A novel system for maintaining Varroa destructor mites on artificial diets and its application for studying mites as a vector for honey bee viruses

Authors: POSADA-FLOREZ, FRANCISCO - Orise Fellow; RYABOV, EUGENE - Volunteer; HEERMAN, MATTHEW - Volunteer; Chen, Yanping - Judy; Evans, Jay; SONENSHINE, DANIEL - Volunteer; Cook, Steven

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/6/2020
Publication Date: 11/24/2020
Citation: Posada-Florez, F., Ryabov, E., Heerman, M., Chen, Y., Evans, J.D., Sonenshine, D., Cook, S.C. 2020. A novel system for maintaining Varroa destructor mites on artificial diets and its application for studying mites as a vector for honey bee viruses. PLoS ONE. 15(11): e0242688. https://doi.org/10.1371/journal.pone.0242688.
DOI: https://doi.org/10.1371/journal.pone.0242688

Interpretive Summary: Varroa destructor mites are harmful ectoparasites of Apis mellifera honey bees, and together with the Deformed wing-like viruses (DWV-like) they vector, are considered a grave threat to the US beekeeping Industry. Methods to adequately control Varroa mites in large number of honey bee colonies, such as with commercial US beekeepers, have limited efficacy and misuse of chemical treatments has led to mite resistance. Laboratory research to study the mite's biology and physiology has progressed piecemeal; maintaining experimental mites in vitro is labor-intensive and requires access to fresh honey bee pupa hosts, and techniques for maintaining mites free of natural hosts are often pagued with mechanical issues and low mite survival. For studies of Varroa-mediated virus transission, infected pupae and adults have 'background' levels of DWV that makes it difficult to follow vectoring and transmission of particular strains of DWV-like viruses. Recently, genetically tagged, clone-derived cDNA vectors of DWV-like viruses has made it possible to follow vectoring and transmission routes for these viruses. Here, we describe a system for maintaining Varroa mites in vitro including a diet free of host tissues, for use in experimental studies of Varroa mite biology and physiology. We validated its utility with an experimental study of Varroa mite-mediated transmission of genetically tagged, cDNA vector for DWV-like virus, VDV1. The artificial diet was supplemented with VDV1 viral particles, and Varrroa mites having fed on this diet were shown to successfully transmit the virus infcction to their pupa hosts. The advantages of this novel in vitro system for Varroa mite maintenance using a diet free of the honey bee derived components, are its ease of use and convenient manufacture in large numbers.

Technical Abstract: The mite Varroa destructor is one of the most destructive parasites of the honeybee (Apis mellifera) and the primary cause of colony collapse in most regions of the world. These mites cause serious injury to their hosts and serve as the vector for several harmful honeybee viruses. Progress in the research to control these mites has been limited, often due to the difficulty inherent in studying these parasites under controlled in vitro conditions. Previous attempts to rear Varroa by feeding them on an artificial diet have had limited success. Several of the different methods used were plagued by leaking and other mechanical difficulties and, thus far, none have been widely adopted. Here we report a robust system for maintaining Varroa including an artificial diet which does not rely on access to honey bee hosts or their tissues contain honey bee tissue-derived components making it particularly valuable in studying Varroa mite vectoring of honeybee viruses. Our system allowed mites to remain alive for up to 5 days while feeding on a purely artificial diet. We demonstrated that when Varroa mites were reared on the artificial diet supplemented with the particles of honeybee viruses, the cDNA clone-derived genetically tagged VDV1 and the wild-type DWV, are acquired by the mites from the diet and could be transmitted to recipient honey bee pupae. The advantages of this novel in vitro system for Varroa mite maintenance using a diet free of the honey bee derived components, are its ease of use and convenient manufacture in large numbers. These features of this novel device are important for experimental studies independent of the honeybee host, and Varroa-vectored honey bee pathogens, as well as mite survival, nutritional needs, metabolic activity and related biological functions.