Marching, marching, munching, munching, the relentless fire ant is still waging guerrilla warfare in the South, and all 18 of the region's U.S. senators want Uncle Sam to mount a new search-and-destroy mission against it.

They are pressuring the Environmental Protection Agency to grant a conditional-use permit for aerial spraying and ground applications of ferriamicide, a controversial pesticide conceded to be a risk to human health.

An EPA spokesman said yesterday that the agency is reviewing an application from Mississippi, which manufactures the compound at a state-owned plant, and will make a decision in several weeks.

If EPA approves ferriamicide, Mississippi can market its chemical in the nine southern states where the fire ant has infested more than 230 million acres of farm, forest, parks and yards. The ant slipped into Alabama from South America about 50 years ago and has been foraging across Dixie ever since.

Ferriamicide is made from Mirex, a potent pesticide used widely for 15 years before Mississippi voluntarily canceled its use in 1977 just as EPA was about to ban the product. Mirex caused cancer in laboratory animals and was considered a danger to humans.

EPA's spokesman said "a helluva lot of pressure" has come from Capitol Hill, as was the case in 1978 when southern legislators orchestrated a "grass-roots" letter-writing campaign to EPA for approval of ferriamicide.

During that dispute, EPA held that ferriamicide was just as toxic as Mirex but found that it degraded quickly and posed no significant long-term risks to human health. EPA approved ferriamicide but put strict limits on its use.

Use of ferriamicide was delayed after the Environmental Defense Fund sued. Then Canadian research indicated that ferriamicide was more toxic than Mirex, the issue was bogged down in debates between scientists and Mississippi finally gave up on plans to make and distribute its product.

The fire ant has gone right on marching, but there is a different twist to the story this time. Whatever EPA decides, the welfare of a private chemical firm, American Cyanamid, will be affected. Since Round One, Cyanamid has marketed a new and much more expensive pesticide effective against the fire ant.

Cyanamid concedes it is watching EPA closely, and it has hired former Georgia congressman Dawson Mathis to help make the case for Amdro, the firm's new product. Mathis, from a fire ant-infested area, was a champion of Mirex before he left Congress.

"We have no view on ferriamicide," a Cyanamid spokesman said, "but since 1979, none of the facts has changed. We'd just like to see that any new product goes through the same review processes, through the same hoops that Amdro had to go through."

In the view of the southern senators, the fire ant's continuing depredations do not leave time for putting ferriamicide through time-consuming hoops. Sen. Thad Cochran (R-Miss.) and 16 others wrote a joint letter this month asking EPA administrator Anne M. Gorsuch to approve ferriamicide.

Sen. Mack Mattingly (R-Ga.), declining to take sides for a particular pesticide, wrote separately to Gorsuch.

An aide to Cochran said research conducted by Mississippi State University has changed some facts about ferriamicide. Dr. Carl Alley, an MSU researcher, said yesterday that tests indicate the compound degrades more quickly than the Canadian studies had indicated.

On the other side, Rep. George E. Brown Jr. (D-Calif.), chairman of the House Agriculture subcommittee that oversees agricultural-pesticide legislation, also has written Gorsuch, urging "painstaking and authoritative scientific analysis" before she makes up her mind on ferriamicide.

Rep. Doug Walgren (D-Pa.), chairman of a House science subcommittee, told Gorsuch in a letter yesterday that he is "extremely disturbed" that EPA would consider approving ferriamicide "without adequate scientific evidence that the pesticide is safe."

Rep. E (Kika) de la Garza (D-Tex.), chairman of the Agriculture Committee, said this week that he is not taking sides but opposes EPA "waiving any existing safeguards." He said, "The fire ant is out of control, but we want to protect against any potential long-lasting damage to humans and animals."

The fire ant, notorious for years in the South, builds large mounds, some three feet high or more, which obstruct and damage farm implements. Its bite infects animals and humans. Pesticides seem to stop it, but only temporarily.

The dispute about ferriamicide is not likely to be the last heard about the fire ant this year. Southerners are upset about President Reagan's plan to cut $3.3 million from the Department of Agriculture's fire ant eradication budget.

As fate would have it, Cochran is chairman of the Senate Appropriations subcommittee for agriculture. His House counterpart is Rep. Jamie L. Whitten (D-Miss.). And Mississippi, which makes ferriamicide, is infested with fire ants from border to border.

"I would say you'll see the money back in the budget for fire ants," a Cochran aide said.