The history of injurious insect introductions from one country to another have shown almost invariably that the introduced species has become more abundant and more destructive in its new home. This comes about because of the upset in the balance of nature which in the insect's native home has reached more or less an equilibrium. The history of insect introductions indicates that as a rule a great many years have to elapse before parasites and predators in the insect's new home become sufficiently adapted to the new host and numerous enough to restore the balance of nature. This has led to extensive work in the biological control in this country, principally by the Bureau of Entomology and a few of the states, in which an effort is made to hasten the return of the equilibrium between the insect and its enemies observed in its native home. If we understand the main object of this work correctly it is to discover those species of insects which are responsible for holding the injurious insects under reasonable control in its native home and to import these parasitic insects into the new home with the hope that they will take hold in the new environment and in a comparatively short time re-establish the desired equilibrium. The gypsy moth and the Japanese beetle are good examples of insects introduced into this country where investigational work of this kind is being pursued. Other examples might be cited.
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Pepper et al. (1934) studied this question.