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Page:Popular Science Monthly Volume 72.djvu/357

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353
INSECTS INJURIOUS TO AGRICULTURE

complete their development within the eggs. The phytophagous insect is then killed within the egg, and the plant thus completely escapes its depredations.

Much more frequently it does not immediately stop the growth of its host; it introduces its egg into the phytophagous insect and in the more or less advanced stages of its development, either during the embryonic period (Encyrtus fuscicollis, divers Platygasters), or frequently during the larval or nymphal period (Ichneumonids, Braconids, etc.). The phytophagous insect which carries in its interior the larva of the parasite, continues to grow and to feed upon vegetation, and is killed by the parasitic insect only when the latter has reached its full development, and when the host has done all the damage it is capable of doing during its existence. The benefit accomplished by the parasite is manifest only in the following generation, and consists in the suppression of the descendants that would have been mothered by the phytophagous larvæ, if they had been able to develop until their transformation into adult insects.

Whether they belong to one or another of these catagories, the predatory and parasitic insects play a regulating role that is useful and remarkable. When, on account of cultural conditions or climatic circumstances or other influences, the phytophagous species tends to increase beyond the average, it thus furnishes conditions eminently favorable to the multiplication of the parasitic species, and that in its turn causes the phytophagous form to decrease.

In a very interesting, but insufficiently known work, Bellevoye and Laurent (1897) have shown that it is not necessary that the parasite should have a greater fecundity than the phytophagous species in order to bring the latter back to its normal condition when it has exceeded it. As paradoxical as is this assertion, with a fecundity simply equal and even inferior, it may rapidly reach the point of annihilation, if other factors and other conditions do not interfere to interrupt this action. All other things being equal, nothing prevents the development of the parasites, so that by their work a greater and greater quantity of the plant-feeding species are destroyed each year. In order to state this fact precisely let us, with the authors just cited, take as simple an example as possible, that of an invasion of the caterpillars of Bombyx. Suppose that at a given period the proportion of parasitized caterpillars is one fourth, and that the parasites have placed a single egg in each caterpillar. Of 8 chrysalids, 6 will give out Bombyx and 2, parasites. We will suppose that of the 6 moths there are 3 males and 3 females; and of the 2 parasites, 1 male and 1 female. Let us suppose that the fecundity of the parasitic species is equal to that of the host species, and that the number of eggs laid by a female of each of the two species is 100.