Today's ag school boosts science, business - but enrollment is down.

''The old cliche that the only good bug is a dead bug just doesn't hold.'' Theodore Wilson, an associate professor of entomology at the University of California (UC) campus here, has just put the philosophy of his pest-control research in as succinct layman's terms as possible.

Mr. Wilson is describing integrated pest management, a system that does not forswear the use of chemical pesticides, but which combines their use with an understanding of insects and crop economics.

IPM, as it is sometimes called, is one of several key areas of continuing research at the UC Davis College of Agriculture and Environmental Sciences, one of the largest colleges of agriculture in the US.

In another campus building on the flat-as-a-pancake floor of the Sacramento Valley, professors are helping to develop a statewide, computerized irrigation system; in yet another, they're perfecting a controversial method of food preservation using low levels of radiation.

Seated in a cramped office limited by partitions covered with books, charts, and outdoor photography, the bearded Mr. Wilson looks much like the student research assistant he was only a few years ago. ''(IPM) is a systems approach to pest control, which means knowing how the plant grows, and how and when it responds to other organisms and predators,'' he says. ''This way we know better when a little spray will do a lot of good, and when all the spray in the world will do very little.''

In the same way that the smart farmer is learning to ''manage,'' rather than liquidate, pests, Wilson adds, today's student of agriculture is learning to understand better the interrelationship of what goes on in the fields with nature, the dominant urban society - and the farmer's ever-shrinking margin of profit.

UC Davis is perhaps best known for its role in developing the tomato harvester, which cuts down drastically on the labor required to harvest a tomato field. (The harvester is the subject of a lawsuit against UC, charging that the machine and other mechanization projects put farm workers out of jobs and serve the economic interests of large agribusiness concerns.)

But according to Michael J. Lewis, associate dean of the college, the field of agriculture as taught today is much more than new machine development - or learning to milk a cow or plant seeds, for that matter.

''As in so many other careers, the emphasis will be on learning to function with information,'' he says. ''The farmer, as much as any other professional, is being flooded with new ideas and products.'' He adds that as computer use grows to help set everything from irrigation schedules to fertilizer use, ''the 'seat-of-the-pants' farmer is going to be less and less successful.''

According to Mr. Lewis, one of the main concerns of agriculture colleges is falling enrollment. A recent report by the National Association of State Universities and Land Grant Colleges shows undergraduate enrollment in agriculture dropped almost 20 percent since 1978, from 100,000 to about 80,000. The drop in enrolling freshmen is even greater.

Lewis notes that, although the undergraduate decline is less drastic at Davis than at many other schools, it still has deleterious effects. ''For example,'' he says, ''we figure that for every 20 students or so lost, we lose (the funding for) a faculty member.'' At a re-search-oriented institution such as Davis, a decrease in professorships is particularly worrisome.

Cuts in funding or positions would adversely affect two ''hot'' areas of university research: genetics and plant breeding. ''There is a great need to maintain the progress already made in developing better plants,'' says Bill Raines, chairman of the Department of Agronomy and Range Science. ''We need to remain at the forefront of these fields, not dragging up the rear.''

Lewis cites urbanization and alienation from agriculture as reasons for falling interest in agriculture education. ''They see us as 'down on the farm,' rather than the link to what is actually a very urban, very powerful industry.'' Much of the misconception is strengthened in high school, he says, where ''agriculture means 4-H and the FFA (Future Farmers of America). Those programs are fine and wonderful, but they only give one aspect of what agriculture education is all about.''

The task of agriculture schools, he adds, is to spread the word that they are ''a very good place to get a strong scientific education.'' A run through the Davis college's 44 majors is enough to disabuse anyone of the notion that this is a hayseed education. Among the offerings: animal science, agricultural economics, nutrition, textiles, environmental policy, and genetics.

One area where Davis sees no enrollment decline is in its University Extension courses in farm-related subjects, which annually serve about 4,000 adults.

Many of the classes focus on single crops - almonds, grapes, grains - but others treat general topics such as pesticide use. ''The classes are not just for farmers, but for anyone from pest control advisers to food processers and bankers,'' says James Lapsley, a continuing-education specialist. Echoing a theme that pervades any discussion of agriculture education here, Mr. Lapsley notes: ''The people who come to us, right down to the guy whose family has been in nuts for generations, are the progressive ones. The unprogressives won't be around much longer.''