Scientists believe they have turned back the first U.S. invasion of killer bees but expect a new offensive in the spring.

Authorities Monday had not confirmed any new swarms in the week since they trapped the first of the Africanized killer bees that crossed into the United States from Mexico."I'm going on the assumption that we won't find another swarm" until spring, said Elba Quintero, area honey bee program manager for the U.S. Department of Agriculture's Animal and Plant Health Inspection Service.

"The weather is getting colder, and bees do not normally move in winter time."

USDA teams on Friday started a search for bees within two miles of where the first swarm of 3,000 was found and destroyed, near the city of Hidalgo. They are taking samples of all managed and wild-bee colonies in the area.

U.S. and Mexican officials in the past three years had placed hundreds of traps baited with a sex lure to track the bees, which have been moving slowly north since 1957, when they escaped from a breeding experiment in Brazil.

Killer bees earned their nickname because of their habit of attacking intruders in swarms and chasing their prey for long distances.

Beekeepers and farmers fear the killer bees will take over domestic hives and reduce honey production and pollination of important crops.

The swarm last week prompted a quarantine on movement of bees out of the two-mile radius where the killer variety was found. The quarantine also prohibits movement of bees from Texas' eight southernmost counties.

USDA trapped two swarms during the weekend along the Rio Grande less than a mile from where the Africanized swarm was caught, but the bees probably are not Africanized, Ms. Quintero said. Test results were expected today.

To the naked eye, Africanized bees are impossible to distinguish from the more docile European bees commonly raised in the Americas.