INSECT DISEASES AND MICROBIAL CONTROL 147 



MALARIAL AND TRYPANOSOME PARASITES 



In connection with the recent interest in antibiotic substances, con- 

 siderable work has also been done on the effect of these substances upon 

 different strains of Plasmodium causing malaria and upon different 

 trypanosomes causing various tropical diseases. Because of the war, 

 however, the results thus obtained have not yet been published. They 

 are highly interesting and offer promise of added application of these 

 substances. 



Weinman found (967) that the general correlation between the 

 gram-stain of bacteria and their sensitivity to gramicidin also extends 

 to protozoa {Leishmaniay Tryfanosoma) and to the Leftosfira tested. 

 Tyrocidine had a marked effect, in concentration of 5 y per ml., upon 

 the flagellates 5 they remained active for many hours, gradually losing 

 their motility} a few escaped giving rise to delayed growth. 



INSECT DISEASES AND MICROBIAL CONTROL 



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 (867). 



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- 



