38 TECHNICAL BULLETIN 149, II. S. DEPT. OF AGRICULTURE 



cause of death, all of the methods of infecting other than the punc- 

 ture method gave negative results. 



In summing up the evidence from the experimental inoculations 

 with yeasts and the observations on bees, it appears that ordinarily 

 yeasts do not infect the blood or tissues of healthy, uninjured bees, 

 although they are capable of becoming actively pathogenic when 

 they gain entrance to the blood or tissues. 



ft has been shown that certain species of fungi are pathogenic to 

 honeybees of all ages. Some of these attack adult bees in nature 

 with considerable frequency and reach a degree of economic im- 

 portance when conditions favor the production of large numbers of 

 spores. Others attack bees so rarely under normal conditions that 

 their importance as causative agents of disease is negligible, whereas 

 still others that have never been found on bees in nature were found 

 to attack bees when inoculated experimentally. 



The greater number of fungus species that cause diseases of adult 

 bees also attack the brood. This was found to be the case with most 

 of the species or strains of Aspergillus used in these investigations. 

 When inoculated experimentally with a pathogenic Aspergillus, brood 

 is attacked and killed more quickly than are adult bees, though the 

 loss of brood resulting from Aspergillus mycosis is much less than 

 that of adult bees. 



The frequency with which bees are attacked by fungi in nature 

 appears to depend chiefly upon the virulence of the pathogenic 

 species and upon their dispersion. Conditions that favor abundant 

 growth of pathogenic fungi in nature are conducive to the spread 

 of fungous diseases. The fact that brood is rarely attacked can 

 probably be explained by the small probability that larval food will 

 contain a sufficient number of viable spores to cause infection. 



The pathogenicity of a fungus appears to be determined by the 

 ability of its spores and mycelium to resist the action of the intes- 

 tinal fluids within the digestive tract of bees. 



Mycosis of bees reaches its greatest significance with adult workers. 

 Throughout the active season an appreciable number of these are 

 killed by pathogenic fungi, principally by the yellow-green spored 

 Aspergilli. In one case about 40 per cent of a lot of bees that showed 

 symptoms similar to mycosis yielded pathogenic species of Asper- 

 gillus. Usually, however, only about 6 per cent of the suspected bees 

 were infected. 



It is difficult to estimate the importance of mycosis among bees 

 because adult bees generally die away from the hive, and, when the 

 percentage remains small, larvae may be carried out soon after 

 they become infected. The death rate from mycosis during the 

 winter varies with conditions within the hive. In strong colonies 

 under good wintering conditions it is negligible; in weak colonies, 

 when pathogenic fungi have grown over the combs, mycosis may 

 become an important factor in the dwindling and death of the colony. 



In view of the fact indicated by this study, that fungous diseases 

 of bees are not likely to assume the importance of either of the foul 

 broods, methods of control have not been extensively studied. 



Complete control would be difficult to attain, since worker bees 

 may be infected while gathering sweet juices from fruits, food prod- 

 ucts, and many other natural and prepared substances, upon which 



