36 APPLIED ENTOMOLOGY 



species feeding wholly on that kind of plant will be more rare than where 

 its food is abundant. If, on the other hand, there is an abundance of 

 the food plant, there is a greater probability of the survival of more of 

 the insects. But this brings its disadvantages. Increase in the number 

 of the insects will result in more food being needed, and finally this will 

 become insufficient and will be followed by the failure of many to find 

 food, death resulting. In this way a balance may be finally secured, 

 though it will not be permanent, the process being repeated in the 

 subsequent years. 



Weather conditions are also a factor in Nature's control. Some 

 insects find in a wet season conditions favoring the survival of a large 

 proportion of those which appear, while for others such a season produces 

 heavy mortality. A severe winter with many and marked fluctuations 

 of temperature may put an end to the rapid increase of some species 

 which because of preceding favorable winters, has been becoming more 

 abundant. Other meteorological factors also enter into the subject of 

 insect control. 



Birds and other animals which feed on our insects must also be con- 

 sidered in this connection. When insects acceptable to these animals are 

 abundant, more will be eaten and in any case many will be destroyed in 

 this way. Where insectivorous birds have an abundant food supply 

 more will survive, which will result in more individuals to be fed. Thus 

 an abundance of insects may lead to a corresponding increase in abun- 

 dance of their enemies. 



Parasites and diseases play their part too in this competition. The 

 more abundant an insect becomes, the more food is thereby available 

 for its parasites, and fewer of these will fail to find an insect to attack. 

 Finally the parasites may become so numerous that practically all the 

 insects of the kinds they attack will be found and killed. The next 

 generation of parasites following this, will, of course, consist of many 

 more individuals than the one preceding, but now so many of their 

 food insects or " hosts" have been killed in producing them that there are 

 practically none left, and most of these parasites die for lack of food. 

 Thus, under these conditions, a sort of " balance of Nature" develops, 

 and though the scales may tip first to one side and then to the other, 

 this balance is usually preserved if periods of a number of years at least, 

 are considered. 



But when man with his many lines of activity appears in the field, 

 introducing and raising millions of plants of the same kind in small 

 areas, instead of scattering them here and there, thus furnishing enormous 

 quantities of food for insects; and when he brings in many pests from 

 foreign countries, no matter how unintentionally, which in their new 

 home are not beset by the foes present in their native land; and when 



