Swarms of so-called "killer bees" will reach the United States within 11/2 years, and they appear to be a more formidable threat than originally expected, scientists report.

"It's going to keep coming at us no matter what we do," University of Kansas insect ecologist Orley Taylor said at the American Bee Research Conference.Experts predict that the dreaded Africanized bees making their way north through Mexico will reach the Brownsville area at the southern tip of Texas in the early spring of 1990, and possibly reach California four years later.

The bees have killed at least 350 people in Central and South America, Jim Tew, director of the International Beekeeping Program at Ohio State University's Agricultural Technical Institute, said earlier this year.

More than 50 bee experts from as far away as Yugoslavia gathered in Weslaco, a city on the Mexican border, for the conference, which concluded Thursday.

Scientists are trying to determine what substances can be used to repel the bees, said Anita Collins of the U.S. Department of Agriculture's research office here.

Collins said one motivation for her research was being chased for long distances by hundreds of bees while studying them in Venezuela.

"I reached the point where I wanted to say something besides, `Run like hell,' " she said.

Scientists and beekeepers have been worried since the accidental release in 1956 of 20 to 30 swarms of Africanized bees in Brazil. The bees tend to be more defensive around hives and tend to more aggressively attack and pursue people than the gentler bees of European descent raised in the Americas.

That behavior has brought the popular nickname "killer bees," a term shunned by scientists and beekeepers. The prospect of hordes of attacking bees inspired a 1978 horror film, "The Swarm," and comedy skits on "Saturday Night Live."

Scientists had assumed the Africanized bees would become Europeanized by mating with the gentler variety, and thus lose their propensity to sting in potentially deadly swarms.

But in the most surprising finding of the conference, researchers reported that the dilution of Africanized genes appears only to be temporary and that the bees' genetic makeup eventually becomes almost purely Africanized again in the wild swarms that have spread up through Central America and southeastern Mexico.

"This African population is going to reach the U.S. virtually unchanged," Taylor said.

The bees are spreading at the rate of about 300 miles a year and are expected to reach Brownsville in March 1990, said Jose Villa of the Agriculture Department's Agricultural Research Service in Baton Rouge, La., and Juan Labougle of the Universidad Autonoma de Mexico.