Varroa mites are one of the most significant challenges faced by beekeepers worldwide. In fact, studies have shown that unmanaged Varroa infestations can lead to the collapse of over 90% of honeybee colonies within two years. These tiny parasites have become a global threat to apiculture, spreading rapidly and undermining the health of bees and hives.
Understanding how to identify, treat, and prevent Varroa mites is crucial for every beekeeper. These pests are not only harmful to individual bees but also serve as vectors for deadly viruses, exacerbating their impact on the hive. In this comprehensive guide, we’ll explore what Varroa mites are, how they affect your bees, and most importantly, the best strategies to protect your hive and ensure its long-term success.
What Are Varroa Mites?
Varroa mites (Varroa destructor) are external parasites that feed on honeybees. These reddish-brown mites attach to adult bees and their developing brood, feeding on their fat bodies and spreading harmful viruses. They are oval-shaped, roughly 1-2 mm in size, and visible to the naked eye.
Varroa mites reproduce within the brood cells of honeybees. Female mites enter the cells just before they are capped and lay eggs on the developing bee larvae. The offspring feed on the larvae and mature into adults before emerging with the newly hatched bee. This life cycle allows mites to multiply rapidly, particularly during periods of brood production.
Where Do Varroa Mites Come From?
Varroa mites originated from the Asian honeybee (Apis cerana), which has evolved mechanisms to cope with these parasites. However, when Varroa mites were introduced to European honeybees (Apis mellifera), these bees lacked natural defenses against the mites, leading to widespread infestations.
Varroa mites spread through several mechanisms:
Bee-to-Bee Contact: Mites transfer during foraging or robbing events, as bees from different colonies interact while collecting nectar or water. Robbing bees can bring mites back to their hives, leading to rapid infestation.
Drifting Bees: Worker bees or drones can accidentally drift into neighboring hives, carrying mites with them. This often happens in apiaries where multiple hives are placed close together.
Movement of Infested Equipment: Beekeepers can inadvertently spread Varroa mites by using contaminated frames, brood combs, or hive tools. Unchecked equipment from other apiaries poses a high risk.
Swarming: Infested swarms establish new colonies elsewhere, spreading mites to previously uninfested areas. Both primary and secondary swarms can act as vectors for mite dispersion.
Mating Flights: Drones carrying mites can transfer them to other colonies during mating flights, as drones typically visit different hives in search of queens.
Natural Mite Fall: Mites that drop off bees within one hive may find their way to another hive if the apiary has overlapping flight paths or if hives are placed too close together.
These multiple pathways highlight why vigilance is essential, as mites can spread quickly within an apiary and between neighboring colonies.
What Do Varroa Mites Do to Bees?
Varroa mites are highly destructive to honeybees and their colonies. They weaken individual bees by feeding on their fat stores, which are vital for energy, hormone regulation, and immune function. This feeding can:
Reduce bee longevity.
Impair thermoregulation and foraging efficiency.
Cause developmental deformities, especially deformed wings in young bees.
More critically, Varroa mites are vectors for several debilitating viruses, including:
Deformed Wing Virus (DWV): Causes malformed wings and weak bodies.
Chronic Bee Paralysis Virus (CBPV): Leads to paralysis and death.
Acute Bee Paralysis Virus (ABPV): Weakens colonies and accelerates decline.
These viruses, combined with the physical toll of mite infestations, can lead to a weakened colony and eventual collapse if untreated.
Are Varroa Mites Harmful to Humans?
No, Varroa mites are not harmful to humans. They are specific to honeybees and do not bite, infest, or spread diseases to humans or other animals. However, the impact of Varroa mites on honeybee populations indirectly affects humans by reducing pollination and honey production.
Signs of Varroa Mites in a Hive
Common signs of a Varroa mite infestation include:
Deformed or Stunted Wings: Bees with crumpled or undersized wings are often a clear indication of a Varroa-related issue. The deformed wing virus (DWV), transmitted by Varroa mites, causes these deformities, leaving affected bees unable to fly and perform their duties within the hive. This can significantly reduce the worker population and productivity.
Reduced Adult Bee Population: A declining number of adult bees in the hive may suggest a Varroa mite infestation. The mites weaken individual bees and spread diseases, causing premature deaths and leading to a noticeable drop in hive activity. Colonies with dwindling populations often struggle to maintain necessary tasks like foraging and brood rearing.
Sunken or Perforated Brood Cappings: Brood cells infested with Varroa mites often have cappings that appear sunken or punctured. This is because mites enter the cells before capping and feed on the developing larvae. This visible sign indicates the need for immediate intervention to prevent further damage to the brood.
Increased Mortality: Dead bees found near the hive entrance or on the hive floor can be a sign of high Varroa mite levels. The combination of weakened immune systems and viral infections transmitted by the mites contributes to the increased mortality rates. Checking for other signs of infestation can confirm if Varroa mites are the cause. See our article about other reasons why there a dead bees around your hive.
Visible Mites: The presence of small, reddish-brown dots on adult bees is a direct sign of Varroa mites. These are often visible on the thorax or abdomen of the bee. Spotting mites on bees during inspections is a clear indicator of infestation, especially when combined with other symptoms.
Poor Brood Pattern: Gaps in the brood pattern, where some cells are empty while others are capped, suggest that larvae have died before reaching maturity. This irregular pattern is often caused by mites feeding on the brood and transmitting harmful viruses, leading to developmental failures and compromised colony growth.
How to Check for Varroa Mites
Regular monitoring is essential to keep Varroa mites under control. How often you check depends on the season and the conditions in your hive:
Spring and Summer: During these active seasons, check for mites every 2-3 weeks. Increased brood production during this time provides a perfect environment for mites to reproduce. Regular monitoring ensures early detection and timely treatment.
Fall: Before overwintering, it is crucial to conduct thorough checks. Varroa mite populations typically peak in late summer and fall. Test every 2 weeks to ensure colonies go into winter with minimal mite levels.
Winter: Monitoring can be reduced, but it is still important to check mite levels during broodless periods, as treatments like oxalic acid vaporization are most effective when no brood is present. A single test in mid-winter may suffice.
Monitoring methods include:
Sugar Shake Test:
Collect about 300 bees (roughly half a cup) from a brood frame.
Place them in a jar with powdered sugar and shake gently.
Mites will dislodge and fall through the mesh lid onto a white surface for counting.
Alcohol Wash:
Similar to the sugar shake but uses alcohol to kill and dislodge mites.
This method is more accurate but results in the death of the sampled bees.
Sticky Board Test:
Place a sticky board under a screened bottom board to collect falling mites.
Leave for 24-48 hours and count the mites on the board.
Drone Brood Inspection:
Remove and uncap drone brood cells, as mites prefer drones for reproduction.
Count the mites found within the cells.
By maintaining a consistent monitoring schedule, you can detect mite infestations early and apply the appropriate treatments before the population becomes unmanageable.
How to Treat Varroa Mites in a Hive
Treating Varroa mites requires an integrated approach, often referred to as Integrated Pest Management (IPM). Common treatments include:
Chemical Treatments:
Miticides: Products like Apivar (amitraz), Apiguard (thymol), and CheckMite+ (coumaphos) are effective against mites. Rotate chemicals to prevent resistance.
Oxalic Acid: Delivered via vaporization or dribbling, this method is effective during broodless periods. For more information, see our article on How to Use Oxalic Acid Vaporization for Bees.
Mechanical Controls:
Drone Comb Removal: Mites prefer drone brood. Removing and freezing capped drone comb reduces mite populations.
Screened Bottom Boards: These allow mites to fall out of the hive, preventing reinfestation.
Brood Interruption: Temporarily removing or caging the queen disrupts the mite life cycle.
How to Treat Bees for Varroa Mites Naturally
For beekeepers seeking organic or natural methods, these options are effective:
Thymol-Based Products: Derived from thyme, thymol treatments like Apiguard are effective in warm climates and are safe for bees when applied correctly.
Formic Acid: Naturally occurring and penetrates capped brood, killing mites within cells. Products like Formic Pro provide an efficient solution without contaminating honey.
Essential Oils: Lemongrass, spearmint, and tea tree oils can reduce mite levels when mixed into feeding supplements. They also promote overall bee health and can be part of a broader hive care strategy.
Sugar Dusting: Regular powdered sugar dusting dislodges mites from adult bees and stimulates grooming behavior. This simple method is non-toxic but requires frequent application to remain effective.
Heat Treatment: Some devices heat hives to a temperature lethal to mites but safe for bees. This approach is chemical-free and environmentally friendly but can be labor-intensive and requires specialized equipment.
Why Choose Natural Treatments?
Some beekeepers opt for natural treatments due to concerns about chemical residues in honey and the environment. Natural methods align with sustainable beekeeping practices and may improve colony health over the long term. These approaches are especially appealing for hobbyists or those marketing organic honey.
Pros and Cons of Natural Treatments
Pros:
Avoids chemical residues in hive products.
Safer for the environment and non-target species.
Supports sustainable and organic beekeeping practices.
Reduces the risk of mites developing resistance to treatments.
Cons:
Often less immediately effective than chemical treatments.
Requires more frequent applications and monitoring.
Some methods, like heat treatment, demand specialized equipment.
Results can vary based on environmental conditions and hive strength.
When Is It Too Late to Treat for Varroa Mites?
It may be too late to save a hive when:
The colony’s population has significantly dwindled, leaving insufficient bees to sustain brood.
The majority of bees show severe signs of mite damage, such as deformed wings.
Queen failure occurs due to stress or mite-induced viruses.
When a hive reaches this stage, it is often better to focus on protecting remaining hives and preventing the spread of mites. Here are key steps to take:
Isolate the Collapsing Hive: Move the failing hive away from the healthy ones to prevent robbing, as robbing bees can carry mites back to their colonies.
Disinfect Equipment: Sterilize all equipment used in the failing hive, including frames, tools, and boxes, to avoid contamination.
Strengthen Nearby Hives: Provide supplemental feeding (sugar syrup or pollen substitutes) and ensure strong populations in nearby colonies to help them withstand potential mite pressures.
Monitor Healthy Hives Closely: Increase the frequency of Varroa mite checks in nearby hives, testing every 1-2 weeks to catch infestations early.
Apply Preventative Treatments: Treat healthy hives with appropriate miticides or natural methods to lower their risk of infestation.
Reduce Robbing Opportunities: Avoid open feeding and keep hive entrances reduced to help guard against invading bees from collapsing colonies.
How to Prevent Varroa Mites
By taking these proactive steps, you can mitigate the risk to your apiary and prevent Varroa mites from spreading further.
Prevention is the most effective strategy against Varroa mites. Steps include:
Regular Monitoring: Perform mite counts at least once per season. In the winter season, consider inspecting the hive using a thermal imaging camera.
Using Screened Bottom Boards: Reduces the chance of reinfestation.
Rotating Treatments: Prevents mites from developing resistance to specific chemicals.
Maintaining Strong Colonies: Healthy colonies are more resilient to infestations.
Reducing Robbing: Avoid open feeding and ensure hives are secure to reduce bee-to-bee mite transfer.
Selective Breeding: Use queen lines bred for hygienic behavior, which helps bees remove mites naturally. See our article about the best bee breeds for beekeeping.
Varroa Mite Treatment Schedule
A seasonal treatment schedule can help keep mites under control:
Spring:
Test and treat before the main nectar flow begins. Use softer treatments like thymol to avoid contaminating honey.
Summer:
Monitor mite levels during peak activity. Consider mechanical controls if counts are low or targeted chemical treatments if they rise.
Fall:
Treat aggressively after honey harvest to knock down mite levels before winter.
Use oxalic acid or formic acid for thorough control.
Winter:
Use oxalic acid vaporization during broodless periods for maximum impact.
Final Thoughts
Varroa mites are a persistent and dangerous threat to honeybees, but with knowledge, vigilance, and the right tools, beekeepers can manage them effectively. Regular monitoring, timely treatments, and preventive measures are essential for maintaining healthy and productive hives. By staying proactive, you can protect your bees and ensure a thriving apiary for years to come.