LOSSES CAUSED BY INSECTS: NATURE'S CONTROL METHODS 35 



wild and scattered. An insect finding in any of these a food acceptable 

 to it, would at once also find a more abundant supply, and a rapid multi- 

 plication would become possible, resulting in their increase to injurious 

 abundance. A second factor has been the introduction of many insects, 

 from foreign countries. In the United States such forms have sometimes 

 entirely failed to maintain themselves. Unfortunately, as has more fre- 

 quently happened, they have found all conditions favorable to a rapid 

 increase, unchecked by their enemies which in most cases have not also 

 been brought to this country with them. A third factor has been that 

 with the increasing occupation of this country, much of its wild bird life 

 has either been destroyed or has been driven away from the neighborhood 

 of man. Many insect feeders among birds, once quite common, must now 

 be sought in remote woodlands and thickets, and rarely show themselves 

 near settlements. Some kinds have adjusted themselves to the new 

 conditions and among these may be mentioned the robin, chipping 

 sparrow, blue bird and a few others. But to too great a degree the insec- 

 tivorous birds are becoming either fewer in number or afraid to visit the 

 settled districts where cats and people are numerous, even though in such 

 places the gardens and trees may be thickly populated with insects. 



With modern agricultural methods distinctly favoring a rapid in- 

 crease of insects by providing an enormous acreage of a single crop; 1 with 

 an addition to our worst native pests of at least as many more from other 

 countries, which have escaped their enemies by coming here; and with 

 our birds becoming less effective in their work, it is only natural that losses 

 by the attacks of insects should be great and increasing in importance. 



Control by Natural Methods. In countries undisturbed by man 

 and his industries it is probable that destruction or serious injury from 

 insect attacks would usually be rather small, particularly in a series 

 of years. The saying in Physics that "Nature abhors a vacuum" 

 seems to be paralleled in Biology by the paraphrase, "Nature abhors 

 extermination." Accordingly, insects appear to be "more or less com- 

 pletely held in balance by natural factors, some of which may be briefly 

 considered here. 



Plants of various kinds form the food of most of the insects which 

 we regard as pests, and in a country entirely under natural conditions, 

 plants of any one kind are liable to be more or less scattered, no large 

 number being close together. Under such conditions a search for the 

 proper food plant is necessary to an insect as a preliminary to egg- 

 laying, and in many instances these may be too scarce to provide for all 

 the insects. In any case, where the food supply is scanty, an insect 



1 As an example of this, apple orchards containing thousands of trees are now 

 common. It is stated that one year in a single valley in California, there were three 

 wheat fields each containing over twenty thousand acres. 



