Diseases of bees / Ministry of Agriculture and Fisheries.

Date:
1948
Catalogue details

Licence: Public Domain Mark

Credit: Diseases of bees / Ministry of Agriculture and Fisheries. Source: Wellcome Collection.

    6/36 (page 2)
    Cause. This disease is caused by a microscopic spore-forming organism known as Bacillus larvae. Spores from some previously existing source of infection, such:as contaminated honey robbed from another colony already weakened by the disease (see below, under ‘‘ Spread,’’ p. 3), become mixed with the brood food fed to the young larva by the nurse bees. The spores germinate within the body of the larva and give rise to bacteria, which then proceed to grow and multiply at a great rate, feeding at the expense of the tissues of the larva itself. Soon after the larva has been sealed over in its _ cell by the bees working on the comb, it collapses and dies. When this happens the food supply of the bacteria is no longer maintained, and their growth and multiplication ceases. Each bacterium then transforms itself back into the spore stage. The spores so produced, now many times more numerous than those which caused the larva to become infected in the first place, remain dormant until some of them, in their turn, after being distributed throughout the hive by house-cleaning bees attempting to clean out the cell containing the dead remains of the larva, are enabled to complete their life cycle by being mixed with the brood food of another larva. The whole process then starts again and is repeated indefinitely. More and more larvae become infected, the proportion of the brood which emerges gradually becomes less and less, and sooner or later the colony dies out from sheer lack of enough bees to keep it going. The spores are very resistant to exposure to extremes of heat and cold, and to disinfectants. They will retain their powers of germination for many years if left undisturbed in old combs kept in store, or in derelict hives, skeps or boxes. Symptoms. The collapse of the infected larva takes place within the sealed cell, after the cocoon has been spun, and is therefore not to be seen merely by looking at the surface of the comb. Associated with this collapse, however, are changes in the appearance of the cell capping, and these changes are of primary importance as a first indication of the presence of abnormal brood in the hive (see Fig. 2). The capping becomes moist and darkens in colour, and as the larva continues to shrink the capping is drawn down into the mouth of the cell, its originally slightly-domed shape becoming reversed. The worker bees nibble holes in the sunken capping and eventually remove it altogether, leaving an open cell containing the remains of the dead larva (see Figs. 3 and 4). The collapse of the larva is accompanied by a change in colour from the normal pearly-white of healthy brood to a creamy brown, light at first, then becoming darker. The consistency of the larval remains is very slimy at this stage and if a match-stick is thrust through the sunken capping into the cell, twisted round and then withdrawn, the slimy mass will pull out in the form of a glistening, mucus-like, brown thread or “‘ rope’ (Fig. 5). The “‘ rope ”’ may be light or medium brown, according to the length of time which has elapsed since the death of the larva, and it has an opaque, milky appearance. It is therefore very aptly described as being coffee-coloured. The ropy condition is succeeded by a tacky stage, as the larval remains in the cell gradually dry up, and the colour changes to dark brown. Further drying leads to the final stage, which is a very dark brown, rather rough scale lying fe