Natural Enemies of Insects

That insects have a host of natural enemies which constantly prey on them is as certainty. If it were not so, and if our pests reached the full limit of their powers of multiplication unchecked, there would shortly be no living plant left on the face of the earth, and no trace of animal life. Insects possess preeminently the ability to multiply rapidly and to spread widely. Therefore it has been computed that the progeny of one plant louse in a single season, if allowed to multiply at the maximum rate and if none suffered accidental death, would make a mass of matter equal in weight to that of the earth.

Among the higher animals that destroy noxious insects birds are entitled undoubtedly to first rank. Few of us appreciate their services. Scores of species depend largely on insects for their food during a part of the year, if not throughout all of it. Among our best friends are the swallows, chickadees, cuckoos, the kingbird, catbird, robin, bluebird, and the woodpeckers; but this list is merely suggestive.

Birds are peculiarly fitted for dealing with outbreaks of injurious insects. Possessed of the power of flight they can flock to places where insect pests that they enjoy are in abundance. At the same time they are not bound to maintain a species at reasonable abundance in order to protect their source of food and keep it from disappearing entirely, as is the case with many insect parasites.

Toads are entitled to prominent rank as destroyers of insects. The number of specimens consumed by them in a season is enormous. Other animals that live on insects to a considerable extent are skunks, moles, and field mice.

Efficient enemies of aquatic insects, or such as spend part of their life beneath the water, are various species of fishes.

Predaceous and Parasitic Insects

The greatest inroads in the ranks of injurious insects are made by other members of the same great class itself, by the predaceous and parasitic insects.

In general, we speak of predaceous insects as those that attack and feed on other insects or animals of various species, but are not dependent on a single individual host for their existence. Thus the dragon fly, poised in air and waiting to pounce on some unwary gnat or fly, is predaceous.

A parasitic insect, on the other hand, usually is highly specialized for existence on some particular species of host, and has reached such dependence on its host that if the latter dies before the parasite has completed its life round, the parasite perishes. Many parasitic insects live within the bodies of their host. A familiar example is found in the species that lays its eggs in the body of the tomato worm, the parasitic grubs finally gnawing their way to the surface, where they spin tiny, white cocoons on the body of their host.

For most of us, the large amount of work of parasitic and predaceous species in destroying insect pests' passes unnoticed. It is brought to mind when we see or hear of a bad outbreak of some injurious insect, and later observe that the threatening species has suddenly grown scarce, sometimes seemingly disappeared from the face of the earth in the very localities where it had been abundant. If we were to follow up such cases carefully, we should find, as a rule, that as soon as the threatening species began to grow excessively numerous, someone of its enemies, stimulated by the abundance of food, increased so rapidly that with the next generation or the next season the injurious species would possibly be wiped out of existence.

In truth, there is a sort of natural balance between the numbers of a given species of insect and those of its enemies. If the insect increases abnormally, the parasites are stimulated to heavy increase and the numbers of the host are rapidly diminished. If, on the other hand, the host decreases abnormally, the parasites perish from lack of food, and therefore, freed temporarily from their attack, the host is enabled to increase once more.

Consideration of the above law helps greatly to explain the fact that injurious species imported from a foreign country are so often intolerable pests. We have brought over the host without its enemies. Finding conditions here congenial, it multiplies to excessive numbers, escaping the attack of the parasites that would have taken it in hand in its native home.

Occasionally, it has been found possible to import artificially the parasites of an introduced pest, and to establish them successfully in this country. But the venture is tedious and exceedingly difficult. Some condition of weather or temperature is found unfavorable to an essential parasite; or it is discovered that in its new home the parasite itself is attacked by some enemy from which it was free in its native land. Rarely, however, the experiment is successful; and the few instances that do work out satisfactorily abundantly repay for the labor and cost of all.

The most extensive attempt at the introduction of parasitic and predaceous enemies from abroad for the control of a menacing pest is that now in progress, under the auspices of the Bureau of Entomology of the United States Department of Agriculture, in the fight against the gypsy moth. At least ten or twelve species must be collected, imported, and colonized successfully in order to make this work a complete success; but there is excellent prospect of ultimate accomplishment; and meanwhile, no other known measures will avail to limit the ravages of the pest.

Among our valuable families of predaceous insects are the ground beetles, tiger beetles, and lady bird beetles in the order Coleoptera; the assassin bugs and many aquatic forms in the order Hemiptera; the dragon flies constituting the order Odanata; and several families in the two winged flies or Diptera.

Two orders furnish the majority of our parasitic species, the Diptera and the Hymenoptera. The number of parasites within these orders is enormous. The principal families include the Tachina flies and Syrphus flies in the Diptera, and the Ichneumons, Braconids, and Chlcids in the Hymenoptera.

Fungous and Bacterial Diseases of Insects

Fungous diseases of various kinds attack insects, and occasionally are responsible for their death in large numbers. A familiar example is seen at the close of every summer when many of our common house flies may be observed clinging to the ceiling, their abdomens distended and covered with a fine, powdery substance, while a ring of the same light powder surrounds the insect's body on the ceiling.

Nearly every season specimens of the common tent caterpillar may be found hanging from a leaf or twig, the body misshapen and covered with a powdery growth. Millions of brown tail caterpillars have been killed in the New England states in the last two years by a fungous disease.

Attempts have been made to propagate these diseases artificially. A fair measure of success has sometimes attended these efforts, but the undertaking is difficult, owing to the fact that certain conditions of weather and moisture appear to be essential to the growth of the fungus.

Caterpillars of some species are attacked also by bacteria of one or more species. Thus, the gypsy caterpillar occasionally dies off in large numbers from the effect of a disease due to a species of such organism. The same or a similar disease attacks also the common tent caterpil