Varroa destructor induces changes in the expression of immunity-related genes during the development of Apis mellifera worker and drone broods

Ontogeny of immunity and natural viral infection in Apis mellifera drones and workers

The most common viral diseases affecting honey bees (Apis mellifera) in Israel include deformed wing viruses (DWV-A and DWV-B) and acute paralysis viruses (ABPV and IAPV). These viruses are transmitted within and between colonies, both horizontally and vertically. All members of the colony contribute to this transmission, on the other hand individual and social immunity, particularly hygienic behaviour, may affect the outcome of the process. In this study, we evaluated the ontogeny of natural infections of DWV-A, DWV-B, ABPV and IAPV, their prevalence and loads, in workers and drones from high (H) and low (L) hygienic colonies. In parallel, we evaluated the expression of two immune genes: peptidoglycan recognition protein S2(PGRP-S2) and hymenoptaecin. The prevalence of DWV-B and IAPV increased with age and was higher in workers than in drones. ABPV was not detected in drones. The expression of both immune genes was significantly affected by age and sex. Drones from H colonies had higher expression of these genes. The increased expression of immune genes with drones' age, particularly in hygienic colonies, suggest additional value of honey bee breeding for hygienic behaviour for sustainable beekeeping.

Effects of Genetic Origin of Honeybees and Climate on Prevalence and Infestation Levels of Varroa

The objective of this study was to evaluate the effect of honeybee genetic origin, climate type and the interactions between these variables on the prevalence and infestation levels of Varroa in a large population of honeybee colonies (n = 1134). For each colony, the morphotype, haplotype and climate type were determined. No differences between the Africanized, European and Hybrid morphotypes were found for the prevalence and infestation levels of Varroa (p > 0.05). Differences between honeybee haplotypes were found for the prevalence of Varroa (p < 0.05), and the prevalence was higher in the African haplotype than in the European haplotype. No differences between honeybee haplotypes were found for the infestation levels of Varroa (p > 0.05). Differences were found between climate type for the prevalence and infestation levels of Varroa (p < 0.05): the temperate sub-humid climate had a higher prevalence and higher infestation levels than the semi-warm climate and the warm sub-humid climate. Correlations between the infestation levels of Varroa and mean annual temperature, mean annual precipitation, winter precipitation and Lang index were found.

Honey Bee Larval and Adult Microbiome Life Stages Are Effectively Decoupled with Vertical Transmission Overcoming Early Life Perturbations

Microbiomes provide a range of benefits to their hosts which can lead to the coevolution of a joint ecological niche. However, holometabolous insects, some of the most successful organisms on Earth, occupy different niches throughout development, with larvae and adults being physiologically and morphologically highly distinct. Furthermore, transition between the stages usually involves the loss of the gut microbiome since the gut is remodeled during pupation. Most eusocial organisms appear to have evolved a workaround to this problem by sharing their communal microbiome across generations. However, whether this vertical microbiome transmission can overcome perturbations of the larval microbiome remains untested. Honey bees have a relatively simple, conserved, coevolved adult microbiome which is socially transmitted and affects many aspects of their biology. In contrast, larval microbiomes are more variable, with less clear roles. Here, we manipulated the gut microbiome of in vitro-reared larvae, and after pupation of the larvae, we inoculated the emerged bees with adult microbiome to test whether adult and larval microbiome stages may be coupled (e.g., through immune priming). Larval treatments differed in bacterial composition and abundance, depending on diet, which also drove larval gene expression. Nonetheless, adults converged on the typical core taxa and showed limited gene expression variation. This work demonstrates that honey bee adult and larval stages are effectively microbiologically decoupled, and the core adult microbiome is remarkably stable to early developmental perturbations. Combined with the transmission of the microbiome in early adulthood, this allows the formation of long-term host-microbiome associations. IMPORTANCE This work investigated host-microbiome interactions during a crucial developmental stage-the transition from larvae to adults, which is a challenge to both, the insect host and its microbiome. Using the honey bee as a tractable model system, we showed that microbiome transfer after emergence overrides any variation in the larvae, indicating that larval and adult microbiome stages are effectively decoupled. Together with the reliable vertical transfer in the eusocial system, this decoupling ensures that the adults are colonized with a consistent and derived microbiome after eclosion. Taken all together, our data provide additional support that the evolution of sociality, at least in the honey bee system tested here, is linked with host-microbiome relationships.

Advances and perspectives in selecting resistance traits against the parasitic mite Varroa destructor in honey bees

Background

In spite of the implementation of control strategies in honey bee (Apis mellifera) keeping, the invasive parasitic mite Varroa destructor remains one of the main causes of colony losses in numerous countries. Therefore, this parasite represents a serious threat to beekeeping and agro-ecosystems that benefit from the pollination services provided by honey bees. To maintain their stocks, beekeepers have to treat their colonies with acaricides every year. Selecting lineages that are resistant to infestations is deemed to be a more sustainable approach.

Review

Over the last three decades, numerous selection programs have been initiated to improve the host–parasite relationship and to support honey bee survival in the presence of the parasite without the need for acaricide treatments. Although resistance traits have been included in the selection strategy of honey bees, it has not been possible to globally solve the V. destructor problem. In this study, we review the literature on the reasons that have potentially limited the success of such selection programs. We compile the available information to assess the relevance of selected traits and the potential environmental effects that distort trait expression and colony survival. Limitations to the implementation of these traits in the field are also discussed.

Conclusions

Improving our knowledge of the mechanisms underlying resistance to V. destructor to increase trait relevance, optimizing selection programs to reduce environmental effects, and communicating selection outcomes are all crucial to efforts aiming at establishing a balanced relationship between the invasive parasite and its new host.

Varroa destructor: how does it harm Apis mellifera honey bees and what can be done about it?

Since its migration from the Asian honey bee (Apis cerana) to the European honey bee (Apis mellifera), the ectoparasitic mite Varroa destructor has emerged as a major issue for beekeeping worldwide. Due to a short history of coevolution, the host–parasite relationship between A. mellifera and V. destructor is unbalanced, with honey bees suffering infestation effects at the individual, colony and population levels. Several control solutions have been developed to tackle the colony and production losses due to Varroa, but the burden caused by the mite in combination with other biotic and abiotic factors continues to increase, weakening the beekeeping industry. In this synthetic review, we highlight the main advances made between 2015 and 2020 on V. destructor biology and its impact on the health of the honey bee, A. mellifera. We also describe the main control solutions that are currently available to fight the mite and place a special focus on new methodological developments, which point to integrated pest management strategies for the control of Varroa in honey bee colonies.