such as the control of the cottony cushion-scale of California by the introduction 

 of the Nozi'us cardinali's lady-bird from Australia, and of the black scale of the 

 olive by the Scutcllista chalcid from South Africa. They have either forgotten or 

 have not heard of the many partial successes and absolute failures. 



The reasons why the natural method of control has not had more cases of 

 success are the extreme complexity of most of the problems, the lack of sufficient 

 knowledge of the factors that operate, and the difficulty of devising measures of 

 control. 



The natural agencies of control may be grouped into four: 



] . — ^Climate. 



2. — Food supply. 



3. — Predatory animals. 



4. — Parasitic animals and plants. 



Climate: The effects of changing and unnusual conditions of heat and cold, 

 snows and rains, humidity and dryness, and other seasonal changes have long 

 been known to be important factors in regulating the number of insects. 



A variable winter is more fatal to most forms of insect life than a contin- 

 uously severe or a continuously mild winter. When insects go into hibernation 

 they become torpid and are able to resist quite low degrees of cold, but if thaws 

 occur they may become partially active for a short time. With every change of 

 this nature the insect loses vitality and this loss may prevent transformation in 

 the spring. Moreover, the effects of thaws in breaking up larval and pupal cells 

 in the ground are often quite marked. 



Sudden changes of temperature of say 30° range within a few hours, which 

 are quite common in our latitude, are very fatal to aphids and many caterpillars 

 during their early stages. 



Another feature of the environment of insects is the different degree of hu- 

 midity demanded by each species or genus. Some insects like the thrips, chinch 

 bug, wheat midge and red spiders are more abundant under dry conditions, while 

 other insects like the plant and Hessian fly develop best under moist conditions. 



Observations seem to show that climatic changes are often fatal to those 

 insects that are living on the fringe of their distributional range. Under favorable 

 conditions some insects may migrate northward and even do much damage, but 

 such movements may be termed incursions, and are temporary and spasmodic in 

 their character. For example, the San Jose scale appeared in this locality some 

 years ago but its presence has not been noted since. 



Again, the San Jose scale was for many years an aggressive insect, multiply- 

 ing rapidly under natural conditions on its numerous food plants, south of the 

 latitude of Quebec. During the severe winter of 1917-18, however, a large per- 

 centage of the scales was killed over large areas in New York State and Southern 

 Ontario. So far as can be observed, the only factor that operated more trongly 

 than usual was that of low temperature conditions which lasted for a longer 

 period than usual. 



