INSECTS AND HUMAN WELFARE — DUNCAN 343 



tracts of country, utterly useless on account of the dense growth of 

 the weed, have been brought into production. The prickly pear ter- 

 ritory has been transformed from a wilderness to a scene of prosperous 

 endeavor." 



Probably not many plants possess the qualifications for becoming 

 pests of the magnitude of the cactus, but we have no way of judging 

 beforehand. There are other weed plants besides cactus which, though 

 perhaps less objectionable, have nevertheless become major pests in 

 countries into which they have been introduced. Such are lantana in 

 Hawaii and Australia, blackberry and gorse in New Zealand, and St. 

 John's Wort in California. Success has been only partiall}^ attained 

 to date in controlling these weeds but the measure of success that has 

 been attained has been accomplished largely through the use of insects. 



In the countries to which they are native these plants are not pests. 

 They fit )iormally into the flora of those regions in mutual adjustment 

 with other organisms. The factors that determine their normal popu- 

 lation levels, as is true of plants everywhere, undoubtedly are several, 

 but among these are the insects that feed upon them. Dr. Brues has 

 pointed out in "Insect Dietary" that insects are a major factor in de- 

 termining population levels in plant life. We perhaps cannot even 

 guess accurately what the consequences would be if insects were to be 

 totall)^ removed from the realm of plaiit life, but we can be sure that 

 it would be some sort of chaos and that man would be numbered among 

 the victims of such a disastrous happening. 



We are accustomed to looking upon insects that attack trees as in- 

 jurious and requiring control, and certainly often, though not always, 

 this view is the correct one. Yet it is equally certain that the majority 

 of insects to be found in a forest are not detrimental to the forest and 

 that many, if not the majority, actually benefit the forest in one way 

 or another. F. C. Craighead (2) in "An Annotated List of the Impor- 

 tant North Ameiican Forest Insects" lists less than 200 kinds of really 

 destructive species distributed over many families, yet W. J. Cham- 

 berlin (1) records 575 species of bark and timber beetles in but two 

 families. The great majority of these are forest species but the great 

 majority are not destructive. 



Chamberlin (loc. cit.) says, "When Micrasis, Carphohor-us, Pity- 

 ophthoruft^ P'ttyokteines^ Lytnantor^ HypotheneTnus^ or any of the 

 other similar species attack the lower limbs of trees and kill them, 

 they are but hastening natural pruning which results in a clean bole 

 and a better grade of lumber." 



Doane, Van Dyke, Chamberlin, and Burke (3) say that "In every 

 heavily stocked young forest there are thousands of trees that must 

 die and pass out of the picture before the forest reaches matu- 

 rity. * * * Nature takes care of this * * * need through 



