Chalkbrood is a mycological brood disease of the Western honey bee (Apis mellifera), caused by the fungus Ascosphaera apis. The aim of this study was the investigation of the pathology of artificially reared Apis mellifera larvae, experimentally infected with A. apis spores (1.0 × 10³ spores/larva). Non-infected larvae served as control. Five living larvae and every dead larva were collected daily (day 1–7 p.i.). All larvae were macroscopically measured, photographed, formalin-fixed, and histologically processed (hematoxylin-eosin stain, Grocott silvering). Histological sections were digitized, and the size of the larvae was measured (mouth-after length, area) and statistically analyzed. Twenty-six larvae from the collected larvae (n = 64; 23 dead, 3 alive) showed histological signs of infection from 3 d p.i. onwards. The dead larvae showed macroscopically white/brown deposits, indistinct segmentation, and a lack of body elongation. Infected larvae were significantly smaller than the controls on days 3 p.i. (p < 0.05), 4 p.i. (p < 0.001), and 6 p.i. (p < 0.05). The early time of death, the low number of transitional stages, and the strong penetration of the larval carcass with fungal mycelium indicate a rapid and fulminant infection process, which is probably relevant for spreading the disease within the colony.

Stonebrood (Aspergillus sp.) is a rare; poorly described disease of the Western honey bee (Apis mellifera) that can affect adult bees and brood. This study describes the pathogenesis using artificially reared pathogen free Apis mellifera larvae, experimentally infected (5 x 10² spores/larva) with Aspergillus flavus. Between day 1 and 5 p.i. (larval age 4 until 8 days), 5 uninfected control larvae, up to 5 infected living larvae, and up to 5 infected dead larvae were collected and examined macroscopically. Subsequently the larvae were photographed, fixed (4% formaldehyde), and processed for histological examination (hematoxylin-eosin stain, Grocott silvering). Sections were digitized, measured (area, thickness), and statistically analyzed. 19 of 43 collected infected larvae showed signs of infection (germinating spores/fungal mycelium): dead larvae (from day 2 p.i.) showed clear histological and macroscopic signs of infection, while larvae collected alive (from day 1 p.i.) were only locally affected. Infected larvae were significantly smaller (day 2 p.i.: p < 0.001, 4 p.i.: p < 0.01, 5 p.i.: p < 0.01) than uninfected larvae (control group). The short incubation period and the early death during pathogenesis could explain why the disease often goes unnoticed: the worker bees remove dead larvae when mycelium has not yet infiltrated the brood cell.

Stonebrood (Aspergillus sp.) is a rare, poorly described disease of the Western honey bee (Apis mellifera) that can affect adult bees and brood. This study describes the pathogenesis using artificially reared pathogen-free Apis mellifera larvae, experimentally infected (5 × 10² spores/larva) with Aspergillus flavus. Between days 1 and 5 p.i. (larval age 4 until 8 days), five uninfected control larvae, up to five infected living larvae, and up to five infected dead larvae were examined macroscopically. Subsequently, the larvae were photographed, fixed (4% formaldehyde), and processed for histological examination (hematoxylin–eosin stain, Grocott silvering). Sections were digitized, measured (area, thickness), and statistically analyzed. In total, 19 of the 43 collected infected larvae showed signs of infection (germinating spores/fungal mycelium): dead larvae (from day 2 p.i.) showed clear histological and macroscopic signs of infection, while larvae collected alive (from day 1 p.i.) were only locally affected. Infected larvae were significantly smaller (day 2 p.i.: p < 0.001, 4 p.i.: p < 0.01, 5 p.i.: p < 0.01) than uninfected larvae (control group). Our study shows that the pathogenesis of stonebrood is characterized by a short period between Aspergillus germination and the onset of disease (about one day), and a rapid larval death.