FROM superfreeze to superbug.The state that produces half the nation's fruits and vegetables has exchanged one disaster for another - and the cost may show up on holiday produce bills. Nearly one year after the coldest temperatures in history decimated California's citrus crop, Gov. Pete Wilson Monday declared a state of emergency in Imperial and Riverside Counties for infestations of the poinsettia whitefly. A new strain of the sweet-potato whitefly that has bothered farmers here for decades, the pest has been blamed for putting 2,500 farm workers out of work and destroying $90 million in crops - from cantaloupes to broccoli, cauliflower to cotton. The infestation is being called the worst natural disaster ever for the two counties. "There is great concern statewide because the number of host plants this bug will attack is enormous," says Bob Krauter, spokesman for the California Farm Bureau Federation. "When it gets done with one crop, it just moves on to the next." Each the size of a pencil tip, the flies gather into flurries and descend on plants like flecks of white paint flung from a brush. The bugs drain fruits, stalks, and leaves of fluid. Besides leaving a gooey excretion on lemons, oranges, and grapefruits, they clog picking machinery for cotton. Eighty percent of such crops as cantaloupes have been ruined in the two counties, which also produce half the country's winter vegetables. Large tracts of carrots and alfalfa have been written off completely. And infestations have been reported in several locales across Arizona and Mexico, as well. "The nation can count on higher prices for winter vegetables - but we don't know how much or for how long," Mr. Krauter says. Ben Abatti Sr., a farmer in Imperial Valley since 1956 has never seen a worse situation for agriculture. Loss of 1,900 acres of cantaloupe will cost him $1.8 million. The fly is particularly pernicious because it does not respond to pesticides and has no known predators. "We're doing everything we can to keep them from coming here," says Gary Kunkel, chief deputy commissioner for Tulare County Agriculture Commission, where just a few flies have been seen in greenhouses. Since the fly is known to seek out weeds to last through colder weather, such plants on roadsides, ditches, and near buildings are being tilled under, as are affected crops. Experts at state universities say predators such as lady beetles have helped lower numbers of other kinds of whiteflies. More research is being done into a stingless wasp, which has been effective against the ash whitefly. The University of California at Riverside unveiled plans Monday for a five-year research program for the whitefly. A concerted statewide effort in the early 1980s helped minimize damage from the common whitefly. Similar formalized efforts have already begun in several counties. Farmer Abatti says he is relying on a low-interest loan from the Federal Emergency Management Association for next season's planting. Beyond such relief, the governor's declaration is expected to do little beyond aid unemployed workers with special insurance.