After Bureau of Entomologj and Plant Quaiantine, U.S.D.A. 



A DOUBLE-FACED ENEMY 



The destructive black stem rust of wheat spreads rapidly through the summer by 

 means of spores, in two or three successive generations. The two-celled spores that 

 survive the winter cannot infect wheat. In the spring these spores produce short 

 hyphae, which bear multitudes of rather tender spores, which are also indifferent 

 to wheat 



eliminate mosquitoes, which depend upon wetness for their early stages — 

 egg, larva and pupa. And in doing so, as we all probably know, we interfere 

 with the continuity of the malaria parasite or of the yellow-fever virus (see 

 table, p. 620). 



Fighting Fire with Fire Biologists have found that a most effective 

 way of fighting an epidemic is with a counter-epidemic. Thus, since a trouble- 

 some species is probably kept in check in its native habitat by its natural 

 enemies, we can restore a disturbed balance by finding the natural enemy of 

 our pest. 



It has been possible to control the destructive Hessian fly by means of the 

 parasitic insect Polygnotus. The gypsy moth has been a constant source of 

 destruction to various cultivated crops since about 1870; it seems to be 

 coming under control with the introduction of the calosoma beetle from France 

 (see illustration, p. 596). One of the first suggestions that insects could be 



594 



