36 



thus showing that in some parts of the State the parasitic flies have 

 been of material assistance in destroying this pest. 



Causes Governing OutbreaJcs of the Army Worm. 



In view of the abundance of the army worm during the present 

 summer it may be profitable to briefly review the causes leading to 

 such outbreaks and the influences which control the same. The 

 army worm is an insect which appears as a pest only at extended 

 intervals or times. This seems chiefly due to the warfare waged 

 upon it by its natural enemies, mainly parasites, predaceous insects 

 and birds, and also to some extent to climatic conditions. 



Parasites often confine themselves to one host and their abun- 

 dance is regulated by the numbers of the host, while birds as a 

 rule seem to prefer the most abundant form of acceptable insect 

 food. The climate affects insect increase either through the abun- 

 dance or scarity of the food supply or through conditions favorable 

 or unfavorable to the multiplication of natural enemies. 



The relationship between an insect and its natural enemies is 

 seldom equally balanced ; that is to say, the one form or the other 

 is the more abundant in point of numbers or more effective by rea- 

 son of peculiarities favorable to its increase. Hence, in years 

 when the army worm is not present in any considerable number 

 its parasites are limited in their multiplication by the scarcity of 

 the host, while the birds give their chief attention to other insects 

 more abundant. Thus the army worm increases slowly in numbers 

 from year to year. Finally there comes a season especially fav- 

 orable for its development and the worms appear in numbers sufll- 

 cient to attract some degree of attention. They are followed the 

 next year by a more or less wide-spread outbreak,* Then from the 

 very fact of their great numbers there is an abundance of food 

 offered their natural enemies, and the latter increase to such an 

 extent as to destroy the greater part of the army worms, where- 

 upon the insect apparently disappears. This shifting relation be- 

 tween the host insect and its natural enemies is best expressed 

 by the term so frequently used by biologists, " the balance of 

 nature," and this " balance " is the factor which controls all insect 

 outbreaks. It would seem probable that the localities suffering 

 most from the army worm this year will be exempt next season 



* I am informed by Secretary Wm. R. Sessions that from careful observations ex- 

 tending over a considerable period of years he has reached the conclusion that army 

 worm outbreaks usually follow a warm dry spring. It seems quite probable that, as 

 pointed out by Mr. Sessions, such conditions of warmth and dryness are favorable 

 to the emerging of the moths and the hatching of the eggs. 



