The grim reaper of the federal budget is on the loose again at the Department of Agriculture and this time around he's eyeing such things as the golden nematode, the witchweed, the Mormon cricket and, yes, even the city gardener and little Fido.

Year II of Fiscal Austerity, President Reagan-style, means that a whole raft of programs that have attained lives of their own at the USDA are having their collective tickets punched.

Congress, of course, will be heard from before this is over. But for the time being, the administration wants to cut USDA spending by 20 percent. That's a lot of money--more of a cut, in fact, than any other department that would still be around would face. USDA's current $29.4 billion budget would drop to $23.5 billion under the president's proposals.

The biggest part of these savings would come in rural lending (down $4.3 billion), food stamp changes (down $1.7 billion) and anticipated lower costs for crop-support programs, among other fiscal biggies.

But to get the budget down to $23.5 billion, the money movers looked under some heavy rocks at USDA and put their covetous eyes on some things most people never heard of, such as efforts to control the witchweed, the cricket and the nematode.

On the grounds that the pests are not economically damaging and that states can act on their own without federal aid, USDA proposes to save $8.8 million by ending research on controlling noxious weeds, scabies, grasshoppers and Mormon crickets, golden nematodes and the witchweed.

USDA would discontinue its regulatory, survey and control of the pink bollworm (saving $2.4 million), reduce federal contributions to control of the imported fire ant (saving $3.3 million) and save $3.4 million more by giving states, industry and humane societies primary enforcement of animal welfare activities (that's where Fido comes in). And, $2.2 million would be saved by turning over to Puerto Rico the responsibility for eradicating cattle fever ticks there.

And so it goes, throughout the department's budget for fiscal 1983. Dozens of activities taken for granted by the barn and bean community would disappear. Many others will remain, but users will have to pay a fee for them.

For example, commodity market news reports (now costing $1.4 million) will be put on a subscription basis. Some $2.5 million more would be saved by altering egg product inspections and increasing fees. Some Economic Research Service publications will be discontinued, saving about $1 million; others will be available for a fee, saving an estimated $1.2 million.

The Extension Service, which provides advice and guidance to bigtime farmers as well as backyard putterers through its county-agent system, would see its $3 million urban-gardening program killed, if the budget proposals go through. Ditto a $1 million farm safety program.

From small cuts, obviously, come big budget savings. But of all the USDA branches, the Forest Service would take it on the chin the hardest. The equivalent of 1,751 jobs would be eliminated from the forestry rolls, some of which are researchers working at eight research facilities that are scheduled to be closed.

Budget-making is not just a matter of cutting, however. The new USDA spending roadmap envisions major new income from the sale of timber and minerals from the national forests. USDA will offer 12.3 billion board feet of timber for sale next fiscal year, compared with 11 billion board feet this year. "Special emphasis," as the budget puts it, will be given to expanded leasing of coal, oil, geothermal energy and strategic minerals in the forests.

The budget is even-handed, though, and it doesn't overlook those who may want to contemplate the trees before they're cut or hike the trails before the bulldozers move in. Additional fees, totaling about $36 million, would be charged the public for use of national forest recreation areas.

The idea, a USDA official explained, is that the administration does not want the publicly operated recreation facilities to compete with private enterprise. He added: "The focus of this administration is on revenue-producing areas. We are emphasizing timber and mineral development. We will spend more money on this, but part of it is to try to eliminate the backlog of applications for exploration and development."

But sometimes for every cost saving, there is a new cost. Public-affairs activities (that is, press agentry), for instance, will cost $876,000 less. But new legislation and increased litigation are expected to cost an additional $867,000.

It'll cost a bit more to keep the watchdogs on duty, too. Secretary John R. Block says he'll need an additional $691,000 to operate his office and he's asking for an extra $3.1 million to keep his inspector general vigilant.