10 BULLETIN 671, U. S. DEPARTMENT OF AGRICULTURE. 



the water medium. Many of the spores stain slightly about the 

 periphery, which aids somewhat in the observation. Good results 

 may be obtained from an unstained water mount. The vegetative 

 forms of Bacillus larvae, when they are present, are observed to be 

 slender rods, which tend to occur in chains. 



CULTI-KKS. 



In culturing affected larvae it is the absence of growth on the agar 

 plates that is important in the diagnosis of American foulbrood. 

 This is because the spores of Bacillus larvae will not germinate and 

 grow on the ordinary media of the laboratory, and other growth is 

 absent because there are seldom secondary invaders present. Occa- 

 sionally there will be a spreading growth of B. vulgatus, or B. mesen- 

 tericus, and very rarely of B. alvei. On quite rare occasions a con- 

 siderable number of colonies of B. vulgatus or B. mesentericus have 

 been found. As both of these species form spores, as a rule, within 

 24 hours, their differentiation from B. alvei usually can be made in 

 this way. A. report on a sample of American foulbrood, therefore, 

 nearly always can be made within a day. 



SACBROOD. 



Sacbrood is an infectious brood disease of bees caused by a filter- 

 able virus (White, 1913 and 1917). 



GROSS CHARACTERS. 



(a) Character of caps and regularity of the brood. — Larvae usually 

 die after capping in this disease, some of the dead brood being un- 

 capped by the bees later. Occasionally the caps are punctured. An 

 area of comb affected with sacbrood therefore presents an irregularity. 

 So far as the age of the affected larva? themselves is concerned, there 

 is considerable uniformity owing to the fact that death in this disease 

 occurs after sealing during the two-day period of rest just preceding 

 pupation. The affected brood, however, is interspersed among healthy 

 brood of varying age, which adds to the irregular appearance of the 

 affected comb (PL II, fig. 1). 



(b) Proportion of affected brood. — As a rule there is not a large 

 proportion of affected brood in a given comb area. Often there will 

 be an affected larva only here and there. 



(c) Position within the cell. — The position of the affected larva 1 is 

 that of extension lengthwise along the floor of the cell, against which 

 the dorsal portion of the larva lies. The head is turned upward, to- 

 ward the roof of the cell. 



(d) Age. — The brood dies after it has been sealed. 



(e) Color. — Usually by the time brood is received for diagnosis the 

 color of the affected larva? is brown or quite dark — often almost 

 black. If the brood is in the earlier stages of decay, however, the 



