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Page:Popular Science Monthly Volume 72.djvu/367

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INSECTS INJURIOUS TO AGRICULTURE

thus assisting in their spread. In any event it suffices to say that actually it appears applicable in a rather limited number of cases.

 

2. Perturbations brought about by Accidental Importations. Reestablishment of the Equilibrium by the Introduction of Predaceous Insects and Parasites

The point of view becomes quite different if, in place of considering the perturbations which man has provoked by the substitution of homogeneous cultures for the primitive vegetation of the soil, we look at what he has accomplished in accidentally introducing, by commerce, a plant-feeding insect into a country where it had not previously existed and where it finds conditions favoring its development. It is readily understood that the chances are great that this species will be introduced without the procession of parasites and predatory species which limit its propagation in its original home. It can notably be imported without the parasites which are especially adapted to live at its expense, or often, indeed, without a single one of its natural enemies, and then finding itself unhampered in its multiplication, the injurious species takes prodigious strides and becomes a scourge infinitely more redoubtable than in its own country.

In such a case everything indicates the value of an endeavor to reestablish the equilibrium by introducing into the invaded country all the auxiliaries capable of checking the plague.

In the United States it has been ascertained that nearly one half of the injurious insects of the first importance are of exotic origin and have been accidentally imported into the country, so it is not astonishing that it is America which has started the method which consists in fighting the enemies of agriculture by means of their parasites and that in that country it has taken on a prime importance. After several fruitless efforts with different insects, Riley, in 1883, succeeded in bringing about the first true acclimatization of a beneficial insect, in importing from England into the United States a small hymenopteron of the family Braconidæ, Apanteles glomeratus, which is a parasite of the larvæ of the cabbage butterfly (Pieris brassicæ). This experiment, however, was only against an enemy of secondary importance, and in order to popularize the method a striking success was necessary—an unprecedented triumph against one of the most redoubtable enemies of cultivated plants. This occurred with a small Coccinellid, Novius cardinalis, which brought about this decisive victory. The history of this insect and the work which it has accomplished in the country to which it was introduced is of such importance that we will give a somewhat detailed recital.