16 BULLETIN 804, U. S. DEPARTMENT OF AGRICULTURE 



In the first group, colonies G and F, it is quite apparent that 

 the Italian bees, colony F, made the better showing, even though the 

 hybrids were the stronger colony in the beginning. As may be 

 seen from a comparison of the two plots in figures 2 and 3, in 

 colony G wherever there was a lag in the house cleaning there was 

 marked increase in the number of larvae remaining over more than 

 one observation period, and these increased until strengthening 

 treatment was started. On the other hand, in colony F these were 

 removed almost entirely by the time the strengthening treatment 

 was started. The Italians did not allow the disease to appear as 

 soon or to spread as rapidly, cleaned house better, left fewer larvae 

 to dry down to the brownish rubbery scales, and responded to the 

 increased honey flow and treatment much more readily. 



In the second group, colonies H, A, and I, the Italian colonies 

 A and I again made the best showing. With the added diluting 

 effect of the honey flow, they allowed the disease to gain no foothold 

 whatever, while the hybrids, though aided by the honey flow, soon 

 succumbed and allowed the disease to gain on them. It is evident 

 that the Italian bees are much more vigorous house cleaners. In 

 several instances, toward the end of the egg laying of the old queen, 

 and well along in the progress of the disease, cells were noted on 

 these diagrams which had previously contained diseased larvae, but 

 which had been cleaned out, and then in which disease had reap- 

 peared after other eggs had been laid and hatched in them. They 

 were cells in which fresh nectar had not been placed between the two 

 series of larvae. 



It was also noted that as the honey flow increased and as the 

 brood became more scattered from the effects of the disease, more 

 and more fresh nectar was placed in the brood nest in cells from 

 which dead larvae had been removed. Most of this nectar, however, 

 was moved up later, particularly after the bees began preparing the 

 brood nest for a new queen in the process of treatment. That the ad- 

 vent of a heavy honey flow was effective in controlling the disease is 

 evident, particularly in the length of time between the infection and 

 the first appearance of disease. The data, however, show little dif- 

 ference in the resistance to infection, or so-called immunity, being 

 slightly in favor of the Italians, if there is any difference at all. 



Disregarding the effect of the honey flow, the period of incubation 

 of the disease is apparently between 3 and 4 days. However, it was 

 noted that after Bacillus pluton was first observed it was anywhere 

 from 24 to 48 hours before many characteristically diseased larvae 

 were observed. Therefore, the actual period of incubation is prob- 

 ably from 24 to 48 hours. 



