Winter Treatment with Oxalic Acid - Complete Guide for Beekeepers

The winter treatment with oxalic acid is one of the most important dates in the beekeeping year. During the broodless phase, the acid achieves the highest efficacy against the Varroa mite. Those who hit the right timing and work cleanly lay the foundation for healthy colonies in spring.

This article is your complete guide: From the broodlessness check through the comparison of application methods to the EU documentation requirements - everything you need to know as a beekeeper.

Note: The approved products, dosages, and legal requirements described in this article are specific to Germany and the EU. If you keep bees in a different country, check your local veterinary medicine regulations for approved Varroa treatments.

Why Oxalic Acid in Winter?

The Varroa mite reproduces in capped brood. In winter, when the queen takes a brood break, all mites are on the bees - so-called phoretic mites. Only then can oxalic acid effectively reach them, as it works exclusively through contact and does not penetrate capped brood cells.

The difference is dramatic:

• Efficacy broodless: 90-95%
• Efficacy with brood: only 30-50%
• Conclusion: The timing is more critical than the method

The winter treatment is the foundation of Varroa management. It cannot replace the summer treatment, but without it, colonies start spring with a mite load that can lead to catastrophe by summer.

Understanding Varroa - Why the Winter Treatment Is So Important

To grasp the significance of the winter treatment, a look at the Varroa reproduction cycle helps:

Exponential Reproduction

A single mite in January can become hundreds of mites by August. The reproduction rate is approximately factor 2-3 per brood cycle (21 days). This means:

Month	Mite Count (Example)	Damage
January	50 mites (after treatment)	No visible damage
April	100-200 mites	Still inconspicuous
June	500-1,500 mites	First weakness possible
August	3,000-8,000 mites	Critical, shortened lifespan
October	5,000-15,000 mites	Colony can collapse

Every mite you do not catch in winter will multiply in spring. That is why the 90-95% efficacy during broodlessness is so decisive - compared to only 30-50% when brood is present.

The Right Timing

The treatment should take place during the broodless phase. Hitting the exact timing is the biggest challenge.

Regional Differences

Region	Typical Period	Particularities
Northern Germany	Mid-November to mid-December	Earlier brood break due to cold
Central Germany	Late November to late December	Classic period
Southern Germany	December to early January	Milder winters, later brood break
Alpine foothills	November to December	Early, reliable brood break
Urban beekeeping	December to January	Heat islands delay brood break

Signals for Broodlessness

• At least 3 weeks after the last feeding
• Ideally after the first extended frost period (several days below 0 degrees)
• The shortest days of the year (around the winter solstice) are often a good indicator
• Climate change note: In mild winters, the brood break can be very short or not occur at all - monitoring is therefore essential

Broodlessness Check in Detail

Verifying broodlessness without disturbing the colony is crucial. Here are three methods from minimally invasive to definitive:

Method 1: Debris Analysis (Non-Invasive)

The most reliable method without interfering with the colony:

1. Insert monitoring board

   Slide the monitoring board (debris tray) under the colony. For open mesh floors, simply insert the slide-in tray. If not permanently present, insert it now.

2. Wait 3-5 days

   Leave the board for at least 3 days. During this time, enough debris accumulates for a meaningful assessment. The colony is not disturbed.

3. Analyze debris

   Carefully pull out the board and analyze the debris pattern: Few light wax particles mean little or no brood. Many dark wax particles (cell caps) indicate brood activity. Chalkbrood mummies show that brood was present.

4. Simultaneously assess mite drop

   Count the fallen mites on the board and divide by the number of days. This gives you the natural daily mite drop - an important indicator of infestation level before treatment.

Method 2: External Observation

Complementing debris analysis:

• Compact bee cluster visible (through mesh floor or window) = brood break likely
• No pollen intake for weeks = no brood to supply
• Outside temperature several days below 5 degrees Celsius = brood break very likely
• No flights even on mild days = colonies in tight winter cluster

Method 3: Mirror Check (Minimally Invasive)

For definitive confirmation:

1. Briefly open the lid (under 30 seconds)
2. Shine a flashlight and mirror between the frame gaps
3. Brood cells visible? Then wait.
4. Only loosely sitting bees on the frame tops? Broodless.

Application Methods Compared

There are three established methods of oxalic acid application. Each has advantages and disadvantages:

Trickling Method in Detail (Most Common)

The most proven method for hobby beekeepers. It is simple, reliable, and requires minimal equipment:

1. Prepare solution

   Use a ready-made, approved solution (e.g., Oxuvar or Bienenwohl) or prepare the solution according to package instructions: 3.5% oxalic acid dihydrate in sugar water (1:1). The solution must be body warm (not above 35 degrees) so it distributes well and does not startle the bees. In winter, keep the bottle warm in your jacket pocket.

2. Calculate dosage

   5 ml per occupied frame gap, maximum 50 ml per colony. Count the occupied gaps from above: If the colony sits on 8 gaps, you need 40 ml. Prepare the right amount in the syringe (50 ml syringe without needle). Mark a dosing guide on the syringe.

3. Put on protective equipment

   Wear acid-resistant nitrile gloves and sealed safety goggles. Long sleeves protect against splashes. Keep a canister of water ready in case solution contacts the skin.

4. Perform treatment

   Briefly open the lid, fold back the cover cloth or inner cover. Trickle the solution evenly into the occupied frame gaps - as directly onto the bees as possible. Start from the outside and work inward. Work quickly (under 1 minute per colony). Close the lid immediately.

5. Document

   Note immediately: Colony number, date, product (including batch number), dosage, treating person, supplier. When using Hivekraft: Record directly in the app - all EU required fields are pre-filled.

Important: The trickling method may only be applied once per winter. A repeat damages the bees through the additional sugar load and acid contact.

Vaporization Method in Detail

For larger operations with specialized equipment - the highest efficacy with the least bee stress:

• Principle: Oxalic acid crystals are heated to approx. 157 degrees and sublimate into a fine vapor in the hive (not above 189 degrees, otherwise the acid decomposes)
• Approved product: In Germany, only Varroxal 0.71 g/g (Andermatt BioVet) may be vaporized - approved since September 2023. Other oxalic acid products (Oxuvar, Bienenwohl) are not approved for vaporization.
• Devices: Only the vaporizers Varrox (gas-powered, ~100 EUR) and Varrox Eddy (battery-powered, ~250 EUR) are recommended per the approval
• Dose: 2 g Varroxal per colony (per package insert)
• Temperature range: Outside temperature between 2 and 10 degrees Celsius (per Varroxal approval)
• Advantage: Can be repeated after 7 days if needed (for incomplete broodlessness). External treatment possible, very bee-friendly.
• Disadvantage: Respiratory protection with P3 filter is mandatory - oxalic acid vapor is corrosive to the lungs

Spray Method

The least common method. It is barely recommended in Germany anymore:

• Principle: 3.5% oxalic acid solution is sprayed on both sides of every occupied frame
• Disadvantage: Every frame must be pulled individually and sprayed - enormous effort and massive disturbance of the winter cluster
• Efficacy: Lower than trickling or vaporizing
• Use case: Only for special management methods (e.g., brood removal in summer with subsequent spray treatment)

Post-Treatment: Monitoring Success

After treatment, monitoring the treatment success is essential:

Debris Check After Treatment

1. Insert monitoring board immediately after treatment
2. Count mite drop after 3, 5, and 7 days
3. Assessment:

Mite Drop (total 7 days)	Assessment	Action
Under 50 mites	Good	No further action
50-500 mites	Acceptable	Plan early spring monitoring
500-1000 mites	Critical	Monitor colony intensively in spring
Over 1000 mites	Alarming	Possible reinvasion, investigate cause

EU Documentation Requirements

The winter treatment is a veterinary medicine application and must be documented without gaps according to EU Regulation 2019/6.

Required Entries in the Colony Record Book

For every winter treatment, the following information must be recorded:

1. Date of treatment
2. Product name - e.g., "Oxuvar 5.7% ad us. vet."
3. Batch number of the product (on every package)
4. Dosage - e.g., "5 ml/frame gap, 35 ml total"
5. Application method - trickling or vaporizing
6. Treated colonies - numbers or names
7. Name of the applicant - who performed the treatment?
8. Supplier - where was the product purchased?
9. Invoice number from the supplier
10. Withdrawal period - as per package insert

Retention Period

• Records must be kept for 5 years
• Digital or analog - both are permitted
• During an inspection, the colony record book must be immediately presentable

Common Mistakes to Avoid

Mistake	Consequence	Solution
Treatment with brood present	Only 30-50% efficacy	Check broodlessness thoroughly
Overdosing	Bee losses, increased winter mortality	Exactly 5 ml per gap, max. 50 ml
Solution too cold	Shock for the bees, poor distribution	Warm solution to body temperature
Double trickling treatment	Massive bee damage	Trickle only ONCE per winter
No documentation	Fine up to 30,000 EUR (Section 89 Para. 6 TAMG)	Enter into colony record book immediately
Technical oxalic acid used	Violation of veterinary medicine law	Use only approved veterinary medicines
No respiratory protection when vaporizing	Lung damage	Always wear P3 mask
Treatment in strong wind	Uneven vaporization, applicant endangered	Wait for calm weather
Treated too late (January/February)	Queen may have resumed laying	Optimal: December, after first frost period
Product stored incorrectly	Reduced efficacy	Store cool, dry, frost-free

The Winter Treatment in the Overall Concept

The oxalic acid winter treatment is only one building block of integrated Varroa management. The overall concept throughout the year:

1. Spring (April-May): Drone brood removal as a biotechnical measure
2. Summer (July-August): Main treatment with formic acid after the last harvest
3. Autumn (September): Monitor natural mite drop, follow-up treatment if needed
4. Winter (November-January): Final mite removal with oxalic acid during the broodless phase
5. Year-round monitoring: Regular debris checks, document mite drop

An integrated Varroa concept is like a gear system: Each measure meshes with the next. Those who view the winter treatment as an isolated action miss the crucial point. It is about the interplay of all building blocks throughout the entire year.

With a consistent Varroa strategy, you have the best chances for healthy and strong colonies in the new year.

August: The Most Important Varroa Treatment of the Year Fall Feeding: Amounts, Timing and the Right Method Annual Planning: The Beekeeping Year at a Glance