MICROBIAL CONTROL OF INSECT DISEASES 159 



strains did not adsorb the toxin, as did the susceptible strains} the 

 susceptible trypanosomes were destroyed completely by antiserum, 

 whereas the resistant forms were also resistant to this antiserum. 



A lipid-like substance produced by species of Phycomyces was ac- 

 tive against Tryfanosoma equiferdum in vitro but not in vivo (830a). 



MICROBIAL CONTROL OF INSECT DISEASES 



Insects are subject to attack by various groups of microorganisms, 

 including bacteria, fungi, protozoa, nematodes, and other insects. Many 

 attempts have been made to control insect pests by the use of pure or 

 mixed cultures of microorganisms. In this connection the following re- 

 lationships must be considered: the receptivity of the insect to microbial 

 attack during its various stages of development} the environmental 

 conditions favoring the attack on the insect by the disease-producing 

 organism; the influence of environment upon the virulence of the at- 

 tacking microbe; the manner in which the parasite attacks the host; the 

 coordination of the optimum activity of the disease-producing agent 

 with the abundance of the host and the proper stage of its develop- 

 ment. 



The microbial agents that keep in check the spread of insects, some of 

 which are highly injurious to plants and animals, are far more impor- 

 tant than any other methods of control. These microbial agents can be 

 classified into three groups, depending upon the nature of the host: (a) 

 microbes that attack economically useful insects and that must be con- 

 trolled in order to avoid important losses from disease; (b) microbes 

 that attack injurious insects and that must therefore be favored and en- 

 couraged; (c) microbial agents infectious to plants, animals, and man 

 that are spread by insects. 



Various bacterial diseases that formerly caused considerable destruc- 

 tion of silkworms and bees have been controlled, once the nature of the 

 organisms concerned was established. One of Pasteur's important con- 

 tributions to microbiology was the control of Flacheria among silk- 

 worms. However, most of the problems of control of injurious insects 

 have been difficult to solve. A great number of bacterial, fungus, and 

 virus diseases of insects are now known, but the many attempts to em- 



