LOS ANGELES, JULY 8 -- After months of unusually low border traffic following passage of a tough new immigration law, illegal aliens again are beginning to pour in from Mexico in response to western growers' complaints of severe labor shortages.

Immigration and Naturalization Service figures show a striking traffic increase in the border sector south of San Diego, where Border Patrol apprehensions of undocumented aliens climbed from 29,080 in May to 44,039 last month.

"We've had a complete turnaround," said Gene Smithburg, assistant chief Border Patrol agent for that area. The pace continues to accelerate, he said, with 11,000 apprehensions in the first six days of July.

Border Patrol intelligence officers also report May-to-June upsurges along most of the Mexican border, although none approach the size of the new influx on the Pacific coast. In the El Paso, Tex., sector, where 18 illegal immigrants from Mexico suffocated in a sealed railroad boxcar last week, apprehensions rose from 16,777 to 19,064.

"The western growers got out their message very visibly that they needed labor," said INS spokesman Duke Austin, referring to reports from California, Oregon and Washington of harvests in danger of going unpicked. The loss of some early fruit crops provoked a bidding war among growers for available workers, who began to see a rise in wages and sent word home that work was easy to come by despite the new law.

INS figures show apprehensions across the entire Mexican border rising from 69,615 in May to 93,790 in June, a period that in most previous years showed a decline after a traditional early spring surge. Roger Conner, executive director of the Federation for American Immigration Reform (FAIR), said the increase indicates the collapse of the "bluff" inherent in the provision of the law that imposes fines and possible jail sentences for employers who knowingly hire illegal aliens.

Conner and several INS officials said the flow of undocumented workers began to drop sharply in November, when the law was signed, because Mexican workers feared immediate arrest and read predictions of mass deportations. After hearing that those things are not happening and that U.S. farm jobs are going begging, they are heading north again, although later in the year than usual.

The apprehension totals for June and the fiscal year remain below last year, which set a record for Border Patrol arrests. This June's figure is 29.7 percent below that of last June, and the October through June total of 805,636 apprehensions is 31.4 percent below the same period the previous year.

But Conner and some INS officials argue that the numbers will continue to return to the old levels until undocumented workers see the new employer sanctions enforced. "It's like spitting on the sidewalk," one official said. "A law isn't really a law unless its enforced."

Conner said that western growers had exaggerated their labor shortage to try to weaken the bill and force new concessions.

INS officials suggested that the new border surge was much smaller in Texas because the economy remained depressed there. But Conner said he thought "in Texas, the grower did not attempt to seize the opportunity to evade the law with an artificial claim of shortages."

Apprehension figures are not completely reliable indicators of illegal border traffic because some aliens are caught more than once in a single month and many escape arrest altogether. Researchers have concluded, however, that a significant rise or fall in apprehensions does reflect a real change in flow across the border.