To spray or not to spray: That is the question many gardeners - especially new ones - ask about insects in their vegetable gardens. The answer: It all depends.

Insects likely to be most troublesome are spider mites on beans and other plants, Mexican bean beetles on beans, and aphids on peppers, tomatoes and potatoes.

If the infestation is not very heavy, many insects can be washed away with a hose. Bean beetles can be picked off by hand.

The problem with spraying is that it also kills beneficial insects that help control predators.

But if the crop is seriously endangered, spraying is advisable, using sevin for Mexican bean beetles and spectracide for mites and aphids.

The Mexican bean beetle is yellowish brown with 16 black spots on its wing covers.The lady beetle that feeds on aphids, and sometimes on other small insects, is orange or red with 12 black spots. Aphids that feed on bean, beet, spinach and other plants are green to bluish black, or sometimes greenish to pink or red, and tiny. Mites are the smallest creatures visible in the garden; they arereddish brown or pale in color and may be covered with a fine web.

It is important when using insecticides to read the directions and precautions on the label and to follow them closely.

Scientists believe about 10 percent of all insects - and there are billions - may be beneficial to mankind. Bees carry pollen to many of our most important crops; ladybird beetles feed on aphids (plant lice). Spraying can kill many of these friends.

Insects have existed for 50 million years and have demonstrated a remarkable ability to adapt to changes in their environment.

After sterile males were released into the environment, the annual number of screwworm infestations in the United States dropped from 50,000 to 150 in eight years. Then, unexpectedly, the number exploded to 90,000 cases. The most plausible explanation is that the females altered their normal habits and mated only with normal males. After great promise, everything had to start over again.

One thing that helps in diagnosing bug problems is knowing the names of plants. In books and pamphlets, most pests are organized on a host plant basis.

Fungicides are used as a preventive measure against plant diseases. They form a barrier and prevent infection. To be successful with fungicides, you must make the right diagnosis and use the right material with the right method at the right time. If this is not done, you risk wasting both time and money.

Herbicides are used to kill unwanted weeds and gasses. Be sure to read the directions on the label.

Federal pesticide laws prohibit the use of a product in any way that is not consistent with label directions. So you must read the label to determine if it is registered for use on the plants that are in trouble.

There is a strong trend now in biological control that involves discovering, importing and using effective natural enemies of pest insects or weeds. For example, in 1975, USDA Agricultural Research Insects Laboratory, Newark, Del., began the introduction into several states of the 7-spotted ladybird beetle, the most important aphid predator in Europe, Asia and North Africa. It is now established in several parts of the United States.

Beetles were released in the area of Byron, Ga., in 1976 and in the spring of 1977 were found to be the most abundant aphid predator on the experimental farm's legumes and fruit trees. CAPTION: Illustration, no caption