MICROBIAL CONTROL OF INSECT DISEASES 161 



ment, especially high humidity and favorable temperature, which are 

 not always found under natural conditions. 



Glasgow (22^ established that some of the caecal bacteria of Het- 

 eroptera show a marked antagonism toward other bacteria and proto- 

 zoan parasites that occur in the intestines of these insects. The caecal 

 system of the insects was removed and dropped into nutrient bouillon, 

 where it remained for a month or more without showing any bacterial 

 growth. This was believed to be proof of the fact that the caecal bac- 

 teria are antagonistic to ordinary saprophytic and parasitic bacteria and 

 prevent their development} also they apparently kill these bacteria 

 when they invade the alimentary canal of the insect. 



According to Duncan (216), the bactericidal principle found in dif- 

 ferent insects and ticks shows differences in regard to the types of bac- 

 teria affected and the degree of their susceptibility. The gut-contents 

 of Argas and Stomoxys show the widest range of action j that of bugs, 

 the least. Spore-forming bacteria are especially affected by material 

 from Stomoxys, whereas staphylococci appear to be more susceptible to 

 the action of Argas material. The gut-contents of ticks was found to 

 have a weak activity upon P. festis, whereas the contents of certain in- 

 sects favored the growth of the latter. This phenomenon may have a 

 bearing upon the function of the plague flea. The action of the lethal 

 principle is greater and more rapid at 37° C. than at room temperature. 

 The lethal principle has been found to be active for at least six months 

 when kept in a dry state. It is thermostable, resisting temperatures as 

 high as 120° C, and is not destroyed by proteolytic enzymes. It appears 

 to be bound to proteins, since it is precipitated from solution by alcohol 

 and acetone, but it is not affected by these reagents. It is insoluble in the 

 common fat solvents. It becomes inactivated when allowed to act upon 

 bacteria and appears to be adsorbed by killed bacteria, even by species 

 that are not destroyed by it. This substance does not have the properties 

 of either bacteriophage or lysozyme. 



The presence in certain insects of a variety of other substances, such 

 as allantoin, which affect bacterial activities has also been established. 

 These observations give rise to the hope that man may in time succeed 

 in developing and utilizing microorganisms for the biological control of 

 injurious insects (881). 



