REVISION MALACOSOMA HUBNER IN NORTH AMERICA 11 



disstria in the years 1897-1899. More recently, Duncan and Hodson 

 (1958), Ghent (1958), Rose (1958), Hildahl and Reeks (1960), and 

 Barter and Cameron (1955) have reported that basal area growth of 

 aspen is greatly reduced by defoliation by M. disstria. Duncan and 

 Hodson reported that a reduction of basal area growth of up to 90 

 percent may occur in the year follov/ing heavy defoliation. Churchill 

 et al. (1964) conclude that recovery of normal basal area growth after 

 the cessation of defoliation appears to take one growing season in the 

 most severe cases. Thus, several years of heavy defoliation would 

 cause a delay of several years until the time a tree is large enough to be 

 of commercial value, regardless of whether the defoliation target is 

 an aspen used for pulpwood in the North or a black cherry used for 

 furniture in the eastern United States. No doubt other host species 

 would be affected similarly by several years of heavy defoliation. 



In a given locality, heavy or complete defoliation usually does 

 not occur for more than 2 to 3 years in succession. Churchill et al. 

 (1964) reported on the long-term effects of defoliation of aspen in 

 Minnesota. Their study has shown increasing mortality with increas- 

 ing defoliation in the combined intermediate, codominant, and 

 dominant vigor class, but not with the suppressed class. They con- 

 clude, therefore, that circumstantial evidence of mortality caused at 

 least indirectly by forest tent caterpillar feeding is provided. Re- 

 ports such as those of Caudell (1902b) attributing death of oak brush 

 in Colorado entirely to repeated defoliation by M. tigris are rare, 

 and usually are not supported by any evidence other than casual observa- 

 tion. Even if hosts are not killed, however, several successive years of 

 heavy defoliation may weaken them enough to permit other factors 

 such as poor site (Churchill et al., 1964), drought (Duncan and Hodson, 

 1958), overgrazing (Clark, 1956a), or bark beetles and borers (Barter 

 and Cameron, 1955) to cause higher mortality rates. 



In the western states tent caterpillars often feed on vaiious shrubs 

 and bushes that are valuable browse species for both domestic stock 

 and big game. Clark (1956a) reported that defoliation by tent cater- 

 pillars in combination with other factors has resulted in serious damage 

 or death to large areas of bitterbrush browse, Purshia tridentata, in 

 parts of northern California. 



In some resort areas of the United States and Canada tent cater- 

 pillars may adversely affect business, since many vacationers are 

 reluctant to fight the s\varm3 of "worms," and are repelled by the bare, 

 "mid-winter" appearance of the forests in late June and early July 

 before a second set of leaves is produced. 



The forest tent caterpillar, M. disstria, often has been so abundant 

 that reportedly it has stopped trains, although this hardly can be 



279-280—68 2 



