Horizontal & Vertical Infections from Varroa, Food & Sexual Activities
Your hives are already infected with Deformed Wing Virus (“DWV”). Low levels of DWV are naturally found in honey bees, and rarely (if ever) cause issues. However, when a colony is infested with mites, symptoms frequently emerge. Mites are vectors, carrying viruses in their bodies and injecting them into bees as they feed on the pupae. DWV infections are fatal and are without treatment. Infected adult bees have deformed wings, a bloated abdomen and discoloured appearance. These bees die a few days after emergence.
Varroa is the major vector of DWV, and will be the focus of the article. Tropilaelaps mites are also vectors, however they are confined to Asia. Further, small hive beetles and drone sperm have also been shown to contain infective DWV particles – Yes, queen bees can get STDs – a fun fact for your next beekeeping event.
Management of Deformed Wing Virus Infections
To the beekeeper, control of DWV begins with Varroa mite control. They are the primary, and most relevant carrier of the virus. When people speak of “chemical management of DWV”, they really mean “Varroa management”. Locke and colleagues found that when colonies are treated with a miticide, two phases of DWV infection emerged:
- A drastic reduction in DWV levels coinciding with the death of mites from the colony, and
- Slight increases in DWV levels, also reported in the untreated colonies (Locke et al,. 2017).
These natural increases are thought to be due to the nurse bees orally transmitting DWV to the larvae or by other adult bees through trophallaxis (oral bee-to-bee feeding). Once treatments were complete, the DWV levels inside colonies went into sub-clinical levels1 with viral loads relevant enough to have sub-lethal effects. In winter, these sublethal effects can lead to insufficient adult bees in spring, which could could collapse the colony as they do not have the population to keep up with the springtime boom.
One common theme throughout this article is the recommendation to constantly monitor Varroa levels; keep them low year-round to avoid the accumulation of irreversibly high DWV levels. Chemical management in the spring and summer alone may not always be sufficient.
Three Chemical Approaches to DWV Management (Hint: it’s all Varroa)
To target DWV, you target their primary vectors: Varroa mites. Previous articles have covered monitoring methods for Varroa (in-the-field testing or using sticky boards) as well as the application of flumethrin inside hives. Chemical treatments are applied on a seasonal basis, based on the infestation severity and colony health.
Thymol Essential Oils
Essential oils like thymol (Apiguard) can be placed inside the hive as strips, gels or vaporizers during the autumn and winter months. Thymol disrupts the mite’s central nervous system by binding to GABA receptors, leading to overstimulation, paralysis and then death (Price & Lummis, 2022)2.
Oxalic Acid Vapourisation
Oxalic acid vapour is used as a Varroa treatment during the broodless periods in autumn and winter, dehydrating the mites’ exoskeleton and soft tissues (Jack et al., 2020). The frequency of applications depend on the severity of mite infestation; it is only effective when mites are roaming the hive (“phoretic stage”), as the vapours cannot penetrate the wax cappings to target the developing Varroa within cells (reviewed in Rademacher & Harz, 2006).
Flumethrin Strips in the Brood Box
Flumethrin affects the nervous system of the mites (Davies et al., 2008), binding to their sodium channels and preventing the cells from closing, leading to excessive muscular contractions. Eventually, paralysis and mite death occurs. Flumethrin is a synthetic pyrethrin, belonging to the class pyrethroids. It is formulated as strips for use in beekeeping, placed in between frames inside the placed within hives.
At high doses, these compounds are toxic to the colony, leading to increased brood removal, increased adult mortality and contamination of the wax, pollen and honey, amongst others. Some treatments can only be administered when there is low or little brood counts as they damage the developing larvae. Others must be applied with the honey supers removed as they are absorbed by the wax and honey. The internet will provide you with the relevant information for your chosen method.
Reducing inter-colony transmission
In addition to the chemical controls listed above, consider:
- Using integrated pest management strategies like drone trapping to minimise reliance on chemical treatments and reduce the chances of mites developing resistance,
- Preventing robbing by reducing hive entrance sizes during periods of nectar dearth to make it easier for weaker colonies to defend themselves,
- Remove or combine weak colonies that are more susceptible to being robbed, as hives prone to robbing will often have higher mite counts.
- Where possible, place hives a few hundred meters apart (after all, the natural beehive is not clustered like that of a backyard beekeeper’s),
- When catching a swarm or obtaining a new colony from unknown sources, quarantine and monitor it for signs of disease and mite infestation before integrating them into an established apiary,
- Keeping strong colonies and providing supplemental nutrition when necessary, so that bees will not succumb to mites.
As far as beekeeping in most parts of the world goes, controlling DWV is done by controlling Varroa. If my dear reader is from a region with Tropilaelaps mites, then the above techniques would work too, however I’m sure there are local practices in place to manage infestations.
Routes of Deformed Wing Virus Transmission
DWV is transmitted to bee larvae and adults primarily through mites. Without mites, the virus has a low infective rate that cannot reach impactful levels inside hives. Mites enable the buildup of high viral concentrations inside the bees and larvae, resulting in the deformed wings, shortened abdomens, and reduced lifespan. Note that the literature defines an “active DWV infection” as one where the replicating virus in present in the head of bees (Yue & Genersch, 2005).
I wrote “mites” and not “Varroa mites” because the Asian-native Tropilaelaps mites (Tropilaelaps mercedesae) also carry DWV, and infect bees but unlike Varroa, who feeds on the fat bodies, Tropilaelaps feeds on the haemolymph (Forsgren et al., 2009). Other DWV transmission routes exist, such as from bee-to-bee, from pollen, from drone sperm and from queen to larvae, covered below.
Horizontal Transmission: from Bee-to-Bee
Along with the feeding action of mites, horizontal transmission of DWV has been implicated in the following:
- Feeding between adult bees via trophallaxis (Fievet et al., 2006),
- A fecal-oral route, as DWV was found in faeces (Chen et al., 2006),
- Through ingestion of infected pupae by nurse bees, which removes their bodies as part of the social immune response (Posada-Florez et al., 2021).
The fastest and surest road to infection was via direct introduction of DWV into the bees’ circulatory system, mimicking the way mites feed (Mockel et al., 2011). The virus was detectable throughout the bee body. By comparison, oral exposure (feeding bees with DWV-infected sugar syrup) resulted in DWV being only detectable in the gut, and no symptomatically-infected bees were observed. Perhaps the bee gut bacteria has a role to play here?
The virus has been detected in royal jelly, brood food, and early larval stages in mite-free colonies (reviewed in Koziy et al., 2019). When infected nurse bees fed uninfected larvae, infection takes hold, but larvae remain asymptomatic even at the time of adult bee emergence (unless high, unrealistic doses are provided; Ryabov et al., 2014). The virus appears unable to replicate in the larval gut, and no obvious infection appears. By contrast, the Varroa-mediated transmission was mediated by one viral strain, DWV^V, suggesting that while many viruses circulate inside a mite, only those with the most favourable infective characteristics will successfully establish themselves in the bee’s body.
If Honey Bees are Vegetarian, Why do They Eat their Larvae?
The default answer is “yes, bees are vegetarian as they only consume plant material”… so why do they eat their young? In short, eating larvae evolved as a way to conserve nutrients during times of shortages, disease outbreaks and stressors so that essential hive functions are maintained (see Schmickl & Crailsheim, 2001). Further, in hives with laying queens, bees will consume any worker-laid eggs to prevent them from developing (Halling & Oldroyd, 2003).
Posada-Florez & colleagues found that Varroa-Sensitive Hygienic bees consumed infected larvae upon removal from the brood cells (Posada-Florez et al., 2021). In one case, cannibalisation of a single infected pupa led to infection of 2,000 worker bees, as the infection spread to other workers through trophallaxis.
As you’ve read above, direct introduction of DWV via mite feeding is essential for the typical DWV symptoms to appear. In this study, the adult bees were infected from cannibalised pupa, and none of the typical symptoms were observed, despite the successful infection detection (Posada-Florez et al., 2021).
Even if DWV was to be orally spread to the pupa, gut-borne infections are not the same as blood-borne ones, and the virus will persist inside the colony, potentially producing sub-lethal effects.
Vertical Transmission: from Queen to Larvae
Vertical transmission refers to the infection passing from mothers (queens) to their offspring (larvae), and has two main causes:
- Egg laid using infected drone sperm, and
- Infected eggs laid as a result of infection from the spermatheca (sperm storage) and/or ovaries of Her Majesty.
When DWV is transmitted vertically, the result is a “covert” infection – the virus is detectable with no visible symptoms accompanying it. (However, the modeling work by Britton & White suggest that vertical transmission can lead to symptomatic, overt infections – I do not agree, as it’d require high doses that are unlikely to be encountered in nature; Britton & White, 2021). This mode of transmission can lead to the establishment of DWV within colonies even in the absence of Varroa mites, underscoring the need for monitoring and managing virus levels in breeding stock (Amiri et al., 2016).
Sexually Transmitted DWV
DWV-infected semen used for artificial insemination of virgin queens resulted in ovarian DWV detection and subsequent transmission to the larva (de Miranda & Fries, 2008). When tissue samples from infected queens were examined, less than 10% showed a medium-level of infection (defined as between 10^3 to 10^7 copies of the virus), and this result was not significant when compared to the control group (Amiri et al., 2016).
Considering all the talk about weakened bees, the study from Amiri & colleagues found that DWV infections do not hinder drones from competing with their uninfected counterparts for mating with the queen, and can successfully inseminate her. Further, they suggest that establishing DWV as a sexually transmitted infection is difficult, since the majority of queens (~84%) had mated with infected drones and did not show any evidence of DWV in their reproductive system.
Whether the drone’s fertility is impacted by the infection, or the process of sexually transmitting DWV is difficult, that is for future research to determine. The authors state that since high levels of DWV reduces the lifespan of worker bees, then high levels of DWV are also likely to reduce the lifespan of queen bees, leading to colony failure (Amiri et al., 2016).
Why Don’t Queens get Deformed Wings?
Whenever there are high levels of DWV in a queen, it cannot be attributed to Varroa, as the life cycle of the queen pupa (~7-8 days) is too short for mites to complete their reproductive lifecycle (~7-10 days). A queen may become infected with DWV by naturally mating with infected drones, or by repeated feedings of royal jelly from infected workers. It is very rare for the queen to have symptomatic DWV infections.
Small Hive Beetles as Vectors for DWV
A sidenote: Huang and colleagues catalogued the microorganisms present in adult and larval small hive beetles and found two pathogenic viruses: DWV and Kakugo virus – the latter being a subtype of DWV, and is thought to cause aggressive bee-haviours (Huang et al., 2019). The authors suggest that the beetles acquired these viruses from their diet of hive materials, and found the DWV capable of replicating inside the adult beetles, which could spread infection by contaminating new hives.
Deformed Wing Virus in Pollen: Clinically (in)significant?
The presence of DWV in pollen likely contributes to its natural occurrence in honey bee colonies (Mazzei et al., 2014). As mentioned above, DWV infection can be established following oral ingestion of contaminated foods, like royal jelly, bee bread and honey. What does this infection mean? Perhaps in stressed colonies, or those with a weaker gut microflora, this oral infection results in negative outcomes for the colony. Until then, the literature on horizontal transmissions will show that oral DWV infections can be symptomatic, but not at field-relevant doses (Desai et al., 2012).
Vector-mediated transmission via the feeding action of mites remains the most impactful route of infection.
Winter Bees as Reservoirs of Springtime DWV Infections in Mite-Free Colonies
A strong implication is placed on the long-lived winter bees, who survive for over 200 days and contain higher levels of fat bodies than the regular bees (reviewed in Doke et al., 2015). Many viruses including DWV, replicate in these fat bodies, so the viral load increases. Further, brood rearing during winter tends to be confined to the small area inside the cluster. Those few larvae are fed via the secretions from the adult bees’ fat bodies, which contain DWV. Hence the authors’ observations that when the winter larvae become adults, they have higher levels of DWV despite the absence of Varroa mites (Locke et al,. 2017).
What About the Other Honey Bee Viruses?
DWV is directly related to Varroa infestations and the cause of colony collapse. Varroa destructor has been implicated as a carrier, or activator of other viral infections like:
- Chronic bee paralysis virus (Dittes et al., 2020)
- Acute bee paralysis virus (Bakonyi et al., 2002)
- Sacbrood virus
- Kashmir bee virus (Shen et al., 2005)
- Israeli acute paralysis virus (Di Prisco et al., 2011)
- And many other novel viruses that are constantly being discovered, like V. destructor viruses (VDVs; Levin et al., 2019)
A commonality between these viruses is being weak and slow-developing, allowing larvae to go through the pupal stage and into adulthood. DWV has lowest virulence from this group, which is why it became the main virus associated with Varroa infestations. Some of the more virulent viruses listed above kill the brood too fast for mites to complete their development on the pupae, and therefore the next generation of mites will not be able to transmit the virus to new hosts.
Future Research: RNA Interference (RNAi) Treatments for Deformed Wing Virus
RNAi has been explored as a potential treatment for DWV. Feeding bees with double-stranded RNA (“dsRNA”) specific to DWV reduced viral levels and wing deformity in DWV-inoculated bees. If scaled successfully, then this method is a promising non-toxic, DWV-specific management approach (Desai et al., 2012). In this study. Around 85% of bees died eight days following oral DWV inoculation and no RNAi treatment, whereas only 15% of bees were dead after receiving RNAi treatment.
These are promising numbers from an oral study, with high doses that are not likely to represent levels acquired naturally from nurse bees or other oral routes. Translating the data to mimic how mites feed naturally is not always successful. For example, Smeele and colleagues performed a similar experiment with mini-hives under laboratory conditions, and showed fluctuating success in the ability of RNAi to control DWV (Smeele et al., 2023). The authors hypothesised that future experiments using higher dsRNA doses may achieve a significant reduction in viral loads, or if alternative methods of administration are used, such as probiotics.
Enter the Gut Bacteria: RNAi Treatments using DWV-dsRNA Expressing Bacteria
One of the core members in the honey bee gut bacteria is Snodgrassella alvi. Using genetic engineering, Leonard and colleagues produced an S. alvi strain with an ability to produce DWV-specific dsRNA, and fed it to bees (Leonard et al., 2020). The bees were then infected with both oral and injected DWV, and those receiving S. alvi bearing DWV-dsRNA had significantly greater survival than control or non-specifc dsRNA-fed bees.
Great news, feeding bees gut bacteria with DWV-specific dsRNA is effective in controlling DWV infections. What about during ongoing mite infections? The final experiment by Leonard and colleagues tested whether S. alvi strains containing anti-Varroa RNAi can protect bees against mites, and found that that yes, they can. The mites that fed on bees who possess the Varroa-specific S. alvi dsRNA died more faster than mites fed on control bees (Leonard et al., 2020). Why did this happen? The bacteria targeted DWV, not mites… the answer is… to be determined.
Perhaps gut symbionts (“beneficial bacteria”) are the way to go when it comes to RNAi delivery in honey bees. Future research will tell us more.
Read the whole post and found puns, typos mistakes? Voice your displeasure with a strongly-worded message to admin@tgbee.com.au, or by filling this form.
Footnotes
- “Sub-clinical” infection refers to the presence of an infective agent with no visible symptoms. Likewise sub-lethal effects are those that harmful but not fatal, and impact the colony’s quality of life. ↩︎
- Thymol acts on the GABA-controlled Varroa RDL receptors, which has a different structure to that of honey bees. When Price & Lummis tested the degree of RDL inhibition with thymol, they found that it affected mites at lower doses than bees, giving thymol the potential to form the basis of a bee-friendly varroacide (note that its varroacide and not miticide – this inhibition is specific to Varroa mites alone; Price & Lummis, 2022) ↩︎