian Krupke. "This tells us what could happen in projected future climates."

For example, temperatures in Iowa, the top U.S. corn producing state, were suitable for corn earworm survival in zero to three years of every 24 years in the 20th Century. But in the 21st Century, that frequency was projected to increase to one to seven out of every 24 years.

More frequent insect pressure means farmers will need to spend more on pesticides or high-priced biotech seeds that help control pests or risk yield losses, researchers said.

More frequent pest problems can also result in more variable yields, which can drive up the cost of crop insurance or disaster relief for farmers, they said.

"Losses due to insect pests, including the resources required to control them, is the biggest cost for corn production," said Corinne Alexander, an agricultural economist at Purdue.

"The European corn borer has been estimated to cost the United States around $1 billion annually, and the corn earworm is responsible for destroying about 2 percent of the corn crop," she said.

The World Meteorological Organization said on Tuesday that 2008 will be the coolest year since 1997, but still the tenth hottest since scientists began recording the data 150 years ago. The 10 warmest years on record have occurred since 1997.