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Page:Popular Science Monthly Volume 72.djvu/422

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eral use of sprays to be stopped. The two methods are not incompatible, for in a given region it is very rare that you can regularly spray all of the trees. This one, or others, will not be treated, and the ladybirds and other useful insects will therefore have a free field to carry on their beneficial work, and centers from which they can be dispersed will be created.

One of the greatest dangers in introductions consists of the possibility of introducing into a region an animal which considered as useful in its original home, is capable of becoming absolutely injurious in the new country into which it is introduced, on account of the conditions of the environment which it encounters. The examples of the sparrow imported from Europe into America and Australia, of the mongoose introduced from the East Indies into the West Indies, of the rabbit imported from Europe into Australia, are too well known to be described. It has been stated that no danger of this sort exists in such cases as these, since parasitic insects of other insects can live only at the expense of these last, and it is the same with predaceous insects. There is no doubt of this, but there exists another danger of a direct character in the importation of the insects which are desired for acclimatization, and that is the danger of importing at the same time either injurious insects sent along as food, or hyperparasites which can prevent the propagation of the useful insects and which becoming acclimatized themselves, endanger even certain useful indigenous species.

It is very easy to take the necessary precautions so that the insects which serve as food during the journey should present no danger, and it will suffice to make sure that they belong to a species existing already in the region where they are to be acclimatized.

The history of the naturalization of Icerya purchasi in Florida shows us that the method of utilization of beneficial insects, practised by incompetent people, may have sad consequences.

As to the danger from hyperparasites, while it is apparently not so serious as the preceding, it is, on the other hand, much more difficult to avoid. Preliminary rearings are necessary before the beneficial species are definitely set at liberty, and all precautions are necessary after the issuing to separate the primary parasites from the hyperparasites. It is for this reason that the application of the method of the utilization of beneficial insects, in order to render all the services which are expected of it, should be carried on indispensably and exclusively by learned men, especially informed concerning insects and their reciprocal and biological relations.

We have shown in this memoir about all that can be drawn from the utilization of predatory and parasitic insects in the struggle against enemies of crops. One conclusion may be drawn also from this study,