MICROBIAL DISEASES OF INSECTS 933 



CAUSAL ORGANISM. No microorganisms have been found either culturally or 

 microscopically. However, experimental evidence shows that the etiological 

 factor is a filtrable virus. The virus contained in a single larva recently dead of the 

 disease has been found sufficient to produce infection in and death of at least 3000 

 larv;e within a week. If the virus from one larva each succeeding time were given 

 the opportunity of increasing 3,ooo-fold, in less than two weeks, theoretically, 

 a sufficient amount of virus would be produced to infect 9,000,000 colonies, more 

 colonies probably than are to be found at present in the United States, and within 

 three weeks enough virus could be produced to inoculate every colony in existence. 

 These figures give an idea of the enormous rapidity with which the sacbrood virus 

 is capable of increasing. However, it is possible that so small an amount of virus 

 may be taken up by an individual larva that no disease results. 



The sacbrood virus is influenced by various physical, chemical and biological 

 agencies, which influences, if applied rightly, may constitute control measures. 

 The virus is killed by heating in water at 60 for ten minutes. In honey, however, 

 it is necessary to employ 70 for ten minutes, while if no heat is applied and the virus 

 is shielded from the sun it remains alive nearly a month. Drying at room tempera- 

 ture for approximately three weeks was not destructive but for longer periods of 

 time it was fatal. Dried virus exposed to the direct rays of the sun was destroyed 

 in from four to seven hours; when suspended in water from four to six hours only was 

 required for destruction, but when suspended in honey fatal effects were obtained on 

 exposure to sunlight for five to six hours. Five days was required to destroy the 

 virus under the influence of fermentative processes in a 10 per cent sugar solution at 

 room temperature, and also in a 20 per cent honey solution at outdoor (summer) 

 temperature. Putrefactive processes (infected larvae crushed and mixed with soil 

 in water) allowed the virus to remain active for approximately ten days. Carbolic 

 acid in 0.5, i.o and 2.0 per cent solution was resisted by the virus for more than three 

 weeks; 4.0 per cent is more effective. However, experiments show that neither 

 this chemical nor quinine should be relied upon as a means of treating sacbrood. 



METHODS OF INFECTION AND CONTROL. The transmission of any 

 brood disease takes place (i) from diseased to healthy brood within a 

 colony and (2) from a diseased colony to a healthy one. As has been 

 shown experimentally the virus of sacbrood produces the disease in all 

 cases when larvae, sick and dead of this disease are picked from the 

 combs, crushed, diluted with sterile water, the suspension filtered by 

 means of a Berkefeld filter and the filtrate so obtained fed to healthy 

 colonies either directly or mixed with sirup. From this fact it is fair 

 to assume that sacbrood may result whenever the food or water used 

 by the bees contains the living virus of the disease. 



The period from time of inoculation to the appearance of the first 

 symptoms of the disease the incubation period is approximately 

 six days, being frequently slightly less. By inoculation the disease 



