INSECTS AND OTHER ORGANISMS 423 



Ladybird beetles have been imported from China to keep down 

 the San Jose scale. Calosoma beetles and several parasitic, wasplike 

 insects have been imported to fight the gypsy moth and the brown- 

 tail moth. This kind of work is growing very rapidly. There are now 

 several stations in this country where insects are cultivated on a large 

 scale, to be sent where needed in controlling injurious insects. 



Spraying of orchard trees, shade trees, and crop plants with various 

 kinds of poisonous mixtures is one of the common means used by 

 farmers in controlling the damage resulting from insect depredations. 



The rotation of crops is used for the purpose of starving out one 

 generation of injurious insects. This can be used against insects that 

 confine themselves to special kinds of food plants. 



In recent years it has been found that insects are subject to fatal 

 diseases caused by species of fungi. Cultures of such fungi have 

 accordingly been used to fight insects. A number of the insects are 

 caught alive and infected with the parasitic fungus, and then set free 

 again. The escaped insects infect their fellows, and millions are thus 

 killed off. This method has been successfully employed against locusts 

 in South Africa and in Yucatan. 



The other natural enemies of insects are toads, frogs, snakes, and, 

 most ii;nportant of all, nearly all kinds of birds. A given amount of 

 money spent in protecting and encouraging the native birds of a 

 region is likely to produce more beneficial results than any other 

 method of fighting insects. 



451. The balance of nature. The number of individuals in 

 a species fluctuates from year to year, partly on account of 

 climatic conditions, as when an early frost destroys plants 

 before the seeds are ripened, or when the frost kills off certain 

 enemies. But much of the fluctuation is due directly to the 

 variation in other species. A good year for ladybirds will mean 

 a poor year for scale lice ; but the following year the shortage 

 in scale lice will reduce the number of ladybirds. The living 

 beings are related to each other in hundreds of ways, so that the 

 elimination or unusual increase in one species is likely to affect 

 the well-being or even the existence of another. Every species 

 has its friends in the living world, as well as its enemies, and 

 when man undertakes to change the face of the earth with 



