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

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.