4i 8 POPULAR SCIENCE MONTHLY 



eral use of sprays to be stopped. The two methods are not incom- 

 patible, for in a given region it is very rare that you can regularly 

 spray all of the trees. This one, or others, will not be treated, and 

 the ladybirds and other useful insects will therefore have a free field 

 to carry on their beneficial work, and centers from which they can be 

 dispersed will be created. 



One of the greatest dangers in introductions consists of the possi- 

 bility of introducing into a region an animal which considered as 

 useful in its original home, is capable of becoming absolutely injurious 

 in the new country into which it is introduced, on account of the con- 

 ditions of the environment which it encounters. The examples of the 

 sparrow imported from Europe into America and Australia, of the 

 mongoose introduced from the East Indies into the West Indies, of 

 the rabbit imported from Europe into Australia, are too well known to 

 be described. It has been stated that no danger of this sort exists in 

 such cases as these, since parasitic insects of other insects can live only 

 at the expense of these last, and it is the same with predaceous insects. 

 There is no doubt of this, but there exists another danger of a direct 

 character in the importation of the insects which are desired for 

 acclimatization, and that is the danger of importing at the same time 

 either injurious insects sent along as food, or hyperparasites which 

 can prevent the propagation of the useful insects and which becoming 

 acclimatized themselves, endanger even certain useful indigenous 

 species. 



It is very easy to take the necessary precautions so that the insects 

 which serve as food during the journey should present no danger, and 

 it will suffice to make sure that they belong to a species existing already 

 in the region where they are to be acclimatized. 



The history of the naturalization of Icerya purcliasi in Florida 

 shows us that the method of utilization of beneficial insects, practised 

 by incompetent people, may have sad consequences. 



As to the danger from hyperparasites, while it is apparently not 

 so serious as the preceding, it is, on the other hand, much more diffi- 

 cult to avoid. Preliminary rearings are necessary before the bene- 

 ficial species are definitely set at liberty, and all precautions are 

 necessary after the issuing to separate the primary parasites from the 

 hyperparasites. It is for this reason that the application of the 

 method of the utilization of beneficial insects, in order to render all 

 the services which are expected of it, should be carried on indispensably 

 and exclusively by learned men, especially informed concerning insects 

 and their reciprocal and biological relations. 



We have shown in this memoir about all that can be drawn from 

 the utilization of predatory and parasitic insects in the struggle against 

 enemies of crops. One conclusion may be drawn also from this study, 



