Bowing to heavy political pressure, the Environmental Protection Agency has authorized three southern states to use a controversial and dangerous pesticide in their battle against bothersome fire ants.

The EPA decision means that Mississippi, Texas and Arkansas will be able to begin aerial spraying of ferriamicide on 14 million acres of infested croplands and wooded areas, where the ants build tractor-impeding mounds and bite humans.

Sen. Thad Cochran (R-Miss.), one of the leaders in the fight for ferriamicide, yesterday praised the EPA decision, saying it would provide welcome relief for beleaguered farmers and land owners.

But Maureen Hinkle of the National Audubon Society said, "This is a discredited chemical that has been given a special exemption because of special pressure applied on the EPA."

Ferriamicide, manufactured by the state of Mississippi, is made from Mirex, a powerful carcinogen that was used to little avail against fire ants for 15 years before Mississippi canceled its use in 1979. Mississippi now contends that ferriamicide, mixed with other ingredients, is not as dangerous as Mirex.

During the Carter administration, with Mirex withdrawn, the EPA approved ferriamicide for use against fire ants but it was not employed after the agency was challenged in court and new scientific data suggested that ferriamicide was more toxic than Mirex.

Pressure for approval continued, however, and EPA last summer convened a scientific symposium to make recommendations on the pesticide. The panel found "no compelling reason" to approve its use and held that the fire ant was not a threat to agricultural production. Other pesticides, though more expensive, are available for fighting the ants, it said.

Although EPA rejected the panel's findings with its decision this week, it still recognized potential perils of ferriamicide by imposing tight limits on its application in the three infested states.