A little time spent giving the garden a good cleanup this fall can pay big dividends next year. It can prevent the over-wintering of insects and plant diseases that would otherwise survive the cold.

The European corn borer, which attacks dahlias, asters, chrysanthemums and gladiolus, spends winter above ground in corn stalks and other big-stemmed plants. Protected it can survive temperatures of 15 to 20 degrees below zero. In the spring those that make it emerge as moths and the females each lay up to 400 eggs, which hatch in early June. Toss what's left of the big-stemmed plants in the garbage can.

Another pest, the iris borer, hatches from eggs laid by moths during the fall on old leaves. Rake the leaves up and get rid of them. This will eliminate many of the eggs. Don't pull off new leaves.

Leaves that fall from apple and cherry trees should be raked up, too. Fungi that cause apple scab and cherry leafspot over-winter on the fallen leaves. While you're at it, pick up the old fruit on the ground under apple, peach and plum trees and remove fruit mummies from the trees. This will lessen fruit rots and insects next year.

Old berries remaining on grape vines should be picked off and disposed of, and the same goes for old leaves on the ground. This will help prevent several diseases, including black rot, from overwintering.

The perennials can use a boost in autumn, too. Cut off old peony stalks about one-fourth inch below the soil surface and get rid of them. This can help prevent botrytis blight, a disease which attacks young shoots when they emerge from the ground in the spring and which causes flower buds to turn brown or black.

Cut off old stems of phlox just below the surface, to help prevent leaf blight next summer.

The hollyhock rust organism overwinters on old leaves and stems: these should be cut off, raked up and removed from the garden.

Dead stems on roses should be cut off and disposed of. Long stems which might loosen the roots in the soil when whipped around by winter winds should be shortened. Rake up and dispose of old leaves on the ground.

Water lilies growing in small pools can be protected with a covering of boards topped with a layer of leaves or straw.

Q: My Irish potatoes were practically ruined by potato beetles this summer. How can they be controlled?

A: The potato beetles problem has been a changing one. Following World War II, excellent control was achieved with DDT, Dieldrin and heptachlor. But the beetles soon developed resistance to them. Next spring contact your county extension agent for advice.