METHODS OF CONTROL OF INSECT PESTS 567 



enemies so that the balance may be kept in a natural way. An 

 historic case is the introduction of a scale insect from Australia 

 to the California citrus groves in 1868. This insect infested so 

 many orchards that a natural enemy to this scale, the ladybird 

 beetle, was later introduced from Australia. Due to the activity 

 of these beetles the scale insect can be kept in check. But man 

 has made a step further in his control of this pest. He has found 

 that fumigating with hydrocyanic gas will kill the scale insects left 

 on the trees. So we find that artificial methods of control are used 

 in addition to nature's method. Insects may be destroyed by 

 (a) picking (as in the case of the potato beetle) or by (6) contact 

 poisons, which kill by covering up the spiracles of the insect so 

 that it cannot breathe, or by (c) stomach poisons, which are sprayed 

 on the leaves to be eaten by the insects. Man has learned to 

 develop plants that mature faster than the insects do or plant his 

 crops early or late, thus escaping insect damage. 



The control of the cotton-boll weevil seems to depend upon 

 early planting so that the crop has an opportunity to ripen before 

 the insects in the boll grow large enough to do harm. Various 

 state and government agencies are at work upon the problem, and 

 ultimately the boll weevil may do more good than harm by bringing 

 about the culture of a type of cotton plant that ripens very early 

 and by forcing the farmers of the South to produce diversified 

 crops, which can be marketed to an advantage if the cotton crop 

 is a failure. Another method is by crop rotation, for in this way 

 insects may be deprived of food plants on which to lay their eggs. 



The spread of the Japanese beetle is being fought by the use of 

 sprays, treatment of the soil, and by the importation of insects 

 that lay their eggs in the larvae of adult beetles. 



Work of Bureau of Agriculture. Most of these methods of 

 destroying insects have been worked out for the farmer by differ- 

 ent bureaus of the Department of Agriculture. The Bureau of 

 Entomology works in harmony with the other divisions of the 

 Department of Agriculture, giving the time of its experts to the 

 problems of controlling insects which, for good or ill, influence 

 man's welfare in this country. The destruction of the malarial 

 mosquito ; the destruction of harmful insects by the introduction 



