QI4 MICROBIAL DISEASES OF INSECTS 



exists: when one of their number becomes weak or where vegetation is 

 scarce both the nymphs and the adults eat one another. 



At the time of depositing the eggs, if the female or even the male 

 is diseased, the eggs will be forcibly soiled with the liquid of the diar- 

 rhoea and the bacillus will be conserved up to the time of hatching upon 

 the eggs or in the mucilaginous matter surrounding the eggs which the 

 locust has provided for their protection. 



A certain number of locusts among every swarm act as healthy 

 "carriers." Carriers among nymphs have never been found. 



The period of life of the insect affects its resistance. The adult 

 locust is individually much more susceptible than the nymph. The 

 habits of life of each, however, have a great influence. The nymphs 

 are continually in contact with the vegetation and with each other as 

 they march in very dense columns; they are endowed with a voracious 

 appetite and undergo in the short period of their larval life, five molts, 

 which are the periods of their least resistance. The winged locusts, 

 to the contrary, passing a large part of their life in the air, are only 

 rarely pressed one against the other, except, for example, when the 

 weather is cold; they also eat much less than the nymph, thus the 

 epizootic will have a greater tendency to become generalized among 

 the bands of nymphs than among the swarms of adult locusts. 



The age of the nymph or locust influences its resistance; the young 

 nymphs have a maximum resistance, but this decreases gradually, 

 reaching its minimum at the time of the last molt; the adult locust has 

 its minimum of resistance at the egg-laying period. 



The period of the molt is not a means of protection against this 

 type of disease, which is a generalized septicemia. 



ARTIFICIAL. The virulence of B. acridiorum decreases very rapidly in culture 

 and in order to obtain the desired destruction of locusts it is absolutely necessary 

 to employ cultures of the highest possible virulence as an attenuated virus immunizes 

 the locust and renders it refractory to a culture of the highest virulence when applied 

 later. 



The virulence is increased by successive passages through locusts or nymphs; 

 twelve series of passages are made using twelve locusts in each series. The cul- 

 ture to be rejuvenated is mixed with a few cubic centimeters of sterile water or broth. 

 Injections are made with a syringe having a very fine sharp-pointed needle. The 

 insect is seized with the left hand, the ventral portion toward the operatoi, and the 

 needle of the syringe inserted between the second and third anterior abdominal 

 segments at the point of intersection with one of the longitudinal ridges, horizon- 



