220 APPLIED ENTOMOLOGY 



to 1,000 eggs in a large cottony mass formed at the hinder end of the 

 body, the upper sm'face of the mass being grooved or fluted (Fig. 216). 

 There are several generations in a season. 



Several of the scale insects treated in this chapter furnish good illustra- 

 tions of the way in which nature works to preserve a balance in the 

 insect world. In the first place it should be noticed that our native 

 scales are often found with tiny circular holes in them showing where 

 parasites after having fed on the insect beneath, have made their escape. 

 Other scales, long in this country, such as the Oyster-shell Scale, now 

 have numerous parasites, some of which are also enemies of other kinds 

 of scales, and in fact may be considered as scale enemies in general, or at 

 least of most scales of the same section. New parasites also appear 

 from time to time as enemies of scales, such as a tiny insect, Prospaltella 

 perniciosi Tower, first discovered about 1912, which at times has done 

 remarkably good work against the San Jose Scale. But when a new 

 scale or other insect native elsewhere, establishes itself in this country, 

 one of the factors at least in its success here, must be that none of its 

 parasites in the locality whence it came, accompanied it in its transfer. 

 If under these circumstances, climatic and other conditions prove 

 satisfactory, we have a case of an insect set free from all restraint, to 

 work its destruction with no check, at least until some insect already 

 present shall select it as a new and satisfactory food. This was evidently 

 the case with Prospaltella and the San Jose Scale, already mentioned. 

 In the meantime, however, years of destruction may elapse before any 

 such check will appear, and the possibility of obtaining its special enemies 

 from its native country appears to offer much in the way of quick relief. 

 This "bug vs. bug" idea as it has been called, has a strong appeal to those 

 suffering losses from the attacks of a newly introduced pest, and it has 

 therefore been widely exploited. 



Probably the first attempt to carry out this idea was the introduction 

 of the Vedalia for the Cottony Cushion Scale, and in this case an un- 

 qualified success resulted. On the other hand, the attempt to establish 

 the Chinese Lady Beetle in this country to control the San Jose Scale 

 has thus far been a failure, and the introduction of the parasite Saitel- 

 lista cyanea Motsch. to work on the Black Scale cannot be regarded as 

 more than partially effective. 



The danger of introducing along with the parasite, its own parasites 

 (secondary parasites) at such a time is great, and therefore this work 

 should be attempted only by those especially trained for it. 



All in all, the "bug vs. bug" idea is one which, though always having 

 many possibilities of success, is also one which will often fail, and there- 

 for cannot be relied upon as a certain panacea for troubles caused by 

 introduced pests. 



