"WHAT we need this month is one week of a nice, sunny, dry spell," says Dr. Mohamed Khan, pesticide coordinator for the District. "Then we can begin to control some of the pests threatening the well-being of our vegetation. Two or three days of sunshine are just not enough."

The rains of the past few weeks have created "some serious problems," says Khan, a plant pathologist. His office has received a lot of calls about infestations by cicadas, gypsy moths, aphids and slugs. However, he says, there is not much that can be done unless the rains stop for a while.

When that dry spell arrives, Khan has the following recommendations:

* Spray the ground and the trunks of trees with Sevin or Carbaryl to fight the periodical cicadas. Keep the children out of the immediate vicinity of the sprayed areas for a few hours, until the spray settles and dries completely.

Also known as 17-year-locusts, periodical cicadas complete their life cycles in 13- to-17-year intervals. But this does not mean that periodical cicadas emerge only every 13- to-17 years. Different broods emerge in different years, and the brood this year is particularly numerous.

If your garden soil has some holes about half an inch in diameter and not more than two inches deep, you have a cicada problem. Cicadas lay their eggs in the ground and on trees, and this year they are everywhere. Young cicadas look like wasps without wings; later they acquire clear, meshed wings. They damage the roots and the new growths of trees and shrubs. They are a danger to lawns and ornamentals, but not to vegetables.

* Spray Sevin or BT against gypsy moths, or trap them manually with burlap bags.

Gypsy moth caterpillars still are around, and in mid-June, the male of the species will be much in evidence. Gypsy moths look like regular caterpillars. They are hairy, and have five pairs of blue dots and six pairs of red dots. They prefer eating the leaves of oak trees, but they attack other trees as well. They leave vegetables alone.

* Apply Sevin or Diazinon against aphids.

Because of the rains and the flush of new growths, aphids are plentiful, sucking on the juices and causing a great deal of damage to vegetables, flowers, bushes and flowering trees.

Use commercial slug baits, or put out your own bait: potato peels or saucers of stale beer.

Commercial slug baits contain arsenic and their use is not recommended in vegetable plots. Because of the mild winter and the rainy spring, slugs are especially numerous this year, and they can devastate salad greens, some herbs and pepper plants.

* Use Orthene spray or Kelthane against mites. Mites attack all kinds of vegetation.

* Before planting, spray the ground with Diazinon to fight squash borers and cucumber beetles.

* Clean up--remove dead leaves and branches so as not to provide room and board to garden pests.

For the same reason, do not apply a heavy mulch now, but wait until the weather turns drier.

Khan practices what he preaches. In his Silver Spring back yard--where he grows tomatoes, peppers and onions, as well as some ornamentals--he maintains "a rigorous schedule of sanitation and fertilization. I try to be timely. To be on time with spraying and fertilizers is very important." He says he tries to avoid chemicals as much as possible, but, in the final analysis, he can't do without them. Drain Your Veggies

The primary victims of the heavy rains are those who garden in low- lying areas or in places with poor drainage. With the exception of watercress, which likes to live on ground as close to a brook as possible, no vegetable benefits from the kind of soggy weather we have been having.

The best way to keep vegetables from "getting their feet wet" and to avoid the rot and the fungi which accompany wet weather is to dig drainage ditches. And any furrow--even if it is hoed only two inches deep--qualifies as a drainage ditch as long as it collects and channels the water away from the plants.