Varroa in autumn
Why, with good varroa management, no further treatment should normally be needed in October – and what to do if the mite infestation is still too high
With good varroa management, no additional treatment should normally be needed in October. That must be our goal. The decisive work against the varroa mite belongs in summer, before the long-lived winter bees are reared.
Anyone who checked their colonies in time and reduced the mite infestation sufficiently in late summer should now have healthy, well-nourished winter bees in the hive. These bees need to be protected. After all, they have to survive for several months and raise the first new generation of bees in spring.
Of course, beekeeping doesn’t always go according to plan. An insufficiently effective summer treatment, an unexpectedly high residual infestation or reinvasion from other colonies can mean that action is still needed in October.
But a late treatment should not become routine.
So the key question is not: What else can I do against varroa in October? But rather: Do I need to intervene at all – and if so, how can I do it as gently as possible?
1. Why October is so decisive for our winter bees
Under normal Central European conditions, a large part of the winter bee generation should already have been reared by October. Depending on the weather, location and colony development, more winter bees may still be added.
These bees differ significantly from the short-lived summer bees. They have special physiological reserves that allow them to survive for several months.
During their development, however, they are particularly vulnerable to the effects of a high varroa infestation.
The mites damage the developing bees and promote the spread and multiplication of dangerous viruses, especially deformed wing virus.
If winter bees have already been severely damaged during their development, a later treatment cannot undo that damage.
A mite killed in October is one mite fewer, of course. But an already damaged winter bee will not become healthy again because of it.
That is why the mite infestation should ideally be low while the winter bees are being reared.
In October, the main goal is to preserve the existing winter bees – not to make up for summer’s omissions with treatments that are as intensive as possible.
2. Why additional treatments can also do harm now
Every intervention in a bee colony means a certain amount of disturbance. This is especially true when combs are pulled, brood areas are altered or veterinary medicines are used.
Of course, a necessary treatment for a high mite infestation makes more sense than continuing to expose the bees to a heavy varroa load.
But in a colony with a low infestation, an additional treatment may not bring enough benefit to justify the burden it entails.
In October in particular, the motto should therefore not be: the more, the better.
Why I would no longer use formic acid now
Formic acid has its place in varroa control. In October, however, I would generally refrain from any further treatment with it.
The often low and fluctuating temperatures make reliable evaporation difficult. At the same time, unfavorable application conditions can put a strain on the bees, the queen and any brood present.
Above all, though, the winter bees already in the hive should be spared as much as possible.
There are other ways to respond to an elevated mite infestation without carrying out a formic acid treatment at this point.
3. Assess the mite infestation first – then decide
Before starting a treatment in October, you should determine as reliably as possible how high the varroa load actually is.
Options include, for example, monitoring the natural mite drop over several days or a suitable standardized infestation measurement on adult bees.
Caution is needed here: the natural mite drop alone is not an exact measure. Amount of brood, colony strength, previous treatments and other factors influence the result.
The terms low, moderate and high infestation also cannot be assessed independently of the season. An infestation level that might still be tolerable in spring can already be problematic in autumn.
Colony strength is particularly important. When the number of bees declines in autumn, the relative mite infestation of the remaining bees can increase considerably.
Infestation levels should therefore always be assessed in the context of colony strength, amount of brood and previous treatment.
4. Which treatment options are still available in October?
If action really is needed, the gentlest and most targeted methods possible should be chosen.
Oxalic acid sublimation when brood-free
If the colony is completely brood-free, a single treatment by sublimating an oxalic acid product approved for this purpose can make a lot of sense.
The mites are then mostly on the adult bees and can be reached by the treatment.
Such a treatment can already serve as the winter (residual) mite treatment. It does not necessarily have to wait until December.
Oxalic acid strips for a moderate infestation or reinvasion
Since 2025, oxalic acid strips have also been approved in Germany in the form of Calistrip Biox.
These can be an additional option, particularly for a moderate mite infestation or renewed introduction of mites in autumn.
The strips are hung between the combs and release their active ingredient over an extended period through contact with the bees.
Two conditions must be met: the bees must still be sufficiently active, and there should not be any significant amount of capped brood.
The strips are not a reliable solution for colonies with a lot of brood. Studies have shown that their efficacy as a sole treatment is insufficient with a normal amount of brood, but that they can deliver very good results with a low infestation and reinvasion.
A treatment with strips should not automatically be combined with further oxalic acid applications either.
Removing or opening the remaining brood
If only a very small amount of capped brood is left in October, removing this brood in a targeted way can make sense.
Another option is to open the remaining capped brood cells with an uncapping fork.
You can then give the bees roughly one to two days to clear out the damaged brood.
It is important to note, however, that this time span is no guarantee of complete brood-freeness. Before the subsequent treatment, you must check that all capped brood cells have been cleared out and that no new brood has been capped.
With larger brood areas, on the other hand, such an intervention would be critical, because a large number of young bees could still be lost.
5. Ten typical situations in October – with recommended actions
Scenario 1: Strong colony, low mite infestation, still some brood
The colony is strong, has sufficient winter stores and shows no noticeable damage. The infestation check confirms a low mite infestation.
Recommendation: Do not treat additionally. Spare the winter bees and plan the winter mite treatment for when the colony is actually brood-free.
Scenario 2: Strong colony, moderate mite infestation, hardly any brood left
The summer treatment basically worked, but the mite infestation is higher than desired. The colony is still active and has only a little emerging brood.
Recommendation: Approved oxalic acid strips can be an option. The success of the treatment must then be checked. Suitability depends on the specific infestation and the application conditions.
Scenario 3: Strong colony, high mite infestation, completely brood-free
The colony is still strong, but the mite load is clearly too high. There is no brood left.
Recommendation: Promptly carry out an approved oxalic acid treatment, for example a single sublimation. Then check its efficacy.
Scenario 4: High mite infestation, only a few capped brood cells left
The queen has largely stopped laying. Only a small remaining patch of brood is present.
Recommendation: Remove the remaining brood in a targeted way or open it with an uncapping fork. After about one to two days, check that it has been completely cleared out. Once brood-freeness is confirmed, a suitable oxalic acid treatment can follow.
Scenario 5: Moderate infestation due to reinvasion
The summer treatment was successful. In October, however, the mite infestation rises again, possibly due to mites being brought in from other colonies.
Recommendation: Check the infestation. If the bees are still active and there is little brood, approved oxalic acid strips can be a sensible option. At the same time, the other colonies in the apiary should be checked.
Scenario 6: Heavy reinvasion, high mite infestation
The mite load rises considerably within a short time. The colony is already brood-free.
Recommendation: Do not wait unnecessarily. A prompt, effective oxalic acid treatment can make more sense here than a longer-lasting strip treatment. Continue monitoring the infestation afterwards, as mites may be brought in again.
Scenario 7: Medium-strength colony, high infestation and several brood combs
The colony still has larger areas of capped brood. At the same time, the mite infestation is clearly elevated.
Recommendation: Do not simply remove all the brood across the board. The loss of emerging bees could weaken the colony further. A suitable, approved treatment or biotechnical measure must be chosen on a case-by-case basis here. Oxalic acid strips alone are not reliable enough with a larger amount of brood.
Scenario 8: Weak colony, but low mite infestation
The colony occupies only a few seams between the combs but shows no noticeable varroa infestation.
Recommendation: Do not treat unnecessarily. Instead, check the queen, food supply, colony strength and possible other causes. If the colony is not fit to overwinter, uniting it with a healthy colony can be considered.
Scenario 9: Weak colony with high infestation and visible damage
The colony is losing bees, has shrunk considerably or shows clear symptoms of virus-related damage.
Recommendation: Assess the state of health and the chances of overwintering. A treatment can reduce the remaining mites but cannot heal winter bees that are already damaged. If the prognosis is very poor, a decision must be made on proper remediation or dissolving the colony, without endangering other colonies.
Scenario 10: Winter mite treatment already successfully completed in October
The colony was brood-free, was treated with an approved oxalic acid product and subsequently shows a low residual infestation.
Recommendation: Do not treat again in December just because of the calendar date. A further treatment should only be carried out if it is necessary based on the infestation and permitted under the approval of the product used.
6. Is a winter treatment always necessary?
For normal beekeeping, an effective winter mite treatment when the colony is brood-free is part of good varroa management.
It should generally be planned unless it has already been carried out successfully at an earlier point.
There is, however, one particular exception: targeted breeding and research programs in which the varroa resistance of bee colonies is studied.
In such programs, other treatment strategies can be pursued under controlled conditions. These include regular infestation measurements, precise documentation of colony development and defined intervention thresholds.
This is not something for beginners, however, and must not be understood as a general recommendation.
The same applies to resistance breeding: a colony that develops well in spring is not automatically varroa-resistant. If the infestation is critical, you must intervene to avoid unnecessary colony losses and endangering other bee colonies.
7. What October reveals about our varroa management
A colony that is healthy and strong in October with a low mite infestation shows that important prerequisites for overwintering have been met.
With a highly infested colony, on the other hand, we should not just think about how we can still reduce the mites now.
We should also ask why it came to this in the first place.
Was the summer treatment too late? Was its efficacy insufficient? Was the infestation underestimated? Or were mites brought in again?
These questions are at least as important for the next beekeeping year as the current treatment.
Because in the long run, the goal should be to control the varroa infestation so early and reliably that our winter bees can grow up as unburdened as possible.
Conclusion: A treatment in October should be the exception, not the rule
With good varroa management, no additional treatment should normally be needed in October.
By this point, the winter bees should already have been largely reared. What they need now above all is peace and quiet, sufficient food stores and the lowest possible mite infestation.
If you nevertheless have to intervene, there are various options available.
For a moderate infestation or reinvasion, approved oxalic acid strips can be an option, provided the bees are still active and there is only little brood.
If the colony is completely brood-free, a single oxalic acid sublimation can make sense. Very small remaining patches of brood can, where appropriate, be removed in a targeted way to achieve this state.
I would avoid an additional formic acid treatment in October wherever possible.
The key is to match the measures to the actual condition of the colony and not to burden the winter bees unnecessarily.
Because the best varroa management in October is the one that already made sure in summer that we have as little as possible to do now.
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