In its unceasing struggle to rid us of multi-legged pests, the Agriculture Department has resorted to hitting below the belt. The victim is the Mexican bean beetle, which USDA hopes to eradicate by introducing what it delicately refers to as "a social disease" that is spread by amorous beetles. "The social disease is actually a parasitic mite that lives on the beetle's body," according to a department report. The mite, an import from Central America, is "100 times smaller than the beetle." It "feeds on the underside of its host's wings and waits for an opportunity to crawl onto another beetle. That chance arrives when the beetles mate." The bean beetle is a major destroyer of soybeans, limas and snap beans. USDA's hopes are based on observations in Central America that "in fields where the beetles were nondestructive, close to 100 percent . . . were infested with mites." Less optimistic, meanwhile, are scientists working for the Energy Department on a better mousetrap. Field mice are a serious energy problem because they gnaw through insulation, wires and other electrical components. DOE sought help from Lawrence Livermore Laboratory in California and scientists there came up with a device that looked foolproof: Bait is placed on the high end of a beam set up like a seesaw; the mouse walks out to get it; the beam tips and the mouse is dumped into a container of water where it drowns. The beam then drops back into place and the trap is ready to go again. This month the scientists called in reporters to show off their device -- only to find that mice seem to be getting smarter. While one climbed out to get the bait, another sat on the near end of the beam to keep it from tipping. Back to the drawing board.