262 



Journal oj Agricultural Research 



Vol. 45, No. 5 



Table 1. — Results of spore-feeding experiments " 



[Duplicate colonies of bees (A and B) were used in the first i years, and triplicate colonies (A, B, and C> 



in 1930] 



' +, Positive American foulbrood; ?+, probable American foulbrood, very slight and unconfirmed and 

 disappearing by end of brood-rearing season; 0, no disease found during season; — *, disease cleaned out by 

 end of brood rearing; — , no recurrence in second season. 



In 1926 a total of 200,000,000 spores fed to a colony was the 

 smallest number that produced disease; in 1927, on the other hand, 

 75,000,000 was the smallest number. However, in the latter year 

 the spores were obtained from another locality in which environ- 

 mental conditions were quite different. In an effort to obtain check 

 results, the feeding experiments were repeated in 1928. Through an 

 error in maldng up the spore dilutions, which was not discovered until 

 too late for rectification, no colony received less than 50,000,000 

 spores. This season one colony of the pair receiving an inoculation of 

 50,000,000 spores became infected. The feeding experiments were 

 repeated again in 1929, with dilutions of spores from 75,000,000 

 down to 100,000 — ^considerably less than the minimum number in 

 1928. Again only one colony of the pair receiving a total of 50,000,000 

 became infected. As a result of two years' experiments this was 

 foimd to be the apparent minimum number of spores of Bacillus larvae 

 capable of producing infection when fed in 1 liter of sugar sirup. In 

 1930 spores from three different locaUties were fed in duplicate to six 

 healthy colonies in dilutions of 50,000,000 and 25,000,000 without 

 prod\icing disease. 



It is therefore apparent that a certain minimum number or mass 

 of spores is required to start the initial action capable of producing 

 American foulbrood in healthy larvae. Under the conditions of these 

 expenments this minimum number was approximately 50,000,000 

 spores of inoculum per liter of sirup. 



SECONDARY OBSERVATIONS 



During the first tliree years of the experiments, or previous to 1929, 

 at which time the experimental colonies were isolated in pairs, certain 

 of the unmoculated control colonies developed disease, 1 out of 13 in 



