Stonebrood is a larval disease usually caused by Aspergillus flavus. Aspergillus fumigatus, A. niger, and sometimes other Aspergillus species are associated with the disease. These fungi are common soil inhabitants that are also pathogenic to adult bees, other insects, mammals, and birds.
The disease is difficult to identify in its early stages of infection. The fungus grows rapidly and forms a characteristic whitish-yellow collar like ring near the head end of the infected larva.
A wet mount prepared from the larva shows mycelia penetrating throughout the insect. After death, the infected larva becomes hardened and quite difficult to crush-hence the name. Eventually, the fungus erupts from the integument of the insect and forms a false skin.
At this stage, the larva may be covered with green, powdery fungal spores. The spores of A. flavus are yellow green, those of A. fumigatus are gray green, and those of A. niger, black. These spores can become so numerous that they fill the comb cells containing the affected larvae.
This disease can usually be diagnosed from gross symptoms, but positive identification of the fungus requires its cultivation in the laboratory and subsequent examination of its conidial heads. Aspergillus spp. can be grown on potato dextrose or Sabouraud dextrose agars. Caution: These fungi can cause respiratory diseases in humans and other animals.