Page:Popular Science Monthly Volume 45.djvu/703

Rh exceptional injury done by the injurious species. Now, the insecticide method of routing this last, under such circumstances, too often involves, also, the destruction of the parasitic and predaceous species, and does more harm than good. This is particularly true of those of our Coccidæ and Aphididæ, and those of our lepidopterous larvæ, which have numerous natural enemies of their own class, and it not only emphasizes the importance of preventive measures which we are all agreed to urge for other cogent reasons, and which do not to the same extent destroy the parasite; but it affords another explanation of the reason why the fight with insecticides must be kept up year after year, and has little cumulative value.

But the problem of the wise encouragement and employment of the natural enemies of injurious insects in their own class is yet more complicated. The general laws governing the interaction of organisms are such that we can only in very exceptional cases derive benefit by interference with it. The indigenous enemies of an indigenous phytophagous species will, cæteris paribus, be better qualified to keep it in check than some newly introduced competitor from a foreign country, and the peculiar circumstances must decide in each case the advisability of the introduction. The multiplication of the foreigner will too often involve the decrease of some indigene. If a certain phytophage is generally disastrous in one section and innocuous in another, by virtue of some particular enemy, it will be safe to transfer and encourage such enemy, and this is particularly true when the phytophage is a foreigner and has been brought over without the enemy which subdues it in its native home. Icerya had some enemies in California, presumably American; but they were not equal to the task of subduing it. Vedalia in the icerya's native home, Australia, was equal to the task, and maintained the same superiority over all others when brought to America. The genus was new to the country, and the species had exceptionally advantageous attributes. But there is very little to be hoped from the miscellaneous introduction of predaceous or parasitic insects for the suppression of a phytophage which they do not suppress in their native home or in the country from which they are brought. The results of the introduction by Mr. A. D. Hopkins of Clerus formicarius to contend with the scolytids, which were ruining the West Virginia pines, were doubtful, for the reason that the indigenous species of the genus were already at work in America. Yet the experiment was safe and desirable because the European clerus is more active and more seemingly effective than our indigenes. The gypsy moth was evidently introduced into Massachusetts without its European natural enemies, and as in some parts of Europe it is often locally checked by such natural