The Mount St. Helens volcano has already caused "the largest devastation in recorded history in this country" and will now give way to more conventional disasters of fire, floods, and a plague of insects unless the area is properly managed, the U.S. Forest Service said yesterday.

Releasing a draft environmental impact statement for eight proposed management alternatives. Forest Service Chief R. Max Peterson said the 9,000 acres of felled and standing dead trees around the volcano not only present "potentially quite high" dangers of fire but will provide food and breeding ground for beetles and other insects that "could b e an epidemic" in healthy forests nearby.

It will take 50 to 150 years or longer for the devastated forest to regenerate, the report said, but much of the economic impact on timber products workers in the Washington state area will be cushioned by a flood of tourists once the volcano cools off.

No one knows when that will be, however, and meanwhile the immediate are is restricted to scientists.

Of its eight proposed management plans, the Forest Service prefers one that calls for establishment of a "geologic" or "interpretive" area of 90,000 acres around the volcano, with additional peripheral areas for recreation, botanic preservation, wildlife management and research for a total of 148,000 acres. Estimated cost would be $180 million, half for road reconstruction and all but about $20 million offset from tourist and timber salvage income. Much of the land is federally owned, but this plan envisions purchase or "cooperative management agreements" for about 30,000 acres.

The other seven alternatives provided different acreage in each category.

The May 18, 1980, blast that eradicated the north face of the mountain in Washington state left fine black sand up to 18 inches deep on the surrounding countryside. The sand, in turn, is covered with 15 inches of pumice and capped with a crust of fine but dense ash up to six thick, the report said.

Highly subject to erosion by both wind and water, this cover has already blown out over the Pacific in one October storm and will continue to cause haze in the Northwest for "a number of years," the report said. There is no scientific verdict on the hazard of breathing the ash, and monitoring stations have not found enough sulfur oxides or other gases to damage human health.

But the eroding ash makes soil stability a problem that is "much more extensive with the fall and winter rains," with mudflows expected to cause "extensive damage" to roads and bridges east of the mountain. A third of the roads in the vicinity are already gone. The area receives 65 to 140 inches of rain a year, and winter flood peaks are predicted to be up to 20 times higher than normal for lack of vegetation, the report said.

Ironically, 8,255 acres that were fertilized and seeded on an emergency basis by helicopter last fall in an effort to stabilize stream banks may become a fire hazard when the grass dries out next summer, the report continued.

Fire worries will continue for a long time, at first because of the decaying wood and frequent lightning storms and later because of heavy tourist traffic and the fact that the new trees will all be the same age and equally vulnerable to fire, the study said.

For those reasons and because of the potential insect problem, the report recommended intensified timber salvage efforts before the spring of 1984, when the beetles are expected to hit their peak. Most of the felled wood can be salvaged for five years from last May, and the study said 762 million board feet, about half of that available, could be retrieved under the favored management plan, bringing $300 million to $500 into the U.S. Treasury.

The proposed plan does not include wilderness area designations, despite environmentalists' recommendations that part of the mountainside be left untouched. Such a classification would require removing all the equipment being used by the 120 scientific projects now under way as well as all monitoring equipment, and "we'd look pretty silly recommending that," Peterson said.