Understanding and Managing Varroa Mites: A Global Beekeeper's Guide

Varroa mites (Varroa destructor) are a ubiquitous and devastating parasite of honeybees (Apis mellifera) globally. These tiny, reddish-brown mites feed on the hemolymph (blood) of adult bees and developing brood, weakening the colony and making it susceptible to other diseases and viruses. Effective Varroa mite management is crucial for the survival and productivity of honeybee colonies worldwide. This guide provides a comprehensive overview of Varroa mites, their impact, monitoring techniques, and integrated pest management (IPM) strategies applicable to beekeepers in diverse environments.

The Varroa Mite Threat: A Global Perspective

Varroa mites originated as parasites of the Asian honeybee (Apis cerana). When Apis mellifera was introduced to regions where Apis cerana already existed, the mites transferred to the new host. Unlike Apis cerana, Apis mellifera has not developed effective defense mechanisms against Varroa mites, making them particularly vulnerable. Today, Varroa mites are found in virtually every beekeeping region of the world, from Europe and North America to South America, Africa, Asia, and Oceania.

The impact of Varroa mites extends beyond individual colonies. They contribute significantly to colony losses, reduced honey production, and increased costs for beekeepers. The economic consequences are substantial, affecting both commercial and hobbyist beekeepers alike.

How Varroa Mites Harm Honeybees

Varroa mites cause harm in several ways:

Direct Feeding: Mites feed on the hemolymph of bees, weakening them and reducing their lifespan. This is particularly damaging to developing brood.
Virus Vectoring: Varroa mites are vectors for numerous honeybee viruses, including Deformed Wing Virus (DWV), Acute Bee Paralysis Virus (ABPV), and Sacbrood Virus (SBV). These viruses can cause significant health problems and colony collapse. The feeding wound also allows bacteria and fungi to enter the bee.
Immunosuppression: Mite infestations can suppress the honeybee's immune system, making them more susceptible to other diseases.

Symptoms of Varroa Mite Infestation

Recognizing the signs of Varroa mite infestation is crucial for timely intervention. Symptoms can vary depending on the severity of the infestation, but common indicators include:

Visible Mites: Adult mites can be seen on adult bees, particularly between the abdominal segments or on the thorax.
Deformed Wings: Deformed wings are a classic symptom of Deformed Wing Virus (DWV), which is often transmitted by Varroa mites.
Brood Abnormalities: Patches of uncapped or dead brood, discolored larvae, and pupae with sunken eyes can indicate a high mite load.
Reduced Bee Population: A noticeable decline in the number of bees in the colony can be a sign of mite-related stress and mortality.
Weakened Bees: Bees may appear lethargic, unable to fly, or have shortened lifespans.
Bees ejecting larvae/pupae from cells. This is a hygienic behaviour that helps to try remove mite infected brood.

Monitoring Varroa Mite Levels: Essential for Effective Control

Regular monitoring of Varroa mite levels is essential for determining the need for treatment and evaluating the effectiveness of control measures. There are several methods for monitoring mite populations, each with its own advantages and disadvantages.

Common Monitoring Methods

Alcohol Wash: This method involves collecting a sample of approximately 300 bees (about ½ cup) from a brood frame, immersing them in alcohol (e.g., rubbing alcohol), and shaking vigorously to dislodge the mites. The mites are then counted, and the mite infestation rate (mites per bee) is calculated.
Sugar Roll: Similar to the alcohol wash, this method involves collecting a bee sample and coating them with powdered sugar. Shaking the bees in a jar causes the mites to detach and fall through a mesh screen. The mites are then counted, and the infestation rate is calculated. This is generally considered less destructive to bees than the alcohol wash.
Sticky Boards: Sticky boards are placed under the screened bottom board of the hive to collect naturally falling mites. The board is left in place for a specified period (e.g., 24-72 hours), and the mites are counted. This method provides an estimate of the mite population but is less accurate than alcohol wash or sugar roll. Natural mite fall will vary depending on the brood cycle so this method should be used in conjunction with the other methods for a more accurate determination of the varroa levels in the colony.
Drone Brood Removal: Varroa mites prefer to reproduce in drone brood cells due to the longer development time. Removing drone brood can disrupt the mite reproductive cycle and reduce mite populations. This is a form of monitoring (looking for mites inside drone brood cells) as well as a control technique.

Interpreting Mite Counts: Establishing Treatment Thresholds

The interpretation of mite counts is crucial for making informed decisions about treatment. Treatment thresholds vary depending on the region, season, and colony strength. General guidelines suggest treating colonies when the mite infestation rate exceeds 3% in the spring or summer and 1-2% in the fall. Consult with local beekeeping associations or agricultural extension services for specific recommendations in your area. Keep in mind that these are *general* guidelines and local conditions can dramatically affect the impact of mites. For example, a colony in a strong nectar flow may be able to tolerate a higher mite load than one that is stressed. A general principle is to treat sooner rather than later when in doubt.

Integrated Pest Management (IPM) for Varroa Mites: A Holistic Approach

Integrated Pest Management (IPM) is a comprehensive approach to pest control that emphasizes prevention, monitoring, and the use of multiple control tactics to minimize environmental impact and promote long-term sustainability. For Varroa mite management, IPM involves a combination of cultural practices, biological control, and chemical treatments.

Cultural Practices

Breeding for Resistance: Selecting and breeding honeybee colonies with genetic resistance to Varroa mites is a promising long-term strategy. Traits such as hygienic behavior (removing mite-infested brood) and Varroa-sensitive hygiene (VSH) can significantly reduce mite populations.
Queen Rearing Practices: Promoting genetic diversity in queen rearing can enhance overall colony health and resilience to Varroa mites.
Drone Brood Removal: As mentioned earlier, removing drone brood can disrupt the mite reproductive cycle.
Small Cell Beekeeping: Some beekeepers advocate for using smaller cell sizes in honeycombs, which may reduce mite reproduction. However, the effectiveness of this method is still debated.
Splitting Colonies: Creating splits (dividing a colony into two or more) can disrupt the mite reproductive cycle and reduce mite populations.

Biological Control

Predatory Mites: Research is ongoing to explore the use of predatory mites that feed on Varroa mites. However, effective biological control agents are not yet widely available.
Fungi: Some fungi species have been shown to be effective against Varroa mites. Research continues in this area.

Chemical Treatments

Chemical treatments are often necessary to control Varroa mite populations, particularly when mite levels exceed treatment thresholds. However, it is crucial to use treatments responsibly to minimize the risk of resistance development and avoid contaminating honey. Always follow label instructions carefully and rotate treatments to prevent resistance.

Types of Chemical Treatments

Synthetic Acaricides: These treatments are typically synthetic chemicals that kill mites but can also be harmful to bees if not used properly. Examples include amitraz (Apivar®) and fluvalinate (Apistan®). Resistance to these chemicals is a growing concern in many regions. Use of these chemicals often requires removal of honey supers.
Organic Acids: These treatments are naturally occurring acids that are generally considered safer for bees and honey than synthetic acaricides. Examples include oxalic acid and formic acid. These treatments are considered safe to use with honey supers on the hive.
Essential Oils: Some essential oils, such as thymol (ApiLife VAR® and ApiGuard®), have shown efficacy against Varroa mites. These treatments can be used as part of an IPM strategy.

Specific Treatment Options

Oxalic Acid: Oxalic acid is a naturally occurring organic acid that is effective against Varroa mites. It is typically applied as a dribble solution (dissolved in sugar syrup) or by vaporization. Oxalic acid is most effective when broodless, as it does not penetrate capped brood cells. This makes it especially useful in late fall or early winter when brood rearing is minimal in many climates. Vapourization must be carried out with appropriate personal protective equipment including a respirator, as oxalic acid vapours are harmful to humans. Dribbling can be carried out on colonies even when they are tightly clustered in winter, whereas vapourizing may cause the cluster to break.

Formic Acid: Formic acid is another naturally occurring organic acid that is effective against Varroa mites, including those inside capped brood cells. It is typically applied using a dispenser that releases formic acid vapors over several days. Formic acid is temperature-sensitive, and its effectiveness can be reduced in hot weather. Proper ventilation is essential when using formic acid to avoid harming the bees. Commercially available options include Mite Away Quick Strips (MAQS) and Formic Pro.

Thymol-based Products: Thymol is a naturally occurring compound found in thyme oil. Thymol-based products, such as ApiLife VAR® and ApiGuard®, are effective against Varroa mites and can be used as part of an IPM strategy. These products are typically applied as a gel or wafer that releases thymol vapors over several weeks. Thymol is temperature-sensitive, and its effectiveness can be reduced in hot weather.

Amitraz (Apivar®): Amitraz is a synthetic acaricide that is effective against Varroa mites. It is typically applied as a plastic strip that is hung in the hive. Amitraz is a powerful chemical, but resistance is a growing concern. It is essential to rotate treatments and follow label instructions carefully. Honey supers must be removed before application.

Treatment Rotation: Preventing Resistance

Rotating treatments is essential to prevent Varroa mites from developing resistance to specific acaricides. A suggested rotation plan might involve using a different treatment each season or year. For example, you might use oxalic acid in the winter, formic acid in the spring, a thymol-based product in the summer, and amitraz in the fall (if resistance is not a concern). Consult with local beekeeping experts for recommendations on the best treatment rotation plan for your region. Some regions have bans on certain chemicals.

Developing a Varroa Mite Management Plan: A Step-by-Step Guide

Developing a comprehensive Varroa mite management plan is crucial for the long-term health and productivity of your honeybee colonies. Here is a step-by-step guide to creating an effective plan:

Educate Yourself: Learn as much as possible about Varroa mites, their impact on honeybees, and effective control strategies.
Monitor Mite Levels Regularly: Use a reliable monitoring method (e.g., alcohol wash, sugar roll) to assess mite populations in your colonies.
Establish Treatment Thresholds: Determine the mite infestation rate at which you will take action. Consult with local beekeeping experts for recommendations.
Choose Appropriate Treatments: Select treatments that are effective, safe for bees and honey, and appropriate for your region and climate.
Rotate Treatments: Implement a treatment rotation plan to prevent resistance development.
Follow Label Instructions Carefully: Always follow label instructions when using chemical treatments.
Monitor Treatment Effectiveness: After applying a treatment, monitor mite levels to assess its effectiveness.
Implement Cultural Practices: Incorporate cultural practices, such as breeding for resistance and drone brood removal, to reduce mite populations naturally.
Keep Records: Maintain detailed records of mite counts, treatments, and colony health. This information will help you track your progress and make informed decisions in the future.
Adapt Your Plan as Needed: Be prepared to adapt your Varroa mite management plan as needed based on your monitoring results, treatment effectiveness, and changing environmental conditions.

Conclusion: A Proactive Approach to Bee Health

Varroa mite management is an ongoing challenge for beekeepers worldwide. By understanding the biology of Varroa mites, monitoring mite levels regularly, implementing integrated pest management strategies, and adapting your plan as needed, you can protect your honeybee colonies and contribute to the long-term health and sustainability of beekeeping. Proactive and informed beekeeping practices are essential for ensuring the survival and prosperity of honeybees, which play a vital role in global pollination and food security. Remember to consult with local beekeeping associations and agricultural extension services for specific recommendations tailored to your region and beekeeping practices. Staying informed about the latest research and best practices in Varroa mite management is crucial for success in beekeeping.