282 



Journal of Agricultural Research 



Vol. 45, No. 5 



demonstrated in 5 c c quantities from a comparatively small per- 

 centage of samples of commercial honey, the numbers are far below 

 the minimum necessary to produce infection when such honey is 

 used in healthy colonies of bees. Before definite conclusions can 

 be drawn, it will be desirable to examine many more samples of coxn- 

 mercial honey and to feed to healthy colonies samples of honey in 

 which the presence of spores has been demonstrated. 



Table 8. — Microscopic examination of dilutions for spores of Bacillus larvae " 



<• -j- indicates that spores were found; — indicates that spores were not found, by microscopic examina- 

 tion; ± indicates that the result was doubtful; ±? indicates that the positive was more doubtful than 

 the negative; — ? indicates that the absence of spores was not definite. 



SUMMARY AND CONCLUSIONS 



As a result of five years' study it has been found that, in order to 

 produce American foulbrood infection in a healthy colony of bees, 

 the sugar sirup used for inoculation must contain a certain initial 

 number of spores of Bacillus larvae. Seventy-three colonies were 

 inoculated during this time with numbers of spores ranging from 

 approximately 5,000,000,000 to 100,000 per colony; 30 of these 

 colonies receiving 50,000,000 spores or less. Of these 30 colonies, 2 

 out of 11 receiving 50,000,000 spores showed infection, but no colony 

 receiving less than that number of spores developed disease. There- 

 fore, the minimum infectious dose of B. larvae for a colony of bees 

 seems to be approximately 50,000,000 spores in 1 liter of sugar sirup. 



PreUminary experiments in which individual bee larvae were given 

 known numbers of spores of Bacillus larvae in 0.01 c c quantities of 

 sugar sirup show that infection can be produced bv this method, 

 but with considerable difficulty. From 50 to 100 larvae were 

 inoculated with each dilution of spores, ranging in number from 

 approxinaately 50,000,000 spores to, theoretically, 1 spore per larva. 

 The minimum infectious dose was found to be 10,000,000 spores per 

 larva fed in 0.01 c c of sugar sirup. These results indicate that the 



