46 BULLETIN 431, U. S. DEPARTMENT OF AGRICULTURE. 



Technically the foregoing studies should be thought of as being 

 preliminary. Questions relating to virulence of the virus, resistance 

 of the bees, technique, and many other factors contribute to make 

 results such as these vary. For practical purposes, however, they 

 are sufficiently complete. In estimating the time necessary for the 

 destruction of the virus in practical apiculture by any of the fore- 

 going tables of results it should be emphasized that the time element 

 should be somewhat increased, inasmuch as the conditions present in 

 the experiments were more favorable for its destruction than would 

 ordinarily be the case in practice. 



MODES OF TRANSMISSION OF SACBROOD. 



The transmission of a brood disease must be thought of as taking 

 place (1) from diseased to healthy brood within a colony and (2) from 

 a diseased colony to a healthy one. The manner in which sacbrood 

 is spread naturally depends directly upon the modes by which the 

 virus of the disease is transmitted. 



As is shown experimentally, the virus of sacbrood produces the 

 disease when it is added directly to the food of young larvae or when 

 it is mixed with sirup and fed to a colony. 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. 



Bees have a tendency to remove diseased or dead larvae from the 

 cells. When the removal is attempted about the time of death, it 

 is done piecemeal. Each fragment removed from such a larva, if 

 fed to a young healthy larva within a week, would most likely 

 produce sacbrood in the larva. Within the hive, therefore, the dis- 

 ease may be transmitted to healthy larvae more or less directly in 

 this way. 



Just what becomes of these bits of tissue removed from the dis- 

 eased larvae, however, is not known. If it were the rule that the 

 tissues of the dead larva after being removed in fragments were fed 

 unaltered to the young healthy larvae within two weeks after its 

 removal, it would seem that the disease would increase rapidly in 

 the colony as a result. Such an increase, however, is unusual, the 

 tendency in a colony being in most cases toward a recovery from 

 the disease. 



This fact leads one to think of other possibilities regarding the 

 destiny of the infected tissues removed as fragments from the dis- 

 eased larvae. If the infective material were fed to the older larvae, 

 death probably would not result. Should it be used by adult bees as 

 food for themselves, the likelihood of the transmission of the dis- 

 ease under such circumstances would apparently be very materially 

 reduced. If the infective material were stored with the honev and 



