Paul Libra, age 13, of St. Paul, Minn., for his question:

WHAT IS THE DIFFERENCE BETWEEN HERBICIDE AND PESTICIDE?

Home gardeners and farmers often use pesticides to protect their plants and animals from insects. Fruit trees must be sprayed regularly to prevent moth larvae from making the ripe fruit wormy. Many cattlemen spray their livestock to protect them from flies, ticks, mites and lice. Insects cause more than $5.5 billion in crop and livestock losses each year in the United States alone.

A pesticide, which is usually called an insecticide, is a substance used to kill insects. Classified according to the way they are made, there are organic, inorganic, botanical and bacterial insecticides.

Herbicides, in the same pest control business as insecticides, are chemical preparations used to kill weeds.

So the difference between a herbicide and a pesticide is clear: both are pest fighters with one specializing in weeds and the other in insects.

Most herbicides are selective, which means they will not harm the crop on which they are sprayed but they will get rid of the weeds. Many are made in the form of gas or liquid and contain carbon, hydrogen and oxygen. One of the first major herbicides developed is called 2 4 D, and it kills broadleaf weeds. Other types include simazine, diuron and trifluralin. Most of the various types of herbicides benefit man greatly.

Although most herbicides are not highly poisonous, they certainly should be stored where pets and small children cannot get to them.

All insecticides, on the other hand, should be considered as poisonous. They should be stored with great care and protective clothing should be worn when the insecticides are used.

Organic insecticides are the type most widely used. Like herbicides, most contain carbon, hydrogen and oxygen atoms. Some also include chlorine, lindane and other chemicals.

Organic insecticides are used on food crops because they do not leave harmful deposits on the food. Inorganic insecticides, which are usually made from minerals, are usually used to protect cotton and livestock, although many are safe enough to also be used on fruit trees.

Insecticides are often sprayed or dusted when used on plants. For animals, various types are available that can be put into tanks for dipping or mixed with insect food.

Both herbicides and insecticides should be chosen carefully and used with great care. Wrong types can kill or injure the plants and animals being treated.

Because of concern over the persistent use of some insecticides, the U.S. government in 1969 announced plans to ban some types. In 1972 it banned almost all uses of DDT, and in 1975 it banned most uses of chlordane.