— 12 — 



Reparding the action of the cHmatological factors on insect life a few facts 

 have been ascertained. Bachmetjew, a Bulgarian, determined the effect of varying 

 temperature and humidity on the metabolic activities of several insects. Humidity 

 modifies the effects of temperature ; there is an optimum humidity for metabo- 

 lism, not only for each species but for each stage of each species and for each 

 stage of each individual. Pierce found that the optimum temperature and humidity 

 for the boll weevil are 83° F. and 65° F. respectively. 



Ewing found that the development of Aphis avcnae is prevented at a cons- 

 tant temperature of 90° F.. and that the optimum temperature for the production 

 of wingless agamic forms is about 65° F. 



Food Supply: This is a factor of considerable importance as most insects 

 tend to miiltiply rapidly when the supply of food is abundant, and to decrease 

 when the supply is scarce. Without wheat there would be no wheat midge, and 

 the practical elimination of certain forest trees has brought about the disappear- 

 ance of their insect pests. 



Some insects feed upon but a single kind of plant, and if the range of cultiva- 

 tion of this plant be extended the insects will also extend their range. They are 

 apparently hardy enough to withstand different climatic conditions. An excellent 

 illustration of an insect of this kind is the cotton boll weevil. It is apparently a 

 potential enemy as far north as cotton will grow. 



Man by clearing forests and making farms where enormous areas of certain 

 crops are grown has made conditions favorable for the development of the insects 

 that feed upon these crops. 



Predatory Animals : These are also an important factor that requires more 

 emphasis. First among these are insectivorous birds. In an address to this Society 

 five years ago on "The Web of Life", I stated as follows : — 



"Facts which convince even the most sceptical are accumulating regarding 

 the valuable role played by birds in the control of noxious insects. Studies of 

 bird diets prove conclusively that the majority of our common birds feed mainly 

 upon insects. Forbes of Illinois states after a careful study of the contents of the 

 stomachs of birds that about two-thirds of the food of birds consists of insects. 

 Well-informed writers tell us that without birds the earth would be uninhabitable 

 after six years, and yet man in his ignorance is constantly destroying these valu- 

 able friends, simply because he finds that they levy an insignificant toll on his 

 'fruits and grains. It is possible that Ave would be better off if certain birds were 

 greatly reduced in numbers, but of this we are not absolutely certain, for the web 

 of life is most complex, and no person knows how far-reaching the results would 

 be. 



"Aside from the fact that birds aid very materially in reducing the numbers 

 of insects when they come as scourges, it is very important to remember that birds 

 nip many incipient scourges in the bud. Their mobility and varied character and 

 habits enable them to move rapidly from place to place and thus maintain the 



