This year US farmers and growers will use an estimated 1.8 billion pounds of chemical pesticides to save millions of dollars' worth of crops that would otherwise have been lost to hordes of hungry insects.
But the hazards such pesticides pose to humans and to wildlife have long spurred efforts to develop alternative ''biological pesticides'' that carry virtually no environmental risk. Signs that these efforts are beginning to have some practical impact include:
* Pennsylvania Gov. Richard Thornburgh's decision to use a biological insecticide called B.t. in the state's 1983 Gypsy Moth suppression program. The decision was based on results of tests this year which convinced state officials that B.t. (Bacillus thuringiensis) was as effective as chemical sprays, and less expensive. By attacking the digestive system of leaf-eating caterpillars, B.t. causes the pests to stop eating and die of starvation.
* The recent completion of a decade of experimentation on ''ecosystem-derived control agents'' for spruceworms and other pests by a University of Massachusetts microbiologist, Dr. Haim Gunner. He says he plans to develop such agents for a large-scale commercial application. The agents work by aggravating the bacteria naturally found in larvae, thereby disrupting the development of the insect.
In addition, the Rockefeller Foundation, long a supporter of research in the field, is giving grants totaling more than $100,000 a year to laboratories at Harvard, Cornell, and the University of California for research on alternatives to chemical pesticides. The grants will allow researchers to follow up on the success of a program that found ways to strengthen the natural defenses of the cotton plant.
The United States Department of Agriculture has been doing research on biological pesticides for years, but Rick Dunkle, pesticide management coordinator for the USDA, has noted a more urgent interest of late. ''It's been taking off the last few years and is now an integral part of our planning program,'' he says.
For his part, Dr. Gunner sees ''the role of biological controls growing enormously in the next 10 years and chemical pesticides shrinking correspondingly.''
Pesticide alternatives work in a number of ways. Some stimulate the spread of bacteria, which occur naturally, on undesirable insects or weeds to kill or contain them. Other varieties of biological controls disrupt insect breeding or strengthen a crop's natural defenses against predators, as was done with cotton.
Despite the apparent promise of biological pesticides, they are not without their quirks, which are sometimes difficult for growers, who are used to ''spray and forget'' pesticides, to adjust to.
When dealing with biological alternatives, the timing of their application is crucial; they can take anywhere from several hours to two days to do the work chemicals sprays do almost instantaneously.
Effectiveness of biological alternatives can also be hampered when chemical sprays are used in adjacent fields. The need for community cooperation when employing the alternatives has slowed their acceptance by farmers and others.
For those reasons some agricultural specialists say they believe biological controls should be linked to a system of Integrated Pest Management (IPM), a concept that employs several techniques, including both chemical sprays and biological alternatives to achieve the most effective result.
But Fred L. Patterson, chairman of the Council for Agricultural Science and Technology's task force on IPM, says that the long-term nature of biological alternatives development tends to obscure the successes the alternatives have already enjoyed, as in the case of wheat and cotton crops.
At the same time, he says, ''production would decrease and costs would rise if alternatives were forced on farmers right now. It takes a lot of research to make even minor changes.''