134 GYPSY AND BEOWN-TAIL MOTHS. [Jan. 



multitude of parasites, and but little food for them. As a 

 result, the flies die off; and the few remaining army worms 

 increase slowly in swamps and along the banks of streams, 

 until after the lapse of years their numbers are sufficient to lay 

 the foundation for another army worm outbreak, only to be 

 followed by the development of a multitude of parasites to 

 check it. This shifting relationship is an illustration of the 

 "balance of nature," so called, the tilting adjustment of 

 an insect to its enemies. 



The canker worm and the tent caterpillar give equally 

 good illustrations, except that the period of their ravages 

 usually extends over two or three years before it is checked. 

 This is practically what takes place with the gypsy moth and 

 brown-tail moth in Europe. As stated elsewhere, their 

 periodic outbreaks run for a few years and then subside, to 

 reappear after the lapse of several years. The birds figure 

 to a considerable extent in checking these insects, and no 

 doubt several fungous or bacterial diseases are also of ser- 

 vice, but the principal check seems to be exerted by para- 

 sites and predaceous insects. 



But the story does not end with simple or primary insect 

 parasitism. Several of these parasites, particularly the 

 larger ones, when they are safely established in the vitals 

 of their hosts, are attacked in turn by certain minute forms 

 of insect life, secondary parasites. Then we have an 

 exhibition of the complicated phenomenon of secondary 

 parasitism, which is almost as much to be dreaded as the 

 original attack of the caterpillar; for if the true (primary) 

 parasites are destroyed, the principal check on the cater- 

 pillar's development is removed. To differentiate between 

 and sort out the beneficial primary from the highly injurious 

 secondary parasites calls for the highest skill of the well- 

 trained specialist. I have thought it best to go into this 

 matter somewhat at length, so that there may be on record 

 one of the most important difficulties and dangers of the 

 promiscuous importation of parasites of the gypsy moth. 



The problem, then, of successfully importing gypsy and 

 brown-tail moth parasites depends on several factors, among 

 which the following are the most important : 



