IXSECTS AFFECTIXG PARK AND WOODLAND TREES 253 



MORE niPORTANT FORP:ST TREE PESTS 

 This division is more or less arbitrary' and is adopted solely because of 

 its convenience. It is exceedingly difficult to distinguish between the more 

 and less important species, and the following sejjaration is luuloubtedly 

 influenced somewhat by personal opinion. It must not be forgotten that 

 most of the species listed as important enemies of shade trees, also feed, as 

 previously pointed out, on forest trees, anil that sometimes these earlier 

 noticed forms ma\' be exceedingh" tlestructi\"e in forest land. Our fall 

 webworm, H )• p h a n t r i a text or Ham, for example, is occasional!)' 

 very abundant on forest trees and causes a considerable amount of injur)-. 

 The spin)' elm caterpillar, li u v a n e s s a ant i op a Linn., lives b)- pref- 

 erence on willows and poplars, occasionally defoliating extensive areas, 

 and the depredations of the forest tent caterpillar, M a 1 a c o s o m a 

 clisstria Hiilin.. are t(,)o well known to re<]uire niore than mention in this 

 connection. 



Conditions are such in this country that we must rely very largely on 

 natural agents of one kind or another to prevent serious injury to ffirest 

 trees. This will ordinarily be accomplished through the activities of 

 various predaceous and parasitic forms, which rarely attract attention 

 because of their abundance. P'ungous diseases and unfavorable climatic 

 conditions also plav an important part in checking insect ravages. .Some 

 of our native species, in spite of these checks, are occasionally very injurious 

 over large areas. One of the most striking cases is tliat of the forest 

 tent caterpillar, a species which feeds very largely on hartl maples, and at 

 irregular and rather widely separated intervals becomes so enormously 

 abundant as to defoliate extensive areas year after \ear. spreading there- 

 from to oiu' shade trees. 



The dangerous nature of introduced species has been general!)' recog- 

 nized and it is well known that a considerable percentage of tlie more 

 serious enemies of genera! agricultural crops have come to us froni abroad. 

 It is fortunate that comparatively few destructive forest pests ha\e been 



