Honey Bee Mites

Mite Treatments for Honey Bees

Varroa mites are the biggest threat to bee populations worldwide. Mites feed on the haemolymph (bee blood) fat body tissue of developing bee larva (& adult bees; see Ramsey et al., 2019), weakening them and infecting them with pathogens during the feeding process. The emerging adult bee exhibits stunned growth, body deformation and a shortened lifespan. Severe infestation can lead to colony collapse as hive productivity diminishes.

Varroa treatment ranges from chemical administration, such as oxalic acid strips, to integrated pest management and organic methods. The Western honey bee, Apis mellifera, is unable to manage varroa like its natural host the Asian honey bee Apis cerana, and human intervention is required.

This article examines the major Varroa mite species, Varroa destructor, and its effects on honey bees. Both chemical and non-chemical treatments will be evaluated on their ability to control Varroa mites. Varroa is not the only mite that infests honey bees. Tracheal mites, Braula flies & Tropilaelaps mites will also be examined.

Lifecycle of a Varroa Mite

There are four Varroa species identified: V. destructor, V. jacobsoni, V. underwoodi and V. sinhai. The latter two are less studied and have a lesser impact on Apis mellifera compared to V. destructor and V. jacobsoni; they are endemic parasites associated with the Asian honey bees and are less relevant to global beekeeping.

The lifecycle of V. destructor and V. jacobsoni are similar and are tied tohe beees’ capped brood phase (pupal). A single adult female mite (called “foundress”) enters a brood cell containing a developing larva, feeds on its fat body and lays eggs. As the adult bee emerges from its capped cell, the foundress and her daughter mites exit it with it. The original female can continue the cycle by entering new brood cells or infesting other bees. The reproductive cycle is synchronized depending on brood type – a mite’s reproductive phase can last 10-12 days if infesting a worker brood, and 14-16 days in drone cells.

A more detail description of the lifecycle is as follows:

1. Within a colony, the foundresses are attached to bees, feeding on their fat body. This is known as the Phoretic phase, where mites move between bees, spreading throughout the hive.

   1. Mites that are attached to foraging bees can remove themselves at flowers and jump on the next forager to arrive, potentially introducing herself to new colonies.

2. The foundress enters the cell prior to the capping of a drone or worker brood and feeds on the late-stage larva’s fat body. The first egg is laid within 60-70 hours of the foundress’ feeding.

3. The foundress lays between 4-6 eggs, one male and the remainder females. Eggs hatch into mite larva within three days and begin feeding on the pupa’s fat bodies.

4. Mite larva undergo two moultings prior to adulthood. This process takes between 5-7 days for males, and 6-8 for females.

   1. The first molting produces the eight-legged protonymph form. This is followed by the duetonymph and then the adult mite.

5. Once the mites mature, the females mate with their male sibling, who dies inside the cell. The resulting mated female mites and original mother mite await for the bee to emerge from its cell.

6. Once the bee emerges, the phoretic phase restarts with an increased number of mites. Daughter mites can attach themselves onto adult bees, whilst the foundress can go on to infest more brood.

   1. Foundresses reproduce several times throughout their life, resulting in exponential growth for the Varroa mite populations.

Effects of Varroa Mite Infestations on Honey Bee Colonies

V. destructor has a more significant effect on its host A. mellifera. Direct damage is done to bees as the mites feed, causing physical injury and weakness. Feeding also leads to the transmission of pathogens from within the mite’s body, resulting in deformed wings, a lower bodyweight and a shorter lifespan.

Varroa Mites as a Viral Disease Vector

Mites are a carrier for several viral diseases. When Varroa Varroa mites feed on bee tissues, they directly transmit viruses like well-studied being Deformed Wing Virus (DWV), Acute Bee Paralysis Virus, and Israeli Acute Paralysis Virus. The increased viral burden is one major cause of reduced bee health and colony failure.

• The Deformed Wing Virus spreads in several ways, and is compounded by the feeding of Varroa mites: Infected queens lay infected eggs, Adults can transmit it to others bees and larvae through contaminated food. Symptoms appear in the emerging bee, leading to deformed wings, reduced body size, shortened lifespan and abnormal behaviour; they are unable to fly.

• The Israeli Acute Paralysis Virus is spread by mites in a similar manner, resulting in rapid paralysis and death in the infected honey bees. Trembling (bees shake their wings rapidly while remaining in place), disoriented movement and an inability to fly are characteristic of this infection.

  • The Acute Bee Paralysis Virus shares the same symptoms, with the exception of the trembling. Infected bees tend to have a a darkening of their abdomen.

Viral or Varroa Mite-Mediated Immune System Suppression?

Non-technical readers are welcome to skip this section

The viral proliferation from Varroa mite vectors leads to a decline in immune capacity. A downregulation in genes associated with autophagy is observed, which weakens honey bees’ innate immune response. Two genes, autophagic-specific gene 18 (Atg18) & poly U binding factor 68 Kd (pUf68) are downregulated, which can favour the proliferation of DWV¹. As the viral load increases in infected bees, genes responsible for protein repair (Pmct) and degradation (Nedd8) are upregulated, presumably to counteract the cellular & molecular damage from the DWV RNA. (Navajas et al., 2008).

When afflicted by a moderate number (2-4) of V. destructor, immune-related transcripts of the antimicorbial genes abaecin & defensin are downregulated (Gregory et al., 2005). One such explanation is a reduction in the immune (and subsequent healing) response of the feeding sites. Under heavier infestations however, the immune response returns. There is evidence to suggest that the humoural immune-gene suppression is ephemeral, as older pupae resume their immune response. A higher mite count in the brood chamber could be required to generate the specific stimulus for immune gene upregulation.

Research suggests that DWV is a strong driver of immunosuppression and colony collapse (Nazzi et al., 2012). There are a number of pathogenic bee viruses present in colonies on top of DWV including Black Queen Cell Virus and Sacbrood Virus. The prevalence of the latter two however, fluctuates and declines throughout the season, contrasting the steady increase in DWV prevalence.

Bees from colonies with a high infestation count had significant transcriptional downregulation of dorsal-1A, a member of the protein family NF-κB. RNAi experiments showed that a reduction in dorsal-1A levels led to increased viral replication due to decreasing NF-κB. The results suggest that it is the inhibition of dorsal-1A by DWV viral replication², not Varroa mite feeding, that drives immunosuppression. Both DWV and V. destructor appear to be strong predictive markers for colony collapse in the winter bees (Dainat et al., 2012).

Varroa Mite Infestation and Colony Collapse Disorder

Varroa mite infestations have been identified as one of the four markers involved in Colony Collapse Disorder (CCD). The first case of CCD was identified in 2006 within the U.S., and emerging evidence indicates that these impacts are due to a combination of factors. Varroa contributes to this issue in several ways, such as direct feeding, secondary infections and weakening the bee’s immune system.

Treatments for Varroa Mites

There are many methods available for Varroa control with various degrees of effectiveness. These range from miticides to organic approaches like powdered sugar dusting. The section below is an overview and not a comprehensive how-to – consult the product label(s) and/or other beekeepers.

Current approaches employ a combination of the above methods, each having their own usage conditions such as time of year, presence of honey supers or larvae in the brood nest. It’s worth mentioning that A. mellifera is more vulnerable to Varroa than A. cerana than Apis cerana, and beekeepers are attempting to bridge this gap by selective breeding for hygienic behaviour as part of modern management (e.g. uncapping-recapping cells, increased grooming).

Chemical-Free Varroa Mite Treatment Options

These methods involve the use of natural substances or non-chemical methods to provide Varroa mite treatment.

Drone Brood Removal for Mite Control

Drone brood trapping is an integrated pest management method for Varroa mite control. It takes advantage of the foundress’ preference for reproducing in the larger brood comb. This method is effective for use mild-moderate mite infestations, and in colonies that are strong enough to rear drones. Once a sufficient number of cells are capped, the comb is removed prior to the bees’ hatching.

Disposing of the drone brood can be done in several ways:

• If using a plastic drone frame, the brood can be scraped off with a hive tool and the frame reinserted in the hive. The removed comb is discarded away from the colony to prevent robbing, as it encourages mites to attach onto new bees.

• In wax foundation frames, the comb can be cut out and disposed; the frame being placed into the colony for reuse.

• For beekeepers with additional resources, the capped drone frames can be frozen for 2-3 days to kill mites. This method will require two sets of frames in order to replace the ones removed, and can be performed in 24-day cycles (the time it takes for a drone to emerge).

  • Frames must reach room temperature before being placed into the hives.

Using Powdered Sugar for Varroa Mite Treatments

This method is more effective in checking for Varroa than an actual treatment.

The basis of powdered sugar dusting lays in the worker’s bee’s ability to groom themselves and others. By applying finely powdered sugar, the bees are stimulated into brushing the foreign particles, dislodging mites in the process. Additionally, powdered sugar granules can adhere to the gripping surfaces of the mite’s feet, weakening their hold on the bee and falling to the hive floor.

This is an inexpensive method that does not harm bees; powdered sugar granules do not enter the bee’s respiratory organs, and can be used anytime. There are two disadvantages to this method, as accumulation of sugar can attract ants to hive (bees will not process the sugar during a nectar flow), and in the nectar dearths, the presence of sugar may encourage other colonies to partake in robbing activities.

Using Screened Bottom Boards

While not directly a method to control mites, using screened (rather than solid) bottom boards causes any mites to fall through the mesh and out of the hive. This method can be deemed the “other half” of the powdered sugar dusting method. When mites fall through the mesh, they land onto a bottom board coated in oil or vaseline which prevents them from returning into the hive.

Beekeepers may choose the vaseline method over oil as it enables them to conduct a mite count to evaluate the “daily mite drop”, which can be used to assess a hive’s mite levels and infestation intensity.

Selective Breeding and Genetics: Mite-Resistant Bees

Selectively breeding honey bees with favourable characteristics involves promoting the spread of Varroa-sensitive hygienic behaviours, more frequent grooming or producing mite-tolerant strains. This process involves careful selection and controlled queen mating over successive generations to ensure heredity. A detailed examination will be provided in a future article.

Approved Chemical Treatments for Varroa Mite Populations

Chemical mite control, such as miticides and oils must be carefully administered in order to avoid harming bees and humans, or contaminating hive products. Chemicals are applied on a seasonal basis, based on the infestation severity and colony health.

Essential Oils for Varroa Mite Control

Essential oils, like thymol, eucalyptus or wintergreen oil can be placed inside the hive as strips, gels or vaporizers during the autumn and winter months. They act as a varroa mite treatment by disrupting the mite’s central nervous system or causing respiratory distress. At high doses, they can be toxic to the adult bee. Essential oils should only be administered when there is low or little brood counts as they appear to damage honey bee larvae.

Using Mineral Oil to Treat Varroa Mites

Like essential oils, food-grade mineral oils can used year-round as a varroa mite treatment. This method works by suffocating mites when administered as a spray or fog, coating their breathing pores. It is applied once a week over a 3-4 week period, and overuse has been linked with excess humidity inside bee hives, which can promote pathogen growth.

Vaporization of Oxalic Acid

Oxalic acid is used as a Varroa mite treatment during the broodless periods in autumn and winter, usually as a vapor. It works by dehydrating the mites’ exoskeleton and soft tissues, and the frequency of applications depend on the severity of mite infestation. Oxalic acid can be toxic to both bees and humans so proper personal protective equipment and handling techniques is warranted.

Oxalic acid does not appear to have any negative effect on the wax and honeycomb structure. Beekeepers are advised to remove honey supers prior to application as the oxalic acid residues can taint honey.

Formic Acid: Mite-Away Quick Strips

Like oxalic acid, formic acid is another organic acid used for mite control. This kills mites by disrupting their metabolism and damaging their respiratory systems, with treatments lasting 1-2 weeks, depending on the application method. It can be used during any season as slow-release gel packs, pads, or strips, as long as the daytime temperatures are between 10-29°C. Proper personal protective equipment and following packaging instructions is recommended to avoid damage the adult bees and beekeepers.

High concentrations or improper application of formic acid can lead to acid vapours penetrating into honey, altering its taste and quality, and damaging the wax structure; honey supers should be removed prior to application.

Another pesticide strip not mentioned here is the organophosphate insecticide coumaphos (CheckMite+). Also applied inside the hive at controlled doses, it has the added bonus of killing small hive beetles as well as (non-resistant) Varroa.

How to Detect Varroa Mites in Bee Colonies

Before controlling Varroa mites, beekeepers must detect Varroa mites. Two popular methods of checking for mites in a managed honey bee colony are the powdered sugar shake test and alcohol wash.

To conduct the sugar shake test:

• Collect a few hundred bees from a brood frame and place them in a jar with an open mesh lid (be careful not to trap the queen!).

• Add 1-2 tablespoons of powdered sugar to the jar and let it sit for a few minutes, allowing the sugar to accumulate and adhere to mites.

• Gently shake the jar over a white surface or container filled with water. Any mite and sugar granules will fall through, and the bees can be released back to their hive

  • If mites are present, they will be visible and thus a positive test result.

An alcohol wash kills the tested bees and is generally seen as more effective than the sugar shake test, conducted as follows:

• Collect a few hundred bees from a brood frame and place them in a jar with a lid (be careful not to trap the queen!).

• Add enough alcohol (usually 70% isopropyl alcohol) to cover the bees, close the lid and shake vigorously for about 30 seconds to dislodge any mites.

• Pour the liquid through a mesh or sieve to another container and observe the mites that fall through.

The aforementioned drone brood trappings and sticky bottom board can also be used in the detection of mites. Visual inspection of bees is another method for detecting Varroa, however it is difficult and suitable for experienced beekeepers. It is hard to distinguish a Varroa mite from a Braula fly with the naked eye.

Other Significant Parasitic Mite Populations

Varroa mites are the main subject of this article, however there are other mites that parasite honey bees.

Tropilaelaps Mites: Tropilaelaps clareae & Tropilaelaps mercedesa

Native to Central and South East Asia, the Tropilaelaps mites have a similar lifecycle to Varroa, entering brood cells to feed on fat bodies and laying eggs. The feeding results in body deformities and a shortened lifespan, with severe infections leading to collapse. The Tropilaelaps have a shorter reproductive cycle than varroa mites (10-12 days), meaning that their population can quickly outnumber Varroa mites.

They are lighter-orange colour in appearance, with an elongated, elliptical shape compared to the flat red circle of V. destructor. Treatments for Tropilaelaps are similar, using chemical methods, requeening hives with disease-resistant stock and integrated pest management techniques. Tropilaelaps is a serious pest, however it is geographically limited and not established in many regions, making its overall severity less than Varroa.

Tracheal mites, Acarapis woodi

This is an internal parasite. The female lays eggs inside the trachea of adult bees, where they complete their lifecycle into adult mites. Trachael mites are microscopic and cannot be viewed with the naked eye. They are found in most parts of the world like Asia, Europe and North America. An infestation of A. woodi causes acarine disease, in which infected bees have difficulty breathing, reducing their lifespan and impairing flight.

Infected winter bees tend to have a poorer survival outcome, and reduced honey production. Short-term treatments include chemicals like menthol, whereas long term management involve keeping mite-resistant bee strains and proper beekeeping management.

Braula (Scuttle) Flies, Braula coeca

While not mites, their similar visual appearance to Varroa mites leads to confusion amongst inexperience beekeepers. Found all over the honey-producing parts of the world, they are small wingless flies that are relatively harmless, causing cosmetic damage to honeycomb cappings as they lay eggs. Their eggs develop into larvae that burrow through the wax to feed on the stored honey and pollen.

Proper colony management is the recommended treatment, and chemical interventions do more harm than good. Interestingly, treating Varroa infestations with substances like oxalic acid have led to a large reduction in Braula populations, indirectly removing them from colonies.

The Impact of Honey Bee Mite Parasitism

This article examined the lifecycle of the Varroa mite, V. destructor and its impact on individual bee and colony health. A comparison between the treatment and detection methods were provided, and other mite species that affect honey bee colonies were examined. Varroa may be the biggest threat to honey bees, however these other mite species play a role in damaging honey bee productivity, and their control is essential to maintain honey bee health.

Research into the long term management and treatment of Varroa is ongoing. It is the biggest threat to bee health worldwide, and finding solutions will ensure the survival of honey bees for sustainable honey production and crop pollination.

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Footnotes

1. There are four “master” variants of DVW (Piou et al., 2022), each with a differing geographical distribution and virulence. As it stands, DWV-B variants have a stronger selective advantage over DWV-A, DWV-C & DWV-D. While the emergence of symptoms depends on the bee’s viral load, the symptom severity and lethality of the infection appears to be variant-driven. ↩︎
2. The downregulation of NF-κB is beneficial to mites as it prevents wound healing, allowing the foundress and her offspring to continue feeding through the same hole made in the pupating bee’s cuticle, suggesting that DWV and varroa mites have developed a symbiotic relationship in the feeding stages. ↩︎