NORTH MIAMI, Fla. (AP) _ A female adult Mediterranean fruit fly has been discovered in a North Miami orange tree, but Florida agriculture officials said it was too early to tell if south Florida has its second infestation since last summer.

Workers were placing more insect traps today in an 81-square-mile area around the site where the insect was found Monday to determine whether another infestation has occurred, said Ernest Collins, spokesman for the Florida Department of Agriculture.

A four-month, $1.6 million eradication program ended last September in Miami, where four medflies were discovered in a Little Havana yard. The previous infestation was a 1981 outbreak in Tampa.

Medflies lay their eggs in citrus and other fruit, which ruins the crop.

Dr. Howard Weems Jr., a state Division of Plant Industry expert in Gainesville, confirmed the latest insect was a female medfly after the pest was sent by courier to his laboratory. Tests showed the insect had not mated.

''The single find at this point indicates an invasion rather than an infestation,'' division spokeswoman Lindy Perry said. ''If another fly is found, then we'll have the beginning of an infestation on our hands. Eradication and quarantine measures will become necessary.''

Under the regular trapping program, there are 3,146 insect traps in Dade County that are checked every three weeks. Another 1,200 traps, to be inspected at least once a week, were going up in the area around the latest find. Those in a one-square-mile area around the host tree will be checked daily, Ms. Perry said.

Inspectors also were cutting fruit near the host tree in search of larvae.

''We're hoping this is a single fly, that she was a hitchhiker on a ship or a plane,'' Ms. Perry said. ''The worst thing that could've happened is that she's part of an infestation that's already established in the northeast Miami area.''

Florida combats medfly infestations with aerial spraying of a fly bait laced with the pesticide malathion and stepped-up trapping. Under medfly quarantines, food on which the medfly feeds must be inspected before being shipped out of restricted areas.