570 



BIOLOGY AND HUMAN LIFE 



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

 eration of injurious insects. This can be used against insects that con- 

 fine themselves to special kinds of food plants. 



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

 eases 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 milhons are killed off. This method has 

 been successfully employed against locusts in South Africa and Yucatan. 



Fig. 240. The calosoma beetle (Calosoma sycophanta). (Somewhat 



reduced) 



This beautiful green animal was used by a French scientist in a campaign against the 

 gypsy moth in 1840. In recent years this method of fighting undesirable insects by 

 encouraging the spread of an enemy insect has been rapidly extended, especially in 

 the United States, which leads the world in applied entomology, a, adult: b, larva 

 feeding on pupa of gypsy moth; c, adult feeding on larva (caterpillar) of gypsy moth 



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

 important 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. 



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

 a species changes from year to year, partly on account of cli- 

 matic 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 



