The cell invasion preference of Varroa destructor between the original and new honey bee hosts

A systematic meta-analysis of the efficacy of treatments for a global honey bee pathogen - the Varroa mite

The western honey bee Apis mellifera is the world's most important managed pollinator. However, globally honey bees have been facing increasing colony losses due to a combination of stress factors. Foremost among these is the invasive mite Varroa destructor, which is the pathogen most commonly linked to colony losses. Despite intensive research focus on this problem, V. destructor management remains extremely challenging, with no definitive treatment approach. A variety of synthetic chemical, agriculturally organic chemical, biological, and physical treatments have been trialled and used in V. destructor management, each with benefits and drawbacks. Here, we carried out a first systematic meta-analysis of the comparative effectiveness of V. destructor treatment methods. We incorporated data from 138 studies from 30 countries, across five continents and found that overall, synthetic chemicals show high effectiveness as a treatment due to their ability to suppress the V. destructor mite. In our analyses of honey bee responses, there was some evidence that synthetic chemicals may have an overall negative impact on them, even while suppressing V. destructor. For improving honey bee health measures, biological treatment methods showed the most positive effect, but with a relatively low sample size. Our study highlights that there is a relative lack of studies on biological management methods, with far more focus in the literature on chemical treatments. We show that, until biological methods receive greater research attention, agriculturally organic methods likely provide the best current treatment option, because they are well studied, nearly as effective at suppressing V. destructor as synthetic chemicals, and less associated with negative impacts on honey bees. Varroa destructor remains a major threat to honey bees globally, with the last large V. destructor free zone (Australia) having now been invaded, emphasising the need to fill these knowledge gaps.

Unique brood ester profile in a Varroa destructor resistant population of European honey bee (Apis mellifera)

Varroa destructor is one of the greatest threats to Apis mellifera worldwide and if left untreated will kill a colony in less than three years. A Varroa-resistant population from Gotland, Sweden, has managed to survive for 25 years with little to no Varroa treatment by reducing the mite's reproductive success. The underlying mechanisms of this trait is currently not known, though previous research indicates that it is the honey bee brood, and not adult bee influence, that contributes to this phenotype. As the mite's own reproduction is synchronized with the brood's development though the interception of brood pheromones, it is possible that a change in pheromone profile would disrupt the mite's reproductive timing. To investigate this, we characterized the brood ester pheromone (BEP) profile of our resistant Gotland population compared to a non-resistant control. This was done by extracting and analyzing key cuticular compounds of the BEP using gas chromatography. A significant difference was found immediately after brood capping, indicating a divergence in their pheromonal production at this time point. This is an important step to understanding the mechanisms of the Gotland population's Varroa-resistance and contributes to our global understanding of Varroa destructor infestation and survival.

Large cells suppress the reproduction of Varroa destructor

BACKGROUND

The parasitic mite, Varroa destructor has posed a threat to the health and survival of European honey bees, Apis mellifera worldwide. There is a prevailing belief that small comb cells could provide a management tool against Varroa mites. However, the hypothesis that smaller cells can impede Varroa reproduction has not been fully tested. Here, we tested this hypothesis under laboratory conditions by using two distinct Varroa in vitro rearing systems: one involved gelatin capsules of different sizes, specifically size 00 (0.95 mL) versus size 1 (0.48 mL), and the second consisted of brood comb cells drawn on 3D printed foundations with varying cell sizes, ranging from 5.0 mm to 7.0 mm at 0.5 mm intervals.

RESULTS

The results showed that mother mites in size 00 cells had significantly lower fecundity and fertility compared to those in size 1 cells. Interestingly, the reproductive suppression in larger cells could be reversed by adding an extra worker larva. Similarly, gonopore size of mother mites was smaller in size 00 cells, but restored with another host larva. Furthermore, both the fecundity and fertility of mother mites decreased linearly with the size of brood comb cells.

CONCLUSIONS

Our results suggest that the reproduction of V. destructor is hindered by larger cells, possibly because larger brood cells disperse or weaken host volatile chemical cues that are crucial for Varroa reproduction. The insights derived from this study are expected to hold significant implications for the implementation of Varroa management programs. © 2024 Society of Chemical Industry.

The Role of Honey Bee Derived Aliphatic Esters in the Host-Finding Behavior of Varroa destructor

Varroa destructor is an obligate ectoparasite of honey bees and shifted from its original host Apis cerana to the new host Apis mellifera in the first half of the twentieth century. The host shift has resulted in a great threat to the health and survival of A. mellifera colonies worldwide. Chemical signals play a crucial role in all aspects of the Varroa life cycle, including host finding. However, the chemical cues that affect the host finding behavior of Varroa mites are still not fully understood. In this study, we systematically profiled the headspace volatiles of both worker and drone larvae of the two honey bee species by using solid phase micro-extraction coupled to gas chromatography-mass spectrometry (SPME-GC-MS), and then used electrophysiological recording and Y-tube olfactometer bioassay to study the potential roles of the selected compounds. The chemical profiling showed that there were four aliphatic esters, ethyl myristate (EM), methyl palmitate (MP), ethyl palmitate (EP), and ethyl oleate (EO) commonly detected from all four types of larval hosts. Among them, EM was a new substance identified from honey bee headspace volatiles. Results from electrophysiological recordings indicated that all the aliphatic esters could elicit significant responses of Varroa pit organs on its forelegs. Moreover, behavioral analyses revealed that EM could significantly attract V. destructor at a medium dosage (10 µg), while MP had no observable effect on the mites and both EP and EO were able to repel the parasites. Our findings suggest an important role of host-derived aliphatic esters in Varroa host finding, and provide new chemicals for Varroa monitoring and control.