Plants aren't at the mercy of insects

At a season when insects are feasting, it's worth remembering that plants have strong natural defenses against them. Some of the subtlest, but most effective, weapons are chemicals.

Entomologist John C. Reese at the University of Delaware says that even plants considered susceptible to attack produce chemicals that retard insect growth. Specifically, he has found that varieties of corn susceptible to cutworm attack actually are well protected.

Plants may produce substances that are toxic to attacking insects. They may also reduce their nutritional value for such predators. One of the most surprising tactics, which has not yet been fully proved, is for infested plants to warn others that danger is imminent.

David Rhodes of the University of Washington has been studying this possibility because of what he calls ''funny results'' over the past three years from research with alder and willow trees. He has found that trees attacked by tent caterpillars produce chemicals that make their leaves less digestible or less nutritious to the caterpillars. But, more than this, uninfested trees in the same stand, or in nearby stands, also change their leaf chemistry.

Mr. Rhodes explains: ''If both infested and nearby uninfested trees are changing, that means the infested trees, or the insects themselves, are sending a message to the uninfested trees, which is triggering some defense mechanism.''

While such a message could flow between tree roots, he says he thinks it more likely is wafted through the air. Mr. Rhodes and his associates have even triggered the warning effect in one instance by plucking off about 4 percent of a tree's foliage.

Mr. Rhodes calls all of this ''strongly suggestive.'' But he says it is not proof. Much more research still is needed to identify any messenger chemical and elucidate its role. Nevertheless, there already is both hope for better pest control and a warning to proceed with caution. Plants have evolved complex means of living with insects - means that are nowhere near to being understood. Humans must be careful not to upset them.

Jack C. Schultz, a Dartmouth College biologist, points out that in a natural forest these means are very effective. Spectacular pest outbreaks, such as gypsy moth infestations, are rare. Indeed, he says, the plant-eating insects normally are sparse.

United States Department of Agriculture chemist Robert D. Stipanovic, making the same point, has noted that the natural toxins made by plants usually are weak. They retard insect development, rather than killing them outright. During an American Chemical Society symposium this spring, he warned that insects could develop resistance to the natural plant defenses if humans try to use similar chemicals deliberately. This, he said, ''could negate millions of years of evolutionary adaptation.''

Plants, it seems, are trying to tell us something. In our efforts to combat insect pests, we should take care not to cripple the plants' ability to help themselves.