Effect of oral infection with Kashmir bee virus and Israeli acute paralysis virus on bumblebee (Bombus terrestris) reproductive success

Highlights

• IAPV and KBV infection result in impaired offspring production in Bombus terrestris.

• KBV infection impairs colony startup in micro-colonies.

• It is important to understand viral dynamics between managed, reared and wild bees.

Abstract

Israeli acute paralysis virus (IAPV) together with Acute bee paralysis virus (ABPV) and Kashmir bee virus (KBV) constitute a complex of closely related dicistroviruses. They are infamous for their high mortality after injection in honeybees. These viruses have also been reported in non-Apis hymenopteran pollinators such as bumblebees, which got infected with IAPV when placed in the same greenhouse with IAPV infected honeybee hives. Here we orally infected Bombus terrestris workers with different doses of either IAPV or KBV viral particles. The success of the infection was established by analysis of the bumblebees after the impact studies: 50 days after infection. Doses of 0.5 × 10⁷ and 1 × 10⁷ virus particles per bee were infectious over this period, for IAPV and KBV respectively, while a dose of 0.5 × 10⁶ IAPV particles per bee was not infectious. The impact of virus infection was studied in micro-colonies consisting of 5 bumblebees, one of which becomes a pseudo-queen which proceeds to lay unfertilized (drone) eggs. The impact parameters studied were: the establishment of a laying pseudo-queen, the timing of egg-laying, the number of drones produced, the weight of these drones and worker mortality. In this setup KBV infection resulted in a significant slower colony startup and offspring production, while only the latter can be reported for IAPV. Neither virus increased worker mortality, at the oral doses used. We recommend further studies on how these viruses transmit between different pollinator species. It is also vital to understand how viral prevalence can affect wild bee populations because disturbance of the natural host-virus association may deteriorate the already critically endangered status of many bumblebee species.

Introduction

The Apoidea, encompassing different families of bees, perform a valuable pollination service (Garibaldi et al., 2013). With up to 80% of the plant species being dependent on insect pollination, in particular by bees (Potts et al., 2010). This results in an estimated value of 9.5% of the total economic value of crops that are directly used for human food (Gallai et al., 2009, Potts et al., 2010). Because of a lack of abundance/presence of wild bees, managed bees are used to pollinate crops (Allsopp et al., 2008).

Because different bee species have a similar foraging behavior (gathering pollen and nectar), with overlapping flower networks, sympatric distributions and direct interactions between species or their stored resources, it is very likely that they are exposed to each other’s parasites and pathogens. Indeed, parasite networks between bee species are complex and comprise a mixture of multi-host parasites (e.g. Apicystis bombi (Maharramov et al., 2013), Nosema ceranae (Graystock et al., 2013a), deformed wing virus (DWV) (Fürst et al., 2014)), as well as multi-parasite hosts (Rigaud et al., 2010). However, with the exception of honeybees (Apis spp.), little is known about the parasites and pathogens of pollinators, even less about the extent to which they cross-infect different pollinators, and almost nothing about the damage of such cross-infections to different hosts.

Here we focus on the effects of interspecific transmission of bee viruses. Most of what is known about bee viruses relates to the European honeybee (Apis mellifera) and its sister species (primarily the Asian hive bee; A. cerana), largely through the pioneering work of Bailey and Ball (1991) during the second half of the twentieth century. The evidence increasingly suggests a large degree of commonality of honeybee viruses among the Apis species (Ai et al., 2012, Choe et al., 2012, Kojima et al., 2011, Yañez et al., 2012, Zhang et al., 2012), usually with similar symptoms. Many honeybee viruses have also been detected in other Hymenopteran pollinators, predators and scavengers, initially mostly through incidental observations (Anderson, 1991, Bailey and Ball, 1991) and more recently also through dedicated research (Celle et al., 2008, Evison et al., 2012, Fürst et al., 2014, Genersch et al., 2006, Li et al., 2011, Peng et al., 2011, Singh et al., 2010, Yañez et al., 2012). Bee viruses have also been detected in non-Hymenopteran hosts associated with honeybees (Celle et al., 2008, Dainat et al., 2009, Eyer et al., 2008, Gisder et al., 2009). Honeybees may also be hosts or vectors of certain aphid viruses (Runckel et al., 2011), through the collection of honeydew, or possibly even plant viruses (Li et al., 2014), which could also be transmitted on to other pollinators, through their overlapping contact network with honeybees.

Because of their wide foraging range, large diversity of floral resources visited, long foraging seasons and extensive accumulation of stored pollen and nectar, honeybees are likely to be major factors in any pathogen transmission network involving other (Hymenopteran) pollinators. The worldwide trade in honeybees and bee products coupled with the increasing pathogen prevalence and loads in honeybee colonies, due to a variety of biological and environmental stressors (Genersch et al., 2010a, vanEngelsdorp and Meixner, 2010), could therefore have potentially serious consequences for local wild bee populations (Fürst et al., 2014, McCallum and Dobson, 1995, Meeus et al., 2011).

However, the above mentioned arguments have so far been largely speculative. Other than detecting honeybee pathogens in other insects, and thus establishing possible transmission routes (e.g. (Evison et al., 2012, Li et al., 2011, Peng et al., 2011, Singh et al., 2010), there has been little research as to whether these viruses are actually infectious or, more importantly, cause damage to species other than honeybees. The only recorded exceptions so far are the association of DWV with wing deformities found naturally in both wild and commercially reared bumblebees (Genersch et al., 2006), the reduced survival of bumblebees orally inoculated with DWV (Fürst et al., 2014) and the rapid mortality of bumblebees injected with low doses of Israeli acute paralysis virus (IAPV; Niu et al., 2014). Studies of the effects of interspecific transfer of pollinator viruses are especially important for bumblebees, since bumblebee diversity is diminishing rapidly in many regions of the world (Biesmeijer et al., 2006, Cameron et al., 2011, Potts et al., 2010).

This study concerns the pathogenic effects on bumblebees (Bombus terrestris or the buff-tailed bumblebee) of two dicistroviruses: IAPV and Kashmir bee virus (KBV), which together with Acute bee paralysis virus (ABPV) form a complex of closely related viruses (de Miranda et al., 2010). These three viruses share a similar pathology, all being rapidly lethal after injection in honeybees. In honeybee colonies, they are normally present in low titer as persistent infections. But under certain environmental stresses, such as for example Varroa destructor infestation, they can undergo re-emergence toward an overt infection-type that can contribute to colony failure (Ribière et al., 2008). Injection of low numbers of IAPV particles in bumblebees also resulted in rapid mortality (Niu et al., 2014). However, the most likely natural virus transmission route for bumblebees is oral. We therefore infected newborn bumblebee workers orally with IAPV or KBV and assessed the effects of this on the performance of bumblebee micro-colonies, a standardized method for studying colony development and reproduction.