54 BULLETIN 780, U. S. DEPARTMENT OE AGRICULTURE. 



cent are infected they become weakened and usually die as a result 

 of the infection. Generally speaking, therefore, it may be said that 

 when a colony contains less than 10 per cent of Nosema-infected bees 

 the prognosis is excellent; that when it contains more than 10 and less 

 than 50 per cent the prognosis is fair; that when it contains more 

 than 50 per cent the prognosis is unfavorable; and that when the 

 number of Nosema-infected bees present approaches 100 per cent the 

 prognosis is especially grave. 



In arriving at a decision as to the probable course and outcome of 

 the infection the strength of the colony must also be considered. 

 This factor, indeed, may be the deciding one. As a rule, the stronger 

 the colony, the more favorable is the prognosis. 



In early spring heavy losses among the workers are not replaced 

 and the colony weakens. During the active brood-rearing season, 

 on the other hand, the bees dying of the infection are replaced by 

 young bees. These young bees being free from infection and the 

 transmission of the disease within the hive during summer being 

 slight as a rule, the prognosis at this season of the year is favorable. 



Experimentally it is found that a single inoculation early in the 

 spring will cause a colony to die as a result of the infection produced ; 

 if inoculated somewhat later, however, the colony will weaken 

 appreciably but wiU recover from the infection; if inoculated during 

 the active brood-rearing season the weakening effect resulting from 

 the infection may not be appreciable; if inoculated toward the close 

 of the brood-rearing season the weakness resulting wiU be noticeable, 

 but the colony may winter; and if inoculated later in the autumn 

 or during the winter the colony will die as a result of the infection. 

 It will be seen, therefore, that the prognosis in Nosema-disease in 

 every case is dependent in some measure upon the season of the 

 year, being more favorable in the active brood-rearing season than in 

 any other. Indeed the season may play a major role in determining 

 the course and outcome of the disease. 



The immediate environment of the apiary may possibly play a 

 r61e in determining the prognosis. Opportunity for reinfection from 

 without tends to vary the course and outcome of the disease. In this 

 connection the nature of the water supply should not be overlooked. 



The extent to which the different races of bees vary in their suscep- 

 tibility to the disease, the extent to which individual colonies vary in 

 their susceptibility, and the extent to which different strains of 

 Nosema a-pis vary as to their virulence are not at all definitely knoMTi 

 at the present time. The facts, however, indicate that in no instance 

 is the variation particularly great. Much care should be exercised, 

 therefore, in ascribing variations in losses from the disease to the two 

 phenomena virulence of the germ and resistance of the host. 



