INSECTS INJURIOUS TO AGRICULTURE 363 



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 Intro- 

 duction 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 homo- 

 geneous 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 with- 

 out the parasites which are especially adapted to live at its expense, or 

 often, indeed, without a single one of its natural enemies, and then find- 

 ing 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 astonish- 

 ing 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 Braconidas, Apanteles glomeratus, which is a parasite of 

 the larvae of the cabbage butterfly (Pieris brassicce). 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 car- 

 dinalis, 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. 



