Interactions of Tropilaelaps mercedesae, honey bee viruses and immune response in Apis mellifera

Tropilaelaps mites are the major health threat to Apis mellifera colonies in Asia because of their widespread occurrence, rapid population growth and potential ability to transfer bee viruses. Honey bee immune responses in the presence of feeding mites may occur in response to mite feeding, to the presence of viruses, or to both. In this study, the mRNA expression levels were measured for three antimicrobial peptide encoding genes (abaecin, apidaecin and hymenoptaecin) and a phagocytosis receptor gene (eater) in worker brood infested with different numbers of actively feeding T. mercedesae. Also, all samples were measured for the amount of acute bee paralysis virus (ABPV), black queen cell virus (BQCV), deformed wing virus (DWV), Kashmir bee virus (KBV) and sacbrood virus (SBV). Using an artificial mite inoculation protocol, the analysis showed that apidaecin was significantly down-regulated when tan-bodied pupae were infested with 1-2 mites and when capping of the cells of newly sealed larvae were opened and closed without mite inoculation (o/c) as compared to the control group (undisturbed brood, no mite inoculation). Reduced transcription levels of the eater gene were also recorded in the o/c group. However, an up-regulation of apidaecin and eater genes was observed in highly infested pupae when compared to o/c group. This occurrence is perhaps due to an adaptive response of the bees to higher mite infestations by up-regulating their immune expression. No significant expression differences were detected for abaecin and hymenoptaecin and the viruses ABPV, KBV and SBV were not detected. However, 86.7% of the pupae were infected with DWV, 83.3% were infected with BQCV and 73% were infected by both of these viruses. In addition, the Tropilaelaps-inoculated pupae showed higher levels and incidence of DWV compared to uninfested pupae. The presence of these two honey bee viruses was not related to the number of T. mercedesae infesting the pupae. Also, the presence of variable levels of DWV and low levels of BQCV did not provoke any expression differences for any of the targeted genes. Overall, this research indicates that feeding by Tropilaelaps mites produces an immune response, that the level of viruses did not produce a correlated immune response by the four genes tested and that Tropilaelaps may be a potential vector of DWV but not to a high degree. The data indicated that the major impact of Tropilaelaps infestation is caused by the mite itself.