66 INSECTS AND HUMAN WELFARE 



growing under natural conditions, and with their slow 

 growth, could never have reached their present age if they 

 were regularly subject to damage comparable to that inflicted 

 upon agricultural plants growing continuously in pure stand. 

 These coniferous forests present peculiar conditions in rela- 

 tion to insect damage which are of great importance at the 

 present time, and they afford interesting material for specu- 

 lation as to the damage which may be caused by insects in 

 the future, to any pure forests of other kinds of trees that may 

 be planted for specific purposes. 



A characteristic of the insect damage to coniferous forests 

 is the great fluctuation which it exhibits both in extent and 

 severity from year to year. The appearance in great abun- 

 dance of many of the insect enemies of conifers often shows a 

 striking similarity to the epidemics of certain human diseases, 

 although the reason for their later disappearance is un- 

 doubtedly due to very different causes. One common insect 

 of Europe is of especial interest since it is a close relative of 

 the gipsy moth, already introduced into the United States. 

 This is the nun moth (Fig. 30), feeding in the larval stages 

 upon the foliage of various conifers. When the caterpillars 

 are abundant, they may completely defoliate trees over wide 

 areas and cause them to die in large numbers, as coniferous 

 trees very frequently fail to survive complete defoliation. 

 The more important and extensive "epidemics" of the nun 

 moth experienced in recent times occurred in 1795, 1839 and 

 1890, or approximately fifty years apart. Since this is about 

 the period required for the individual trees to attain a mature 

 size, the appearance of the insects shows a very interesting 

 parallel to the more or less regular appearance from year to 

 year of many insects destructive to annual agricultural 

 plants. In the case of trees of this sort, it is evident that a 

 more frequent occurrence of such outbreaks would seriously 

 menace the continued existence of the host-tree species. The 

 earlier outbreak cited above was suddenly terminated during 

 the third summer as the result of the appearance of a fatal 

 infectious disease of the caterpillars, and such disease, as well 



