Sacbrood, a disease caused by a filterable virus, does not usually cause severe losses. It is most common during the first half of the brood-rearing season and often goes unnoticed, since it usually affects only a small percentage of the brood. In nature, adult bees detect and remove infected larvae very quickly. Therefore, by the time the beekeeper observes the symptoms, the disease may be too severe for the adult worker population to handle.

Figure 37. Sacbrood - side view of prepupa stage. (Photo by M. V. Smith, University of Guelph, Guelph, Ontario.)

Both worker and drone larvae are affected. Pupae may be killed occasionally, whereas adult bees are immune to it. Dead brood often is scattered among healthy brood. The cappings over dead brood are first punctured and later removed by the bees. Death usually occurs after the cell is sealed and the larva has spun its cocoon. The larvae gradually change from pearly white to dull yellow or gray and finally to black. The head of the larva, the first part of the body to change color, becomes black. (See Figure 37.) Larvae die in an upright position with raised heads. (See Figure 38.) Diseased larvae are easily removed intact from

Figure 38. Dead larva infected by sacbrood with typical dark, raised head. Larva may be removed in one piece. (Photo by M. V. Smith, University of Guelph, Guelph, Ontario.)

the cells, unlike those afflicted with American foulbrood. The contents of the larvae are watery, and the tough outer skin appears as a "sack or bag of fluid" when suspended. The dried scale lies flat on the lower side of the cell, with the head end raised and the tail flat on the bottom of the cell. The scales are rough and brittle and do not adhere tightly to the cell wall.

Nurse bees are suspected of transmitting the disease by carrying the virus from cell to cell. It is also believed that robber bees spread the disease by carrying contaminated honey from colony to colony.

Sacbrood usually disappears in the late spring when the honey flow has started. Strong colonies and regular requeening seem most effective in combating this disease. No antibiotic is effective in preventing or controlling sacbrood.