REVISION MALACOSOMA HUBNER IN NORTH AMERICA 9 



oak stands of some of the mountains of central and southern California, 

 and M. calif or nicum in some of the aspen forests of the Rocky Mountains. 

 Because of these outbreaks and because of the old records of outbreaks 

 there can be little doubt that tent caterpillars occurred in outbreak 

 numbers long before any settlers arrived in North America. 



In many parts of North America the original vegetation has been 

 drastically altered by man's activities, and replacement often has 

 been by species that are the preferred hosts of tent caterpillars. The 

 best example of this is the aspen forests of the northern United States 

 and southern Canada, which came in after the conifers had been 

 logged or burned off in many areas. Other favored hosts, such as most 

 species of Prunus and introduced fruit trees, would be rare in mature 

 hardwood forests, but they grow abundantly along fencerows, road- 

 sides, and other sites that have been opened up by man's activities. 

 No direct evidence indicates diat the mere presence of vast new acre- 

 ages of suitable hosts has caused outbreaks of tent caterpillars, but the 

 larger the suitable area the more likely it is that conditions favoring 

 an outbreak would be found somewhere within the area. It seems 

 likely, therefore, that we have more outbreaks than formerly, and 

 more extensive ones, too, since suitable hosts are abundant over larger 

 areas. 



Numerous authors have suggested that tent caterpillars may occur 

 in outbreaks at about 10- year intervals, but outbreaks do not appear 

 to be cyclic, since the lengda of time between reported outbreaks in a 

 given locality varies considerably, being as short as 3 to 5 years, or 

 as long 15 to 20 years or more. For example, Hildahl and Reeks (1960) 

 have reported the recorded outbreaks of M. disstria in Manitoba and 

 Saskatchewan. For 10 selected localities they reported the interval 

 between the first severe infestation at a given locality and the next 

 severe infestation at the same locality to vary from about 6 to 16 years. 

 They also point out that 4 of the 10 localities experienced only one 

 outbreak in a 35-year period. One of these localities had an outbreak 

 in 1938, but had not had another one up to 1959, and others had not 

 had an outbreak for almost as long. 



Outbreaks in a given locality usually persist for 1 to 4 years before 

 being brought under control by various factors such as disease, para- 

 sites, starvation, and weather, or a combination of several factors. 

 Often an outbreak may be collapsing in one area while gaining mo- 

 mentum in another area that may not be any great distance away. 

 Hildahl and Reeks (1960) report that M. disstria occurred in outbreak 

 numbers at some place in Saskatchewan or Manitoba each year from 

 1923 to 1953. Records before 1923 were incomplete, and after 1953 

 outbreaks declined to the point where no defoliation was noted in 



