There exists an intimate relation between all these conditions, and this relation binds in a particularly striking manner the plant-feeding insect and parasites which live at the expense of the latter. The harmony which results from the mutual adaptation of these beings should not be surprising, since it is the condition sine qua non of the existence of the species. From the reciprocal actions which they exercise upon one another, results the equilibrium in which they are maintained.
II. Perturbations brought by Man in the Natural Equilibrium
The intervention of man in disturbing the laws of nature is capable of breaking this natural equilibrium, and of bringing about in the existing order a perturbation from which he is perhaps the first sufferer of serious consequence. This rupture can be occasioned by two principal causes: (1) by new conditions created for insects by cultures; (2) by the accidental carriage of certain species from one country to another.
1. Perturbations provoked by New Conditions created by Cultures; Methods bringing about the reestablishment of the Equilibrium
Man, in planting over a vast extent of country certain plants to the exclusion of others, offers to the insects which live at the expense of these plants conditions eminently favorable to their excessive multiplication; for he diminishes in their favor the difficulties of their struggle for existence and often favors their alimentary specialization, while the food-plant, in the conditions which it finds itself, is not always capable of reacting by defensive adaptations of sufficient compensating value.
In this case, man, in order to regain the equilibrium favorable to his own interests, should have recourse to a regular rotation of crops, destined to interrupt the life cycle of the injurious species, and to all methods possible to increase the resistance of the plant. But also the beneficial insects whose useful role is incomparable should be watched. It is necessary to aid or at least to start their work; and, finally, in any circumstances it is necessary to know them in order to protect them in a judicious way, and above all not to destroy them by inopportune cultural practises.
Protection of Beneficial Insects.—Apropos to the Hessian fly, we
may enter into. "The retarded development of these parasitic larvæ," says Künckel, "enables the successive issuing of adult insects during several years, and is evidently in close correlation with the appearance of the locusts; the latter, decimated, fly from their enemies to reproduce far away; the former awaiting their return to insure the well-being of their progeny; thus is established a regular balance between the multiplication of the locusts and that of their egg-feeding parasites, which assures the perpetuity of both species."