The vectoring competence of the mite Varroa destructor for Deformed wing virus of honey bees is dynamic and affects survival of the mite

Authors: RYABOV, EUGENE - University Of Maryland; Posada-Florez, Francisco; Rogers, Curtis; LAMAS, ZACHARY - Non ARS Employee; Evans, Jay; Chen, Yanping - Judy; Cook, Steven

Submitted to: Frontiers in Insect Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/12/2022
Publication Date: 9/23/2022
Citation: Ryabov, E.V., Posada-Florez, F.J., Rogers, C.W., Lamas, Z.S., Evans, J.D., Chen, Y., Cook, S.C. 2022. The vectoring competence of the mite Varroa destructor for Deformed wing virus of honey bees is dynamic and affects survival of the mite. Frontiers in Insect Science. 2:931352. https://doi.org/10.3389/finsc.2022.931352.
DOI: https://doi.org/10.3389/finsc.2022.931352

Interpretive Summary: Apis mellifera honey bees are subject to harmful parasitism by ectoparasitic mites, Varroa destructor. This mite is a vector of honey bee viruses, including Deformed wing virus (DWV), types A and B, which are one of the top issues of honey bee health. This study investigated the ability, or competence, of Varroa to transmit an overt infection of either DWV A or DWV B during vectoring. Varroa mites were collected from the same set of four honey bee colonies at five occasions from May to October, 2021 and used in infectivity assays, which included placing mites on young honey bee pupae to allow mites to vector DWV for six days. Pupae were subsequently analyzed for the amount of either DWV A or B present. Less than half (39.8%) of tested mites vectored an overt infection (> 10^9 viral copies) in pupae, and both types A and B were vectored at a similar rates. An additional experiment kept collected mites on adult honey bee hosts for 12 days and then their vectoring competence was remeasured in infectivity assays. Mites retained in their phoretic (dispersal) stage showed significantly increased vectoring competence, with 89.8% of mites being able to induce overt infections in pupae. It was noted that the lifespan of mites was affected by the type of DWV that it transmitted to an overt level; The mites that induced overt DWV B but not DWV A infections lived 15.5 days on average, significantly shorter than mites inducing an overt DWV A but not DWV B infection (24.3 day lifespan), or the mites that did not induce an infection of either DWV A or B (21.2 day lifespan). Mites that transmitted high levels of both DWV A and B had an intermediate lifespan of 20.5 days. The negative impact of DWV B on mite survival could be a consequence of DWV B, but not DWV A to replicate in the mites. The results showing increased infectivity of mites kept for prolonged dispersal stage has a direct implication to an IPM strategy, queen caging, used by beekeepers to control Varroa; mites remaining in the colony may become increasingly competent to transmit overt DWV infections in bees, possibly leading to colony death.

Technical Abstract: The ectoparasitic mite, Varroa destructor and the viruses it vectors, including types A and B of Deformed wing virus (DWV), pose a major threat to honey bees, Apis mellifera. Analysis of 256 mites collected from the same set of field colonies on five occasions from May to October 2021 showed that less than a half of them, 39.8% (95% confidence interval (CI): 34.0 - 46.0%), were able to induce an overt-level DWV infection with more than 109 viral genomes per bee in the pupa after 6 days of feeding, with both DWV-A and DWV-B being vectored at similar rates. To investigate the effect of the phoretic (or dispersal) stage on adult bees on the mites’ ability to vector DWV, the mites from two collection events were divided into two groups, one of which was tested immediately for their infectiveness, and the other was kept with adult worker bees in cages for 12 days prior to testing their infectiveness. We found that while 39.2 % (95% CI: 30.0 – 49.1%) of the immediately tested mites induced overt-level infections, 12-day passage on adult bees significantly increased the infectiousness to 89.8% (95% CI: 79.2 – 95.6%). It is likely that Varroa mites that survive brood interruptions in field colonies are increasingly infectious. The mite lifespan was affected by the DWV type it transmitted to pupae. The mites, which induced overt DWV-B but not DWV-A infection had an average lifespan of 15.5 days (95% CI: 11.8 - 19.2 days), which was significantly shorter than those of the mites which induced overt DWV-A but not DWV-B infection, with an average lifespan of 24.3 days (95% CI: 20.2 - 28.5), or the mites which did not induce high levels of DWV-A or DWV-B, with an average survival of 21.2 days (95% CI: 19.0 - 23.5 days). The mites which transmitted high levels of both DWV-A and DWV-B had an intermediate average survival of 20.5 days (95% CI: 15.1 - 25.9 days). The negative impact of DWV-B on mite survival could be a consequence of the ability of DWV-B, but not DWV-A to replicate in Varroa.