Increased Tolerance and Resistance to Virus Infections: A Possible Factor in the Survival of Varroa destructor-Resistant Honey Bees (Apis mellifera)

Abstract

The honey bee ectoparasitic mite, Varroa destructor, has a world-wide distribution and inflicts more damage than all other known apicultural diseases. However, Varroa-induced colony mortality is more accurately a result of secondary virus infections vectored by the mite. This means that honey bee resistance to Varroa may include resistance or tolerance to virus infections. The aim of this study was to see if this is the case for a unique population of mite-resistant (MR) European honey bees on the island of Gotland, Sweden. This population has survived uncontrolled mite infestation for over a decade, developing specific mite-related resistance traits to do so. Using RT-qPCR techniques, we monitored late season virus infections, Varroa mite infestation and honey bee colony population dynamics in the Gotland MR population and compared this to mite-susceptible (MS) colonies in a close by apiary. From summer to autumn the deformed wing virus (DWV) titres increased similarly between the MR and MS populations, while the black queen cell virus (BQCV) and sacbrood virus (SBV) titres decreased substantially in the MR population compared to the MS population by several orders of magnitude. The MR colonies all survived the following winter with high mite infestation, high DWV infection, small colony size and low proportions of autumn brood, while the MS colonies all perished. Possible explanations for these changes in virus titres and their relevance to Varroa resistance and colony winter survival are discussed.

Natural Varroa mite-surviving Apis mellifera honeybee populations

“…yet the mite population growth is slower than in mite-susceptible colonies and the Gotland colonies are able to survive the winters. By contrast, local mite susceptible colonies all perished with drastically high mite infestation only after one season without mite control treatment (>1 mite/bee; Locke et al 2014). Even though they survive, Gotland colonies often have DWV symptomatic adult bees with deformed wings and can have high DWV infections similar to mite-susceptible colonies (Locke et al 2014).…”

“…By contrast, local mite susceptible colonies all perished with drastically high mite infestation only after one season without mite control treatment (>1 mite/bee; Locke et al 2014). Even though they survive, Gotland colonies often have DWV symptomatic adult bees with deformed wings and can have high DWV infections similar to mite-susceptible colonies (Locke et al 2014). This could suggest that the population has also acquired a colony-level tolerance to DWV in addition to their adapted resistance to the mite as they manage to survive with high DWV infections when mite-susceptible colonies perish.…”

“…Autumn mite infestation rates can be high in the Gotland population (Locke et al 2014) relative to the winter mortality threshold for the region (>0.3 mites/bee; Fries et al 2003). yet the mite population growth is slower than in mite-susceptible colonies and the Gotland colonies are able to survive the winters.…”

“…This could suggest that the population has also acquired a colony-level tolerance to DWV in addition to their adapted resistance to the mite as they manage to survive with high DWV infections when mite-susceptible colonies perish. Black queen cell virus (BQCV) and sac brood virus (SBV) infections both decreased dramatically by the autumn in the Gotland mite-resistant colonies but increased in mite-susceptible colonies (Locke et al 2014). Although BQCV and SBV are seldom responsible for colony death, they are both virulent brood diseases that can have quite damaging effects on colony functioning and overall health (Ribière et al 2008;Bailey and Ball 1991;Bailey and Fernando 1972;Anderson and Giacon 1992).…”

Atypical viral and parasitic pattern in Algerian honey bee subspecies Apis mellifera intermissa and A. m. sahariensis

“…However, few mites were found positive for viruses in this survey. With the exception of CBPV and BQCV, which were rarely detected in mite samples elsewhere Locke et al 2014), we expected a high frequency of DWV and IAPV-ABPV in Algerian mite samples. No mites were found positive for IAPV or SBV, while DWV and ABPV were only detected in mites of 10 and 16 % of the A. m. intermissa colonies, respectively.…”