How Insects Choose Their Food Plants

Charles T. Brues.

All forms of animal life need organic materials in order to exist, grow, and reproduce. Some subsist on living, dead, or decaying plants. Others get the foods they require from living or dead animals. Many kinds, including some of the insects, live on a mixed diet of both plant and animal materials. Civilized man has almost endless variety in his diet: Bacteria, yeasts, fungi, roots, berries, fruits, and foliage of plants furnish vegetable food; he eats the flesh of many invertebrate animals such as crustaceans and mollusks, although fish, birds, and mammals commonly furnish his main protein requirements.

No insect selects food in such variety, but a few insects are omnivorous in the sense that they may consume many kinds of plant and animal materials. Most of the more specialized kinds restrict their diet to a limited range particularly the forms that develop as parasites within the bodies of host animals, which almost invariably are other insects.

Such parasitic ones, which are called entomophagous parasites, generally are very specific in the selection of their hosts. They usually lay their eggs on or directly within the body of the host insect and continue from generation to generation to confine their attacks to the same species of hosts.

Predatory insects, which capture living prey just as do the carnivorous birds and mammals, confine their diet to animals smaller or less active than themselves. Quite frequently they also select particular kinds of prey: Some consistently capture aphids, some devour caterpillars, some feed on scale insects, and a few are addicted to a diet of snails. On the other hand, groups like the praying mantids and ant-lions accept and relish a wide range of flesh. Because predators subsist largely on other insects, they depend mainly on the vegetarian kinds, which are the most abundant source of suitable prey.

Parasitic and predatory insects reduce the abundance of plant-eating insect life. Nevertheless, under the conditions that have prevailed in nature for millions of years, their influence has not kept the vast hordes of vegetarian insects from maintaining populations at a high level. Neither has it curtailed their evolutionary differentiation, because they have developed innumerable adaptations in structure and in habits to their environment. Some of the most striking features in this respect relate to the instinctive behavior that determines the selection of food plants.

Farmers always have known that many species of insects feed only on a particular crop or series of crops. They appear season after season and evince an unvarying predilection for the plants that nourished their forebears. There is great variation in the number and variety of food plants they select, but there is a fixity of purpose in their behavior that is far beyond their dietary requirements.

In a search for the causes underlying such selection, we shall consider mainly those species about half of the living species of insects which feed on the flowering plants, particularly those of economic importance, since we have more accurate knowledge of them than we do of most of the insects that are associated with wild plants.

Common in home and market gardens are cabbage, cauliflower, radish, kohlrabi, brussels sprouts, turnips, and collards. These members of the family Cruciferae have a pungent odor and taste because of the presence of chemicals known as mustard oils, which the tissues of the plants secrete. The chemicals attract a series of generally unrelated insects to the plants, on which they may lay their eggs.

Thus, the cabbage butterfly seeks out the cabbage patch in the garden to deposit its eggs. The caterpillars that hatch from the eggs eat and grow to maturity on the plants selected by the parent butterfly. If they are placed on other plants to which they are not accustomed, they go on a hunger strike, doggedly refusing to eat, and finally perish miserably in the midst of plenty. Only if sap of the food plant or mustard oils are smeared on the strange foliage will they recover their appetites and resume feeding. There is a close correlation between the choice made by the butterfly and the fondness of its caterpillar offspring for the kind of food that has been chosen for them.

In some insects, the adult and larval stages feed on the same plants, but the adult cabbage butterfly, like other butterflies and moths, sucks the nectar of various flowers and the laying of its eggs on the larval food plant is not a response to its adult appetite. Any failure of the butterfly to select plants acceptable to its offspring would spell disaster, because the young caterpillars cannot go foraging in search of plants other than those upon which they find themselves. Similar peculiarities prevail among the great variety of diverse insects that restrict their feeding to specific plants.

The sense of smell in adult insects is so much more acute than that of humans that we cannot appreciate its action. In the developmental, growing stages of the higher insects, such as caterpillars and grubs, it is far less acute but equally discriminative, and it is commonly associated with the refusal of any food that lacks the specific stimulus to which their olfactory apparatus is attuned. It is as if a human would eat corn pone only, or cabbage, or onions, or cottage cheese, and never venture a baked potato, hot dog, or ice-cream cone to vary the monotony.

Another example of the association of insects with specific kinds of host plants is the Colorado potato beetle, which spread northward from its native home in Mexico, following its native food plant, a common weed of the potato family. Now widely distributed in the United States, it confines its feeding almost entirely to the foliage of the potato plant. Sometimes it appears on tomato and eggplant, which are related members of the family Solanaceae. Grow a few potatoes in the garden, and the potato beetles will find them sooner or later sooner if your neighbors harbor them, and later if a long journey is required.

The Mexican bean beetle feeds only on the foliage of various sorts of garden beans, cowpeas, soybeans, and related legumes. In recent decades it has extended its range into the northern parts of the United States; wherever it goes, it always seeks out beans. It is hard for humans to appreciate this point, for we cannot perceive anything special about the odor of potato or bean foliage.

The bean beetle is a black sheep of the large family of lady beetles, whose other representatives, eminently predatory, feed voraciously on plant-lice and scale insects, both as larvae and adults. This small group may have become vegetarians in the geological past, for it has had time since to spread around the world and to develop a number of species, each restricted to special plants, such as members of the cotton family in Africa, potatoes in the Orient, and legumes in Europe.

Another American species, the squash beetle, feeds on the foliage of native gourd vines and on several other garden cucurbits. The squash beetle has never reverted to a meat diet; it may well be that its vegetarian habits represent a sudden shift of instinct comparable to the structural mutations that occur sporadically in nature or as the result of experimental techniques.

Of the wild plants, consider the milkweed, which has a milky sap, or latex. The familiar monarch butterfly always lays its eggs on milkweed, which is the only food that its caterpillars will accept. Also on the leaves of milkweed are commonly seen rather large, black-spotted red beetles, which eat the foliage as adults and bore in the roots as larval grubs. Like the monarch butterfly, they are addicted to milkweed and would be starved out of existence without it. In the scheme of nature, they are fortunately provided for by the mother insect, whose unvarying instinct leads her to lay her eggs in the proper site.

An insect that restricts its feeding to a single species of plant is the boll weevil. The larvae of this snout beetle burrow within the flower buds and immature bolls of cotton. It is native to the New World Tropics, whence cotton came into cultivation, and has extended its range into the cotton fields of our Southern States since the beginning of the present century. Thorough search has failed to find any other acceptable food plants, which is a very striking point because another insect, a caterpillar known as the bollworm, similarly bores into the green bolls of cotton but also likes other succulent fruits and vegetables. It can be so abundant in the ripening ears of corn as to rank as a major pest of corn.

Another pest of corn, the European corn borer, now widely naturalized in our country, is a still more general feeder. It bores into all above-ground parts of the plant. It does not stop at that, however, as it appears equally fond of many plants as diverse as dahlias, smartweed, and hemp.

We can group the insects I have mentioned and others as well into three categories. The first group includes the feeders that exercise little choice, depending largely on availability, abundance, texture of foliage, succulence, and the like. Nearly all of them have preferred food plants, however. Thus, the gypsy moth caterpillar feeds on the leaves of a variety of deciduous forest trees, but it is most abundant on oak and on birch, avoids ash, and refuses chestnut, while the older caterpillars in a pinch will consume even tough pine needles after their jaws have become big and strong enough to cope with such material. The common cecropia moth seems to prefer willow leaves, but its diet includes a great variety of our common deciduous trees and shrubs. Many grasshoppers range over a wide variety of low plants. Such insects are known as polyphagous because they accept plants in considerable variety.

The second group, the insects that restrict their feeding to a small and discrete number of usually similar plants, are termed oligophagous. No clear-cut line can be drawn to separate them from the polyphagous forms, but they obviously represent a distinct specialization in food selection, especially when their food plants have some characteristics in common, which we can demonstrate through our own senses or by laboratory methods.

Members of the most highly specialized series, the third group, are referred to as monophagous; that is, they are restricted to a single species of food plant. They are comparatively few in number; indeed, some entomologists believe that none exists in the strictest sense. But to all intents and purposes the boll weevil, whose habits have been minutely studied, falls into this category, and several other insects appear to be just as precise in their tastes. All in all, the vegetarian insects form a vast series in which more or less indiscriminate choice of food becomes more and more restricted and sometime may reach a stage of absolute dependence on a single species of plant.

Such a succession appears to be an evolutionary process, but by no means is it a single progression of changes, as the restriction of food plants appears time and time again in unrelated groups. Rather, it is the indication of an inherent tendency in insects (undoubtedly engendered by their delicate sense of smell) that leads them to live in a world of odors.