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Page:Popular Science Monthly Volume 72.djvu/364

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360
POPULAR SCIENCE MONTHLY

There is no reason why this method should not be adopted on a large scale, replacing the box by a room in which the window is closed by means of a wire screen. A similar method could be employed with a number of other insects, and quite recently (1907) Silvestri has advised it for the olive fly in arranging a plan of defense against this insect, based both upon the protection to parasites and on cultural methods.

Against the injurious scale insects of the Aspidiotus group it will be particularly easy to put into practise a similar method, and it will give good results. It will not be necessary to burn during the winter time the branches taken from attacked trees, but to collect them in the neighborhood of the infested trees. Aspidiotus, which can live only upon living plants and are incapable of traveling, will thus perish by inanition. Parasites living at their expense can easily, upon issuing, gain the fruit trees. In the same way, if one is using insecticidal measures, it will only be necessary to remove the cut-off branches or cut-down trees to sufficient distances so that they will not be touched by the insecticides. It is necessary to have seen the parasites of scale insects at work; it is necessary to have observed that the immense majority of those which cover a tree are often eaten and perforated by one or several regular, round holes through which the parasites emerge, in order to understand how such measures are justified.

Johnson, in 1899, having put some fragments of twigs covered with the San Jose scale in a series of tubes, obtained more than a thousand parasites (Aphelinus fuscipennis Howard) in each of them. Struck by this observation, he recommended the protection of this Aphelinus by the application of such measures as we have mentioned.

Berlese, in 1902, suggested the same method in the struggle against Diaspis pentagona, one of the most dangerous enemies of the mulberry, in Italy.[1]

There still exists a method entirely different from that which precedes, but whose end is also to protect parasites and to assist in their multiplication. It consists in encouraging, or cultivating in the neighborhood of the plantations, wild plants which harbor them.

Thus, for example, parasites of the olive fly do not live exclusively upon that insect, but also upon certain gall insects of the oak and the briar rose. Therefore, it has been recommended to preserve in the neighborhood of the olive groves, bushes or hedges of these plants, or even to transport galls into the olive groves.

Some authors, struck by the eminently useful part played by parasites in certain invasions of insects, have actually advised the abstaining from destructive measures in fear of killing at the same time parasites

  1. Berlese is of the opinion that winter treatments for scale insects should be discontinued, on account of the great number of parasites destroyed by them. We can not adopt this opinion, for reasons which we shall give later on.