ON ISLE OF WIGHT DISEASE IN HIYE BEES— ETIOLOGY. 
753 
upon the age and fertility of the queen. Also, for normal prosperity, there must be 
maintained a definite proportion of nurse bees and foragers. 
In a colony affected with disease of any kind, which significantly affects the 
normal mortality rate, the age incidence of the mortality amongst the workers, 
and the productivity of the queen, are characters of the highest importance as 
affecting the maintenance of the colony as an effective and prosperous unit. These 
two opposing factors struggle with each other — losses from idleness and crawling, and 
mortality due to disease, added to the normal wastage on the one hand, and gains 
from the production of new bees on the other. A young and prolific queen by sheer 
production of new bees may so keep down the proportion of infected and hence more 
or less ineffective members as to render such a colony to some degree profitable. 
Disease may be maintained within a colony in two important ways. It may 
be instituted by the infection of a few members of the colony by contact with a 
single bee carrying mites, which has mingled in the cluster. This may be a stray 
bee from another colony or a member of the stock which has been robbing a diseased 
colony, and such infection may constitute the only one from the outside. In this 
case we may expect that progress will be slow, if indeed the infection does not die 
out. The attacked bees may be old and die away from the hive before transmitting 
Tarsonemud to other members of the colony, or the infection may be so swamped by 
normal increase as to be practically ineffective. Whether a stock once infected is 
doomed sooner or later in every instance we have not sufficient evidence as yet to 
say. Some of the cases quoted, if the samples of bees taken may be regarded as 
representative, appear to have lost the infection. And we do have some evidence 
that extinction maybe delayed for a long time. Rennie and Harvey, No. 1 (1919), 
have already directed attention to cases where the source and time of infection of a 
stock was known in autumn and the usual symptoms did not become evident until 
the following year. 
A second and highly important factor, however, which we are satisfied is very 
frequently in operation, is repeated or multiple infections continued from the outside 
over a considerable period of time. We then have the disease spreading from many 
foci. The drifting of bees into strange hives is common. Once the disease has 
gained some ground, the social instinct of the colony is weakened, both by the 
disturbance of the normal balance of worker types and by the illness of a high 
proportion of bees. Robbing may now take place, and amongst the robbers there 
may be infected bees which will intensify the trouble. This robbing, at first resisted, 
is eventually allowed to become rife, and when this is established we have noticed 
that extinction is practically inevitable. 
Other factors which may tell against the stock are the presence of an indifferent 
queen whose production may be poor, and from whose low racial vigour shorter- 
lived bees result. 
The varying character of the factors shows that a uniform course of spreading of 
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