The infamous "killer bees" that crossed the Texas-Mexico border last month should be able to live year-round in about half the United States, including Washington and much of the East Coast, according to new studies that greatly expand previous estimates on the Africanized bee's range.

Earlier predictions limited the invading bees to the southern parts of Texas, Louisiana, California and Florida. According to the new studies, however, colonies of the unusually aggressive bees could establish year-round residence as far north as Long Island, Pennsylvania, central Ohio, Illinois, Indiana, the southern borders of Iowa and Nebraska, central Colorado, northern Utah, Nevada, western Idaho and Washington state.

The new range estimates are based on the first experiments that examine in great detail the bees' ability to endure cold. When bees get cold, they form a tight cluster to keep warm, with each bee serving as a tiny heater, burning up the calories in honey. Researchers were surprised to learn that Africanized bees are relatively hardy little creatures, capable of surviving 120 consecutive days in temperatures below 40 degrees Fahrenheit.

"We're pretty confident they can survive much farther north than previously thought," said David Roubik, an expert on tropical bees at the Smithsonian Tropical Research Institute in Panama, who performed the studies with bee biologist Edward Southwick of the State University of New York at Brockport and climatologist John Williams of Williams Associates in Brockport. Their study is being published in the current issue of the journal Comparative Biochemistry and Physiology.

The first U.S. swarms of Africanized bees, which have relentlessly expanded their range since they were brought from Africa to Brazil in 1956 and began interbreeding with more-docile European bees, were trapped and killed in the Rio Grande valley of South Texas two weeks ago.

The bees are notorious for their aggressive personality. Sting for sting, they are no more venomous than European honey bees, which are kept by commercial beekeepers and can be found across the United States. Africanized bees differ because they are highly defensive of their hives. They are quicker to rile and more likely to sting en masse. They will also pursue intruders over greater distances. Bee scientists joke that Africanized bees have an "attitude problem."

The work by Roubik and colleagues is based on studies in which Africanized and European bees were placed in refrigerators. When temperatures begin to cool, bees stop leaving the hive to forage. Eventually, the bees mass in a tight bunch to keep the center of the colony, with its queen and her brood, at a constant 86 degrees Fahrenheit.

"As it gets colder and colder, the bees bunch together as tight as physical size permits," Roubik said. "From the outside it doesn't look like they're doing a darn thing. But these big flight muscles are just burning up the honey. They're just like little heaters."

The scientists found that Africanized bees had a distinct advantage over their European counterparts at moderately cool temperatures. At the coldest temperatures, the Europeans had the upper hand.

Earlier work by Orley Taylor of University of Kansas, which suggested a limited range for the bees, was based on temperatures in Africa. Taylor now says the new studies convince him the bees will probably have a substantially greater range.

Taylor said a number of other factors could limit the bees' range. Their greatest challenge will be European bees, which are already well adapted to the temperate climate of the United States and not likely to give up their turf easily. How much interbreeding will occur between the two, and whether the offspring will be gentler, is the subject of intense debate among scientists.