Three airlines agreed yesterday to resume spraying DDT in West Coast-bound airplanes leaving Japanese beetle-infested Dulles International Airport. The airlines were warned that unsprayed flights might be grounded.

By yesterday morning Trans World and American airlines had followed the lead of United Airlines in ordering ground crews to halt spraying out of fear of damage tht the commercially banned pesticide might do to passengers, crew members and the airplanes.

But at a morning meeting arranged by the Air Transport Association, a trade group for major airlines, U. S. Department of Agriculture officials made clear their determination to continue spraying. "The bottom line of the discussion," said one ATA spokesman after the meeting was Agriculture's culture's implied threat to invoke quarantine powers against unsprayed planes.

Fearful of damage the ravenous beetles might do to California's $8.9 billion agricultural crop. Agriculture has been conducting sprayings without public announcement since 1963, according to King Lovinger, a department spokesman."There never was any secret about it," but the department never had any reason to make an announcement about the action, he said.

DDT has been banned from commercial sales since 1970, but the Environmental Protection Agency has approved use of a DDT mist, combined with a popular garden pesticide called "Sevin," aboard the airplanes, Lovinger said. The spray is used in minute quantities aboard empty air-craft - only eight quick squirts per Boeing 707 - and poses no threat to human health, he said.

Shortly after the airlines were told of the limited nature of teh sprayings and given assurances that the spray will not corrode the aircraft, spokesman for the three airlines announced separately that they were resuming the sprayings.

By then one American, one TWA and five United flights had left Dulles unsprayed. Flight crews at three other airfields declared infested with the beetles - Cincinnati's municipal airport, Dover Air Force Base in Delaware and McGuire Air Force Base in New Jersey - continued spraying - yesterday without controversy.

The dusting at Dulles began Sunday and should end by Setp. 1, the windup of the East Coast beetle season, Lovinger said. In previous years, planes leaving Washington National and Baltimore-Washington airports also have been sprayed, he said.

Agriculture officials said yesterday they were baffled by the airlines' sudden opposition to the program, "I don't know why in the world this has happened now," Lovinger said. The department is planning to replace the DDT mist with a new pesticide that does not contain DDT, but Lovinger said he has no idea when the new product will be available.