940 MICROBIAL DISEASES OF INSECTS 



Weakness is a colony symptom which will invariably be manifest 

 if a sufficiently large percentage of the bees of the colony are Nosema- 

 infected and if the infection persists for a sufficient period. When only 

 a small percentage of the bees are infected the weakness resulting may 

 never be apparent. The loss in strength may be gradual or sudden. 



A Nosema-infected colony behaves similarly to a healthy one. 

 The stores are sufficient. The queen does her work well. As the 

 colony dwindles the queen is usually among the last handful of bees. 

 The brood in general is normal in appearance, but in colonies weakened 

 by the disease not infrequently it is seemingly in excess of the amount 

 that can be properly cared for by the adult bees present. The workers, 

 especially the young ones, are most frequently infected, although drones 

 and queens are susceptible. It is not unusual to find from ten to twenty 

 per cent of the workers of diseased colonies infected. An infected, 

 bee manifests no outward symptoms of the disease when seen among the 

 other bees of the colony and it performs functions similar to those per- 

 formed by healthy ones. 



When the stomach of an infected bee is removed it may show marked 

 changes which are characteristic of Nosema-disease. The brownish 

 yellow or dark reddish hue of the normal stomach becomes gradually 

 lost as the disease advances. The organ is often increased in size, 

 the constrictions are less marked, and the transparency is diminished. 

 In late stages of the disease, however, the stomach approaches the 

 normal in size and the constrictions are again well marked. The organ 

 is then white and opaque and the tissues are fraible and easily crushed. 

 When crushed the mass presents a milky appearance. 



Upon microscopic examination, Nosema apis is found in very large 

 numbers in the crushed tissues. The presence of the parasite is almost 

 invariably recognized by its spore form. The presence of Nosema- 

 infected bees in a colony is the one constant colony symptom of the 

 disease: 



CAUSAL ORGANISM. White proves that Nosema apis is the cause of Nosema- 

 disease by a process of elimination. Maiden (1912-1913) who studied the bacter- 

 iology of Nosema-infected bees found that although the number of bacteria in dis- 

 eased bees, was much greater than in normal ones, there was no evidence of a direct 

 etiological relation existing between these bacteria and the disease. White himself 

 found that Nosema-disease is not caused by a filtrable virus. Higher animal 

 parasites and fungi being absent, and the bacteria and the filtrable viruses thus being 

 eliminated, tentatively at least, there remains only one group, the protozoa, and of 



