10 BULLETIN 780, U. S. DEPAETMENT OF AGRICULTURE. 



always necessary to establish the causal relationship , between the 

 germ and the disease. 



Because of the absence of any of the higher animal parasites and 

 of fungi in bees suffering from Nosema-disease these groups of para- 

 sites naturally can be eliminated as possible causal factors. Maiden 

 (1912, 1913) studied the bacteriology of Nosema-infected bees. He 

 found that the number of bacteria in the diseased bees was much 

 greater than in normal ones, the proportion being as 12 to 1. He 

 found, however, no evidence of a direct etiological relation existing 

 between these bacteria and the disease. Whether they play a 

 secondary rdle is a question which admits of much discussion but 

 one which is somewhat foreign to the present paper. 



Some preliminary experiments were made by the writer in regard 

 to the possibility of the presence of a filtrable virus in Nosema- 

 disease. The results obtained indicate that no such virus is present. 



By thus eliminating, at least tentatively, the higher animal para- 

 sites, the fimgi, the bacteria, and the filtrable viruses — groups of 

 parasites which cause diseases in animals — there remains another 

 group, the protozoa. Of this group there is only one species, Nosema 

 apis, that is constantly present in Nosema-disease. Other protozoa 

 are occasionally encountered in adult bees, but when found are 

 present usually in small numbers only. The conclusion is naturally 

 reached, therefore, that Noserna apis is the cause of Nosema-disease. 

 Such a conclusion is in harmony with views generally accepted at the 

 present time in regard to proof necessary to establish the causal re- 

 lation of such a germ to the disease. 



PREDISPOSING CAUSES. 



Experimental inoculations have shown that in general adult bees 

 of all ages are susceptible to Nosema infection. In nature it is foimd 

 that the youngest bees are always free from infection and that the 

 old shiny bees usually are. The absence of Nosema apis in the younger 

 ones may be attributed simply to the fact that they have not yet been 

 infected through the taking of food containing the germ. In the case 

 of the shiny bees it seems probable that they have escaped infection, 

 although it is possible that some of them might have been infected 

 at one time and later recovered. 



The brood does not seem to be at aU susceptible to infection with 

 Nosema apis. In heavily infected colonies the larvse and pupae appar- 

 ently remain healthy. In these studies larvae were inoculated more 

 or less directly by means of a pipette and examinations ^ were made 

 daily following the inoculation. The spores were found mixed with 



1 The examinations were made through fixing and sectioning inoculated larvse. 



