The influence of Nosema (Microspora: Nosematidae) infection on honey bee (Hymenoptera: Apidae) defense against Varroa destructor (Mesostigmata: Varroidae)

Highlights

• High-MMR bees show higher mite mortality than low-MMR when inoculated with Varroa.

• Varroa inoculation increases bee mortality in high-MMR relative to low-MMR bees.

• Mite load was lower in high- than in low-MMR bees when not inoculated with Nosema.

• Similar mite load was found in high-MMR and low-MMR bees when Nosema inoculated.

• Nosema inoculation reduces Varroa mortality in high-MMR bees.

Abstract

The objectives of this study were to quantify the costs and benefits of co-parasitism with Varroa (Varroa destructor Anderson and Trueman) and Nosema (Nosema ceranae Fries and Nosema apis Zander) on honey bees (Apis mellifera L.) with different defense levels. Newly-emerged worker bees from either high-mite-mortality-rate (high-MMR) bees or low-mite-mortality-rate (low-MMR) bees were confined in forty bioassay cages which were either inoculated with Nosema spores [Nosema (+) group] or were left un-inoculated [Nosema (−) group]. Caged-bees were then inoculated with Varroa mites [Varroa (+) group] or were left untreated [Varroa (−) group]. This established four treatment combinations within each Nosema treatment group: (1) low-MMR Varroa (−), (2) high-MMR Varroa (−), (3) low-MMR Varroa (+) and (4) high-MMR Varroa (+), each with five replicates. Overall mite mortality in high-MMR bees (0.12 ± 0.02 mites per day) was significantly greater than in the low-MMR bees (0.06 ± 0.02 mites per day). In the Nosema (−) groups bee mortality was greater in high-MMR bees than low-MMR bees but only when bees had a higher mite burden. Overall, high-MMR bees in the Nosema (−) group showed greater reductions in mean abundance of mites over time compared with low-MMR bees, when inoculated with additional mites. However, high-MMR bees could not reduce mite load as well as in the Nosema (−) group when fed with Nosema spores. Mean abundance of Nosema spores in live bees and dead bees of both strains of bees was significantly greater in the Nosema (+) group. Molecular analyses confirmed the presence of both Nosema species in inoculated bees but N. ceranae was more abundant than N. apis and unlike N. apis increased over the course of the experiment. Collectively, this study showed differential mite mortality rates among different genotypes of bees, however, Nosema infection restrained Varroa removal success in high-MMR bees.

Introduction

Global honey bee (Apis mellifera Linnaeus) colony loss occurs through interactions between different stressors (Cox-Foster et al., 2007, Currie et al., 2010, Neumann and Carreck, 2010). Although Nosema ceranae Fries and Varroa destructor Anderson and Trueman have been recognized as two potential contributors to this global crisis, the main cause of the high colony mortality is still controversial (Cox-Foster et al., 2007, Currie et al., 2010, vanEngelsdorp et al., 2009). Better understanding of the interactions between these parasites is critical to the development of new management strategies to mitigate colony loss.

Nosema species (N. ceranae and N. apis Zander) are intracellular endoparasites of adult honey bees infecting the epithelial cells of the midgut (Forsgren and Fries, 2010). Single- and co-infections of N. ceranae and N. apis occur in both A. mellifera and A. cerana colonies with higher prevalence of N. ceranae (Chen et al., 2009, Klee et al., 2007). This may be associated with faster reproduction of N. ceranae, geographic isolation or the inability of host immune system to cope with N. ceranae infection (Chen et al., 2009, Martin-Hernandez et al., 2011, Shutler et al., 2014). Nosema alters aspects of both honey bee behavior and physiology (Hassanein, 1951, Goblirsch et al., 2013, Higes et al., 2010). Nosema also affects the metabolism of infected bees by degenerating epithelial ventricular cells, thus influencing hemolymph levels of fatty acids and vitellogenin, affecting enzymes secretion and protein content of hypopharyngeal glands (Alaux et al., 2011, Chaimanee et al., 2012, Goblirsch et al., 2013, Matasin et al., 2012, Suwannapong et al., 2010, Wang and Moeller, 1970). Additionally, N. ceranae infection inhibits immune system function in honey bees (Antunez et al., 2009) which may decrease resistance of the host against other pathogens. Collectively, infection with Nosema often prevents bee population build up in spring, decreases bee longevity and consequently reduces colony survival (Botias et al., 2013, Goblirsch et al., 2013, Higes et al., 2009).

The Varroa mite is an external obligate parasite that feeds on both adult and immature stages of bees. In the process of feeding, the mite, affects physiological responses in bees (De Jong et al., 1982, Schneider and Drescher, 1987), vectors a variety of pathogens (Ball, 1985, Chen et al., 2004, de Miranda and Fries, 2008, Liu, 1996) and weakens the immune responses of bees making them more susceptible to diseases (Yang and Cox-Foster, 2005). Collectively, this negatively impacts colony level responses such as brood rearing, foraging activity and honey production (Currie and Gatien, 2006, Garedew et al., 2004, Gatien and Currie, 2003, Kralj and Fuchs, 2006, Ostermann and Currie, 2004, Romero-Vera and Otero-Colina, 2002, Schneider and Drescher, 1987). Varroa is also a major contributor to colony loss, and thus it induces considerable cost to global apiculture (Currie et al., 2010, Guzman-Novoa et al., 2010, Le Conte et al., 2010, Neumann and Carreck, 2010). Female mites typically spend 1–10 days in the “phoretic” stage on adult bees (Boot et al., 1993) where they can be subjected to removal through grooming responses of bees. Several mechanisms of resistance against Varroa have been developed in commercial lines to moderate acaricide use that include increased removal of mites from brood, suppression of mite reproduction and increased rates of mite removal through grooming (Harris, 2007, Ibrahim and Spivak, 2006, Spivak and Reuter, 2001). Currie and Tahmasbi (2008) showed that mite mortality rate (likely associated with active grooming), is affected by genotype of bees and environmental conditions. Interactions between pathogens and parasites could influence potential benefits of host-resistance mechanisms in honey bees or increase the costs associated with defence against V. destructor.

Although the distribution, mean abundance, seasonal cycle and epidemiology of N. ceranae and N. apis are becoming better understood (Copley et al., 2012, Genersch et al., 2010, Klee et al., 2007, Martin-Hernandez et al., 2007, Traver et al., 2012), there is little information on how Nosema infection interacts with mite-resistance responses under different levels of Varroa infestation. Therefore, the goals of this study were to determine the effects of combinations of Varroa and Nosema parasitism on groups of bees established with different defensive capabilities resulting in different mite mortality rates, and to quantify the benefits (in terms of the differences in the levels of mite mortality rates) and potential costs (in terms of differences in bee mortality rates).