48 BULLETIN 431, U. S. DEPARTMENT OF AGEICULTUEE. 



by the fact that colonies m the apiary that were not inoculated 

 experimentally remained free from disease, although many colonies 

 in the apiary were heavily infected at the time. 



Sacbrood has a tendency to weaken a colony in which it is present. 

 Frequently this weakness is noticeable and often marked. Rob- 

 bing, which occurs not infrequently at such a time, results in the 

 transmission of the virus, to some extent at least, directly to healthy 

 colonies. Robbing, therefore, must always be considered as a prob- 

 able means of transmission. 



The modes of transmission of sacbrood within the colony and from 

 colony to colony, as will be seen, are not by any means completely 

 determined. In what way the sacbrood virus is carried over from 

 one brood-rearing season to another is one of the many problems con- 

 cerning this disease that are yet to be solved. The foregoing facts, 

 accompanied by the brief discussions, it is hoped, will - throw some 

 light upon this important phase of the study — the transmission of 

 this disease — and will serve as an aid to later researches. 



DIAGNOSIS OF SACBROOD. 



The diagnosis of sacbrood can be made from the symptoms already 

 described (p. 10). The colony may or may not be noticeably weak- 

 ened. The adult bees are normal in appearance. Scattered here and 

 there on the brood frame among the healthy brood are found dead 

 larvae in the late larval stage. Usually there are only a few of them, 

 yet sometimes there are many. These larvae may be in capped or 

 uncapped cells. When found in uncapped cells, however, the cap- 

 pings had already been removed by the bees after the death of the 

 larvae. The cap over a dead larva in a cell may be found punctured 

 or not. The brood possesses no abnormal odor, or practically none. 



The post-mortem appearances of larvae dead of the disease are espe- 

 cially valuable in making the diagnosis. The larva is found extended 

 lengthwise in the cell and on its dorsal side. Throughout the period 

 of decay it will be found to maintain much of the form and markings 

 of a healthy larva of the age at which it died. Soon after death the 

 larval remains are slightly yellow. After a period they assume a 

 brownish tint. Since the brown color deepens as the process of decay 

 and drying takes place, the remains may be found having any one of 

 a number of shades of brov/n. They may appear at times almost 

 black. 



After death the cuticular portion of the body wall becomes tough- 

 ened, permitting the easy removal of the larva intact from the cell. 

 When removed, the saclike appearance of the remains becomes easily 

 apparent. Upon ruptm'ing the cuticular sac the contents are found 

 to be a brownish, granular-appearing mass suspended in a compara- 



