INSECTS INJURIOUS TO VEGETATION. 389 



Competition between plants is normally very active. As a result any insect species 

 which subsist parasitically at the expense of a plant species may easily bring about 

 complete extermination of the host plant ; not through direct injury, so much as 

 indirectly, by permitting a competing plant species to secure an advantage. A 

 striking case in point is afforded by the gipsy moth in its relations to pine and oak. It 

 is injurious to both, but is ordinarily more injurious to oak than to pine. The two 

 trees are active competitors for space in which to grow in many localities in New 

 England where the gipsy moth has been in a state of outbreak, and where the oak 

 until recently held the upper hand. In such localities it has not infrequently hap- 

 pened that, although the pine trees have been injured to some extent, the pine species 

 has benefited in the end and through the oak species having been injured to a 

 proportionately greater extent. The lives of countless pines have been saved, under 

 certain circumstances and conditions, by the activities of an insect which, under 

 other circumstances and conditions, has proved itself to be a dangerous enemy of 

 trees. " Vegetation " has not been injured by the gipsy moth ; only some plants 

 and trees have been injured to the benefit of their competitors. 



It may not, therefore, be assumed that, because insects are at times injurious to 

 plants, phytophagous insects as a class are to be considered as generally injurious 

 rather than beneficial to green plants as a class, or to vegetation. Such an assumption 

 appears not to accord with fact, nor with the theory of natural evolution and natural 

 control of species, but to have been a logical outgrowth of a belief in supernatural 

 origin and supernatural control. The phrase selected by Dr. Harris as a title for his 

 treatise is aphoristic, and means little more than that insects are sometimes injurious, 

 to plants of economic value, and thereby become obnoxious to man. On the whole 

 the insects inliabitiug a tract of woodland are less injurious to vegetation than a boy 

 w^eeding a carrot bed, or a man pruning an orchard, or cattle grazing in a pasture. 

 With a few exceptions (such as may be afforded by migratory locusts and other very 

 indiscriminate feeders), insects are as beneficial as they are injurious to vegetation, 

 wherever plants are actively competing among themselves for space in which to grow, 

 and this means in most of the well-watered, moderately fertile localities in the 

 temperate and tropical regions. 



The phrase " insects obnoxious to man " is equally open to criticism. The race 

 class and condition of the men, and the conditions and circumstances prevailing at the 

 time and place, have all to be considered. If all mankind is meant to be included, 

 all men must be included, and it may easily be that a large number will be benefited 

 by the misfortunes of a small number. 



