Abstract: To explore why the fungus Ascosphaera apis only infects the honeybee stretched larvae some hours before and after sealing and the related infection mechanism, the inoculation experiments were carried out with different dosages of A. apis spores (0, 1.0×10², 1.0×10³, 1.0×10⁴, 1.0×10⁵ and 1.0×10⁶ spores/mL), inoculation time (3rd, 4th, 5th and 6th instar larva) and low-temperature treatment of 28℃. Furthermore, larvae in different infection stages were sectioned to investigate the infection process of A. apis. The results showed that A. apis pathogenicity is dose-dependent, A. mellifera ligustica larvae could resist the infection of A. apis at the dosages lower than 1.0×10³ spores/mL and the corresponding infection rates at these dosages had no significant difference from the control (P>0.05). The differences of morbidity in different instars of larvae inoculated are due to the difference in spore dosage ingested by different instar larvae. Chilling treatment at the transformation stage significantly increased the infection (P<0.05). In addition, pathologic section showed that the spores in the midgut could not grow due to lack of oxygen, unless they could arrive the hindgut accompanying with faeces. The mycelium in hindgut broke through the gut wall, massively proliferated in body cavity and finally outgrew from the cuticle. Larvae in transformation with the maximal nutrient substances stored and significantly decreased immunity provide A. apis the best opportunity to infect by increasing the possibility of success and decreasing the cost. Altogether, the present work provides a foundation to elucidate the mechanism of A. apis infection and the mechanisms of insect-pathogen interaction

Key words: Honeybee, Ascosphaera apis, Apis mellifera ligustica, infection, spore dosage, inoculation time, pathogenicity