Interactions among Varroa jacobsoni mites, acute paralysis virus, and Paenibacillus larvae larvae and their influence on mortality of larval honeybees in vitro

The mortality of honey bee larvae and pupae reared in vitro caused by various combinations of oral inoculation with Paenibacillus larvae larvae spores, acute paralysis virus (APV), and infestation with Varroa jacobsoni was studied. The effect of the mite itself and the mite acting as a vector of APV on the mortality of larvae and pupae was investigated. Mortality caused by P. l. larvae ranged from 25 to 55% depending on spore dose. Oral inoculation with APV caused 9% mortality, which was not additive to the mortality caused by P. l. larvae. P. l. larvae did not induce the activation of APV infection. The mortality caused by V. jacobsoni itself was 25%, and by mites transmitting APV, 55%. Neither the mites themselves or the mites transmitting APV had an additive effect on mortality caused by P. l. larvae. The study suggests that APV transmitted by mites is the most significant cause of mortality of the treatment combinations tested. The results do not suggest that the mite itself or the mites transmitting APV act as a stress factor provoking clinical symptoms of American foulbrood (AFB) in individual larvae in the in vitro rearing system.