358 POPULAR SCIENCE MONTHLY 



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 har- 

 mony 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 prin- 

 cipal 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 reestablish/merit 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 multi- 

 plication; 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 com- 

 pensating 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 cul- 

 tural practises. 



Protection of Beneficial Insects. — Apropos to the Hessian fly, we 



may enter into. " The retarded development of these parasitic larvae," says 

 Kiinckel, " enables the successive issuing of adult insects during several years, 

 and is evidently in close correlation with the appearance of the locusts; th« 

 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 estab- 

 lished a regular balance between the multiplication of the locusts and that of 

 their egg-feeding parasites, which assures the perpetuity of both species." 



