Varroa Control – Why cull drone cells?

uncapping drone cells

uncapping drone cells

“Varroa mites are innocent, OK” – after all, they are just doing their thing without any malevolent intent. It’s what they do. But – by feeding on the bees’ “blood” the Varroa mites unwittingly carry and inject harmful viruses and other toxic entities.

The key thing in the interaction between the Varroa mites and the bees is that the Varroa carry out their reproduction in the brood cells of our bees. The female Varroa mite enters the brood cell and lays a series of eggs. The first is always male; the second and subsequent eggs are always female. They are laid at consistent time intervals.

Bees have incubation periods that differ between the different forms of bee in a colony.



Varroa mite on larva

Varroa mite on larva

Without going into a lot of science, the incubation period for drones is two days longer than the smaller worker bees and this is long enough for the mite to lay two female eggs. The worker bee cells only yield one female mite. There is only ever one male mite and he stays in the cell and perishes in each generation, so he doesn’t matter.

The mathematics of this are key to the drone problem. For worker bees, there is only time for one female Varroa egg to be laid per cell, so in each succeeding generation there are: – 1->1->1->1->1 females; there is no increase in the female Varroa population and hence the total mite population is stable and controllable at a low level.

However, if there are two female eggs per generation the population grows explosively: – 1->2->4->8->16->32->64………but of course this is for only one mite. If there are ten, then it goes: – 10->20->40->80->160->320->640

If there are 100 to begin with we have: – 100->200->400->800->1600->3200->6400, …….and the colony soon becomes overwhelmed, not by the mites themselves, but by the viruses they carry.

This cannot happen with worker bee brood cells; there is not enough time in the incubation period for two female mite eggs to be laid. The drone brood is where the problem lies and it MUST be monitored. This means taking a sample, which regrettably results in the death of a small part of the drone brood. Despite the injustice to the drone bees, a responsible beekeeper MUST monitor and control the mite-infested drone brood in the hive. Unfair it may be but it is unavoidable if the colony is to survive. Drone bees are innocent victims. They do not deserve to be slaughtered but the numbers are inexorable. We must keep the Varroa numbers as low as we can.

Of course, if the drone brood is found by monitoring (which means killing a few drone embryos) that it is not significantly infested, then no intervention is necessary and none should be undertaken.

what we found in drone brood

what we found in drone brood

In a sample taken from the drone brood of one of our hives we found that about ten percent of the brood were infected with the Varroa mite. This is held to be significant but not disastrous. Five percent is considered to be a light infestation, twenty five percent is severe.

Thymol kills the Varroa mite. Thymol is not a man-made pesticide or insecticide. It is an essential oil produced from Thyme (Thymus vulgaris), which many of us have in our gardens. It is harmless to humans and has been used in medication by many ancient and modern cultures. We must use it.

As for harmony in nature, bees and mites have been co-existing for eons, so why have not all the bees either been wiped out or become resistant? The answer seems to be that in the search for greater honey production the European Bee has been selectively bred to become physically larger and in so doing the incubation period has been prolonged. In the case of the drone brood this has enabled to mite to produce two progeny instead of one. Man has destroyed the balance of nature. (Text written by Peter Horsefield)

How is varroa spread to other colonies? (from: Adventures in Beeland)

  • Foragers carrying mites drifting into, or robbing from, other colonies
  • Drones carrying mites visiting other colonies
  • The beekeeper moving brood frames between hives or transporting infected colonies to new areas
  • Varroa mites have been found on flower-feeding insects such as certain species of bumble-bees, scarab beetles and flower-flies. Although the Varroa mite cannot reproduce on these insects, its presence on them may be a means by which it spreads short distances and finds new honey bee hosts.

Below are a couple of monitoring methods beekeepers can use to help monitor how bad infestation is.

  • Drone trapping

Varroa mites prefer to breed in drone brood. Drones take 24 days to develop whereas workers take 21, so drones give the mites time to fit in more breeding cycles. The mites identify the drone brood by its different smell, which is a result of the more protein rich diet fed to drones.

In England drone trapping can be used as a method of varroa monitoring and/or control during April, May and possibly June. To do this put a drawn super frame into the brood box, to encourage them to build drone comb in the gap underneath, or a frame with drone foundation or drawn drone comb, depending on the size and strength of the colony. Put the super frame at the side of the brood nest, not the middle.

Once the drone comb is sealed, cut it off the super frame. You can do this 2-3 times during the summer. Before you destroy the drone brood you can uncap some to see how many mites are inside.

  • Varroa board monitoring

Draw a grid with several squares on your monitoring board and smear it with vaseline, then put it under your open mesh floor for a week. The grid pattern makes it easier to count mites and the vaseline ensures the mites stick to the vaseline and aren’t blown off when you inspect the board. A week later, count the number of mites and work out an average daily drop count. If there is a lot of debris which makes it hard to count mite numbers, put the debris in a jar with a lid before mixing with methylated spirits and shaking vigorously: once the debris settles, the wax, propolis and other debris will sink, the mites will float.

To assess how bad the problem is based on the daily drop count, the season and the type of hive must be taken into account – UK colony collapse thresholds would be a daily drop of 6 in May, 10 in June, 16 in July and 20 in August. The Beebase varroa calculator is a helpful tool which can tell you how bad the problem is based on the time of year.

What treatments for Varroosis are permitted in the UK? A detailed account of how to carry out four forms of treatment.

  • Shook-swarm

I have a blog post explaining how a shook-swarm works, ‘A successful shook swarming‘. Ideally this is carried out in early spring, in late March – early April. The bees are shaken onto new foundation frames and all the old brood comb, containing lots of mites taking advantage of the new spring brood to breed, is burned. Sugar syrup is fed so the workers can draw out new comb quickly. This is a helpful non-chemical anti-varroa treatment because a large percentage of the mites are destroyed, followed by a short break in the queen laying while new comb is drawn out, which further cuts down on mite reproductive cycles.

  • Icing sugar

The advantages of this method are that it’s cheap and easy to do. It can also be done with supers on, unlike thymol based treatments like Apiguard which might taint the honey with their smell. The icing sugar works in two ways – by reducing the electrostatic charge by which the varroa cling to adult bees and by inducing the bees to groom. A flour dredger or a honey jar with holes punched in a lid work well. Work in pairs to do the treatment, with one person holding out each frame horizontally and another person dusting the sugar over each side.

As the treatment doesn’t kill mites, but only knocks them off, it is only any good in a hive with an open-mesh rather than a solid floor. Since it only affects phoretic mites clinging onto adult bees, which only make up about 30-40% of the mite population, it is a low efficiency treatment and generally only reduces mites by about 20-30%. This may sound good, and is better than nothing, but really an 80% effective treatment (such as Apiguard or oxalic at the appropriate times of the year) is needed to have any real effect on mite numbers.

You cannot rely on sugar dusting alone to keep varroa levels down; if you do your colonies are likely to die. This is true generally of varroa control: you cannot rely on one treatment alone, but should use several different methods throughout the year.

Apiguard, a natural thymol based treatment, can be given in August once your supers have been removed (otherwise your honey will stink of thyme). Starting Apiguard in August allows the hive to produce several generations of healthy bees before going into the winter. Two 50g treatment packs are given, one initially and the second 10-15 days later. Small colonies or nucleuses can be given a half dose.

The treatment works because the worker bees dislike the heavy thymol scent. They start removing the gel to clean the hive and remove the foreign smell, distributing it round the colony and killing off varroa mites in the process. Both adult mites and developing mites inside capped cells are affected, but honey bee larvae are safe. Tape up your varroa monitoring board whilst treating so the fumes stay in the hive. Apiguard should be done while the weather is still warm, as it is most effective – 90-95% effective – in the optimum conditions of an external ambient temperature of more than  15°C and active bees. This is because distribution of the Apiguard gel depends on the bees transporting it round the hive during the process of hive cleaning, and this activity increases as the external temperature rises.

  • Oxalic acid 

This treatment can be carried out once either in December or January whilst brood levels are either non-existent or low. It works by damaging the proboscis of the varroa mites, preventing them from sucking haemolymph from the host bees. Oxalic acid can be purchased pre-mixed in a sugar solution, which is the safest method.

Choose a bright and warm winter’s day when the bees are loosely clustered, so that as they move inside the cluster they distribute the chemical onto the mites. Put a varroa monitoring board over the mesh-floor, as it feels good to count the number of dead mites dropped onto it over the next few days; sometimes I’ve counted over a hundred in a week. Warm the product slightly until it’s lukewarm, remove the hive roof and crown-board, and trickle 5ml over each seam of bees. Do this very quickly to avoid chilling the bees too much. If the colony has been treated before and still has the same queen, it is unwise to use it again as it may harm the queen.

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