INSECTS AFFECTING PARK AND WOODLAND TREES 253 



MORE IMPORTANT FOREST TRKI- FIRSTS 

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

 its convenience. It is exceedingly difficult to distiny-uish between the more 

 and less important species, and the following' separation is imddiiblrdly 

 inriuenced somewhat by personal opinion. It must not be forijotten that 

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

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

 noticed forms may be exceedingly destructive in forest lanil. Our fall 

 webworm, Hyphantria textor Ham, for example, is occasionally 

 very abundant on forest trees and causes a considerable amount of injury. 

 The spiny elm caterpillar, Euvanessa antiopa Linn., lives by pref- 

 erence on willows and poplars, occasionally defoliating extensive areas, 

 and the depredations of the forest tent caterpillar, Malacosoma 

 disstria Hiibn.. are too well known to require more than mention in this 

 connection. 



Conditions are such in this country that we niust rel\- \ery largely on 

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

 trees. This will ordinarily be accomplished through the activities of 

 various predaceous and parasitic forms, which rarely attract attention 

 because of their abundance. Pungous diseases and unfavorable climatic 

 conditions also play 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 that of the forest 

 tent caterpillar, a species which feeds very largely on hard 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 our shade trees. 



The dangerous nature of introduced species has been generally recog- 

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

 serious enemies of general agricultural crops have come to us from abroad- 

 It is fortunate that comparatively few destructive forest pests have been 



