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Page:Popular Science Monthly Volume 72.djvu/363

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359
INSECTS INJURIOUS TO AGRICULTURE

have elsewhere insisted upon the fact that one of the measures most often recommended—the destruction of the stubble remaining in the field after the harvest—may have unfortunate consequences, for doing this in a tardy manner one risks intervening at a moment when all of the flies have emerged and have abandoned the stubble, exposing to destruction only the parasites whose part would have been to stop the invasion the following year.

Kieffer has pointed out a remarkable analogous fact for a Cecidomyiid, namely, Diplosis tritici, attacking not the stubble, but the grains of wheat, and has shown that one of the measures which has been advised—burning the debris after the threshing—has only an injurious effect, for while it is true that this debris contains pupæ of the midges, it should be remembered that the healthy and nonparasitized larvæ of these flies transform in the ground, while those which remain in the heads are, on the contrary, parasitized.

In the cases which we have just mentioned, the protection to be accorded to the parasites consists solely in abstaining from inopportune measures capable of bringing about their destruction without any advantage whatever. In other cases it is an active protection which has been advised, and which comprises operations destined to insure the survival of the parasites.

It is in this way, for example, that Decaux, struck by the multitude of ichneumon flies, or Braconids, which came out of the buds of apple attacked by Anthonomus, advised, in place of immediately burning these buds as was generally done, preserving them in boxes covered with gauze, raising the latter from time to time during the period of issuing of the parasties so as to permit them to escape. In 1880, he put this method into practise and collected in Picardy buds reddened by the Anthonomus from 800 apple trees, amounting to 5 hectoliters; and thus accomplished the destruction of more than a million Anthonomi, and set at liberty about 250,000 parasites which the following year were aids in the destruction of the weevils. The orchards treated being isolated in the middle of cultivated fields, it sufficed to repeat the same operation the following year in order to stop all serious damage during ten years.

This plan started by Decaux has been perfected by Berlese (1902) in order to protect the parasites of the Cochylis. This author recommends the use of boxes with the cover pierced by a window, being also covered by a metal plate perforated with holes 2 mm. in width. In the autumn there is placed in the box nearly full-grown larvæ with the leaves necessary for pupating. In the springtime the parasites will issue through the openings, while the moths perish in the box.[1]

  1. If it is desired to preserve the parasites for study, the perforated plate is covered with a bell glass, in which the parasites accumulate without ever reentering the dark box.