FOREST INSECTS 6.5 



opment of woody tissue, which is practically al).sent in all of 

 the agricultural croj) plants, and which serves as food for 

 quite a considerable variety of insects. On the other hand, 

 the large fleshy roots so characteristic of many food plants 

 are not developed in trees, which are consequently not in- 

 jured by insects of the type that are so destructive to vege- 

 tables like the carrot, radish, etc. With the horticultural trees 

 like the apple and peach, forest trees have more types of in- 

 sect enemies in common, but with the exception of the nut 

 trees, palms and a few others the fruit of forest trees is of 

 little consequence so long as sufficient seed is produced to 

 allow for reproduction. In exceptional cases, certain insects 

 actually do prevent the development of seeds to such a degree 

 as to menace seriously the reproduction of certain species of 

 coniferous trees, but ordinarily their influence upon seed 

 production is not of any great importance. 



Aside from the fundamental differences in material compo- 

 sition between the individual tree and herbaceous agricul- 

 tural plant, and not to consider for the moment, the slow 

 development of the tree, we find another very striking differ- 

 ence in the environment encountered by the insects which 

 feed upon the two types of plants. 



In the previous chapter on agricultural insects, attention 

 has been called to the way in which associations of very di- 

 verse plants have been replaced by large masses of crop 

 plants belonging to single species. This change has upset 

 entirely the balance which exists under natural conditions 

 between the plants and their insect enemies. Many forests 

 are comparable to the miscellaneous association of plants 

 which have given place to cultivated fields, and in fact, much 

 of the present agricultural land was once covered by forests. 

 On the other hand many forests, e.g., those consisting of co- 

 niferous trees, are frequently composed of single species grow- 

 ing to the practical exclusion of all others over very extensive 

 areas. When such is the case, the situation is very similar to 

 that presented by agricultural plants. It is, however, funda- 

 mentally different in at least one respect: these trees are 



