Salivary gland transcriptome of Varroa destructor reveals suitable targets for dsRNA-mediated control of the mite

Varroa destructor is a parasitic mite of honey bees and a major driver of honey bee colony losses. By feeding on the host’s body fluids, this obligate ectoparasite has a strong impact on honey bee physiology, causing the reduction of weight and longevity, and the spread of viral pathogens. Despite its importance, many aspects of Varroa lifestyle are still obscure. During feeding activity on the host, the mite injects salivary secretions only partly identified so far. Here we performed a differential expression analysis between the transcriptome of salivary glands and the rest of the body of V. destructor, identifying 12 transcripts encoding putatively secreted proteins. Using databases such as InterPro, UniProt and NCBI, we functionally annotated this secretome, which includes proteins belonging to cysteine protease, serine protease, endochitinase and phospholipase families. Selected genes were subsequently studied from a functional point of view, by analyzing their impact on mite survival following gene silencing. Mites were soaked in dsRNA solutions targeting chitin-binding, kazal serine protease inhibitor and papain cysteine protease, for a few hours and maintained on honey bee pupae under laboratory conditions. The effect on mortality was checked daily, while gene silencing was assessed at 72 hours after treatment, by qRT-PCR. The dsRNA mix targeting the three genes simultaneously resulted effective in reducing mite survival compared to controls. Further functional characterization of these salivary components will offer new knowledge on the molecular basis of Varroa-bee interactions, on which to develop innovative and eco-sustainable mite control strategies.