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.
Highlights
Honey bee (Apis mellifera) colonies are declining in the United States and in Europe causing economical stress to apiculture and to the agricultural crop production industries that rely on honey bee pollination [1]
Varroa-induced colony collapse is more accurately a result of secondary virus infections vectored by the mite that precipitates a progressive epidemic and colony mortality [8], [9], [10]
The mite infestation rate was negatively correlated with population size (R = 20.56; Fig. S1a) and had a significant explanatory effect on the total number of bees and brood in the colonies (F1,50.3 = 5.99, P = 0.0179)
Summary
Honey bee (Apis mellifera) colonies are declining in the United States and in Europe causing economical stress to apiculture and to the agricultural crop production industries that rely on honey bee pollination [1]. On top of a list of health stressors associated with honey bee colony collapse is the ectoparasitic mite Varroa destructor [2], [3]. This parasite has a world-wide distribution and inflicts more damage and higher economic costs than all other known apicultural diseases [4]. Initially obscure and practically unknown, deformed wing virus (DWV) has become one of the most prevalent viruses world-wide due to its close association with Varroa mite infestation [11], [12]. Varroa-mediated virus transmission during pupal development is directly responsible for the characteristic wing deformities resulting in flightless adults that die shortly after emerging [12], [13]
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