Hikers and birders are treading more warily this spring among the thick vegetation at Santa Ana National Wildlife Refuge. Signs tell why: "Bee Aware. This is an African Bee Area."

A swarm of Africanized "killer bees" captured just outside the border refuge two weeks ago confirmed the onset of a spring invasion from Mexico. It ushered in an unwelcome era in southern Texas.These bees are here to stay, and people must learn to live with and respect them, scientists say.

"Should we be scared?" asked Nancy Colver, a Canadian on a birdwatching trip in the subtropical Lower Rio Grande Valley last week. She and her husband paid little attention to the warning signs as they ventured into the forest.

Scared, no, alert, yes, said Dennis E. Prichard, manager of the 2,088-acre refuge, who advises visitors: "If you see bees, avoid them. Give them a wide berth."

It's a genuine concern at Santa Ana with its 130,000 visitors and ideal habitat for bees. Sooner or later, Prichard believes, the highly defensive bees will attack and possibly kill someone.

Scientists have trapped and destroyed at least 10 swarms of the bees in the past two weeks along a 200-mile stretch of the border from Brownsville to Laredo, two of them in Bensten State Park in Hidalgo County.

A swarm trapped last October near the border city of Hidalgo was the first one trapped in the United States after crossing the Rio Grande from Mexico.

The October swarm prompted a quarantine in the state's eight southernmost counties, from which no bees may be moved. A trapping in Laredo last week extended the quarantine to 11 more counties.

Concern appears to be increasing in the region. Fire departments have been called on to exterminate wild bee swarms, a task they rarely performed in the past.

The Africanized bees are descendants of African queen bees that escaped from a laboratory in Brazil in 1957 and began breeding with the more docile European bees normally used for agriculture in the Americas. They have since spread through much of Latin America.

They retain the aggressive characteristics that European bees had before humans began breeding them to make honey and pollinate crops. Africanized bees earned the "killer" label because of their tendency to launch mass stinging defenses against intruders.

Because they are hard to manage and devote much of their energy to swarming and creating new colonies, farmers and beekeepers fear their economic damage more than their stings.