Open main menu

Page:Popular Science Monthly Volume 72.djvu/365

This page has been proofread, but needs to be validated.
361
INSECTS INJURIOUS TO AGRICULTURE

which they harbor, or the predaceous insects which destroy them. This way of looking at the question is very exaggerated. It is only in case where the parasites constitute restricted and very localized centers of contamination that this idea can hold for these determined points, admitting that it will still be possible to utilize such centers of propagation. In the great majority of cases, on the contrary, it must be said that however useful parasites may be, the fear of destroying them ought never to prevent the undertaking of all measures having for an end the direct destruction of the injurious insect. Parasites act, in fact, only at a more or less long maturity, and admitting that with an invasion of caterpillars the majority or even all harbor in their interior larval parasites, they will none the less accomplish the greater part of their depredations in a manner quite as complete as if they were not parasitized. Should we, then, allow them to devastate a field or orchard in order that the parasites can, the following year, accomplish their beneficent work? An intervention with destructive methods, far from being dangerous, will permit us, on the contrary, always to obtain a double result: first, it will immediately stop the damage and save in a more or less complete manner the products of that year; and, second, it is not likely in the great majority of cases that the caterpillars will be more abundantly parasitized in that particular spot than in any other portion of the country; and in destroying a certain number of non-parasitized caterpillars, one will diminish for the whole region the number of possible adults which would assure the generation of the following year, and that without changing the existing proportion between the parasites and the representatives of the injurious species.

The assertion that insectivorous birds can cause more harm than good by attacking either the useful species or larvæ parasitized by them, does not appear to us well founded and seems to us to be refuted by analogous arguments. In spite of the thesis formerly proposed by Perris, and ably defended of recent days by Berlese and Severin, the protection of insectivorous birds appears to us not at all as susceptible of thwarting the beneficent action of useful insects.

Utilization of Indigenous Insects in the Fight against Indigenous Injurious Species.—Aside from the intelligent protection which should be given to beneficial insects and which, as we have just shown, can be based only upon exact knowledge of their biology and the relations which they have to other organized beings, can man assist in artificially multiplying them, and making of them a forced subject to his will which will serve him at will in the struggle against indigenous enemies of cultivated plants—those which for centuries have devastated our prairies, fields, orchards and forests?

The fungous parasites and microbes have already been brought into our arsenal, from which we draw against the enemies of agriculture. Can we bring in entomophagous insects in their turn?