10 UNITED STATES NATIONAL MUSEUM BULLETIN 2 76 



either province in 1955 or 1956. Beginning in 1957, however, and con- 

 tinuing to the present time (1964), outbreaks again have been present 

 (personal observation and various reports). This record shows that 

 outbreaks have occurred almost continously since 1923, and it takes 

 into account only one species and covers only two Canadian provinces. 

 It would be safe to say that Malacosoma could be found in outbreak 

 numbers somewhere in North America every year, and in most years 

 numerous outbreaks could be located. 



Many observers have reported on the factors which end tent cater- 

 pillar outbreaks, but little study has been done on the factors which 

 permit an outbreak to build up. Hodson (1962) has presented data 

 indicating that two outbreaks of M. disstria in Minnesota were pre- 

 ceded by an interval of 2 to 3 years in which temperatures after hatch- 

 ing were moderate, with no prolonged cold periods below 59° F. 

 (when little or no feeding occurs), and with no temperatures cold 

 enough to severely damage the aspen foliage and result in starvation. 

 In addition, he reported that in both outbreaks there was an uninter- 

 rupted series of 6 years prior to and during the course of the outbreaks 

 when moderate temperature after hatching was the rule. This and 

 other unpublished data have led him to conclude that a series of years 

 in which late springs are followed by moderate temperatures are most 

 favorable for an outbreak of M. disstria. 



Wellington (1952) states that the physical requirements of M. 

 disstria include warm, humid, cloudy weather during much of the larval 

 stage, and he has presented evidence suggesting that two outbreaks of 

 M. disstria north of Lake Superior and Lake Huron (these outbreaks 

 occurred at the same tim^e as those reported by Hodson) were preceded 

 by increases in the annual numbers of cyclonic passages. This agrees, 

 in a general way, with the evidence presented by Hodson since cyclonic 

 systems would tend to be warmer and more moderate than anticyclonic 

 systems. The movement through an area, however, of a single anticyclon- 

 ic system cold enough to freeze the aspen foliage could be dis- 

 astrous to the caterpillars, but it would not stand out as important 

 if only the relative frequencies of cyclonic or anticyclonic systems 

 were compared without considering their individual characteristics. 



The principal damage is caused, of course, by the tremendous 

 numbers of caterpillars that may completely defoliate anything from 

 the backyard apple tree to thousands of square miles of woodland 

 (fig. 330). Trees that have been completely defoliated will put out 

 a new set of leaves, but growth is greatly reduced, and weakening or 

 even the death of the tree may result. As early as 1904, Foster re- 

 ported a great reduction in the size of annual rings produced by a 

 sugar maple in New Hampshire following heavy defoliation by M. 



