DIVISION IX 



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MICROBIAL DISEASES OF INSECTS 



INTRODUCTION* 



Microbial diseases are of interest to the layman from two economic 

 standpoints : 



I. At certain stages of their existence, certain insects have 

 an economic value; for this reason their breeding is desirable and any 

 plague which devastates their numbers should be combated. Pas- 

 teur was the pioneer in this line, not only being the discoverer of the 

 first known bacillary insect disease, flacherie of the silk-worm, but he 

 worked out an efficient method for its scientific control. His work is 

 the more notable since he was handicapped by the lack of suitable 

 methods of isolation and study of the organisms discovered. 



II. Certain insects or their larvae are at times veritable plagues 

 laying waste valuable crops and causing serious hardships, even 

 famine and epidemic disease resulting in many cases. Not infrequently 

 these insects naturally become subject to microbial enemies which make 

 heavy inroads on their numbers, thus checking the insect plague. Such 

 an epizootic occurred among the white grubs in Michigan in 1912. 



The artificial employment of these microbial enemies naturally 

 suggests itself as a means of voluntary control, and such experiments 

 have been carried out successfully on a practical scale. One of the 

 best examples of this is seen in the arrest of the locust epizootic in Mexico 

 and the Argentine Republic by the use of cultures of B. acridiorum. 



Another thing worthy of note which has been mentioned many times 

 by those working with microbial insect diseases, is the fact that these 

 diseases seem to be almost explosive in character; an epizootic among 

 insects caused by a fungus disease is after a comparatively short time 

 entirely wiped out and another disease takes its place; in many places 

 bacterial diseases seem to have almost entirely supplanted the fungus 



* Prepared by Zae Northrup Wyant. except paragraphs on " Miscellaneous Fungus Dis- 

 eases" by C. Thorn. 



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