178 APPLIED ENTOMOLOGY 



That insects like other animals suffer from the attacks of various 

 diseases, is perhaps not generally realized. Yet the list of these diseases 

 is not a small one and our knowledge of them is still extremely limited. 

 Some of them are caused by bacteria and are just as truly germ diseases 

 as are those from which man suffers. Others are caused by parasitic 

 plants which in one way or another enter the body of the insect and grow, 

 consuming the nourishment they find there and finally kill the animal, 

 usually making its body hard and firm, or " mummifying" it. A third 

 type of disease is that known as the "wilt disease," in which neither 

 bacteria nor fungi have been discovered, where the insect "wilts" after a 

 time, becomes soft, and gradually decays. The producing cause of this 

 class of diseases is still unknown, but they are infectious, spreading from 

 one individual to another, and where the insects are abundant and 

 weather conditions are favorable they cause a high mortality. 



Attempts have been made to utilize diseases for the control of insect 

 pests. The Chinch Bug has been the subject of one of the most thorough 

 of these experiments, the fungus already referred to having been cul- 

 tivated for the purpose. It was found that by the use of appropriate 

 methods, cultures of the fungus obtained in the fall could be grown 

 during the winter, and bugs inoculated with it in the spring could be sent 

 out to fields where the insects were abundant, and liberated there to 

 spread the disease. To some extent this was a success, but it was soon 

 found that if the inoculated bugs were set free during dry weather the 

 disease failed to spread rapidly enough to prevent great injury, while if 

 the weather was wet the fungus was in most cases already present and 

 the addition of more diseased bugs at best only hastened its spread 

 somewhat. As a business proposition then, the artificial cultivation 

 and distribution of the fungus has been given up. 



In the case of a bacterial disease of grasshoppers which has at times 

 been observed greatly to reduce the numbers of this insect, somewhat 

 similar results have been obtained. In a few instances some degree 

 of success has been secured by spreading the germs, but here the factor 

 of cannibalism seems to enter into the problem. With species of grass- 

 hoppers which feed considerably on dead or dying individuals, there is 

 some probability of successful treatment in this way, but such species 

 are not numerous, and there also appears to be more or less immunity 

 to the germ in some species. 



The whole problem of control by disease appears to hinge on satis- 

 factory answers to three questions: Can the disease be cultivated so 

 that a supply can be obtained and continued? Can it be introduced 

 successfully into regions where it is needed but not present? Will the 

 disease establish itself there and become effective? 



The answers to the first two of these questions are liable to be affirma- 

 tive ones, though this is not always the case. The third is the most 



