This page was last updated
18 February 2012
Quest for the Good Life Welcome to our pages on Varroa mites and the problems it causes bee keepers. We have
to use combined controls to keep their numbers down and the bee colony as healthy
as possible.
Varroa mites are about the size of a pin head. They live on the adult bees but also share the brood cells with the young bees. In the right conditions, their numbers can grow significantly in the hive and cause a weakening of the colony.
Bee keepers have to control them as best possible. Eradication is perhaps a false hope, but maintaining low numbers is the goal so the bees can continue to thrive.
One of the first things the bee keeper has to do is assess the level of infestation. They really are a difficult thing to see even when you have a frame of several thousand bees in front of you. As part of their life cycle is in the brood cell with the young bee, many are out of sight.
Hives have options of open mesh floors (varroa floors) and these allow the mites to fall outside the hive and perish. This only happens when they are dislodged by the bees. Now mites are good at hanging on so not many will be lost like this. By sliding a board into this floor, bee keepers can catch a sample of these falling mites over a fixed period. Usually the board is made sticky with oil or vaseline so the mites cannot climb back up. The total number of mites are averaged out by the number of days and the mite drop per day calculated.
There are guidelines as to a low, medium or high infestation level based on this mite drop and the time of year.
This picture shows the board we use. It has caught all of the hive debris over a week. This one was taken during a period of varroa treatment so the dead mite numbers are higher than normal. We divide the board up into 16 squares. This makes it easier to photograph at macro level each square. The mites can then be counted in each square.
You can see all of the debris that falls out of the hive over the course of a week. Much of this is pollen dropped accidentally, pieces of beeswax and body parts! Of course varroa mites are also in this.
It can be difficult to count all of the mites in this mess, but by zooming in photographically it allows you to see more detail. Other methods include floating the mites in meths and then counting them.
This close up of a single square shows how much debris there is in a small area. By zooming in you can count the mites, square by square and build a total estimate for the board. This is divided by the number of days the board was in place to get an average daily mite drop.
With the build up of mites in the autumn, this is a useful time to use control methods to reduce numbers.
There are many different types of controls, some chemical, some made from essential oils and even more home made recipes too. Some of the older chemical ones are loosing their effectiveness as the mites are becoming resistant. This means that in a total population of mites these treatments have killed most but left behind some that are not affected by it in the same way. These go on to breed and create more resistant offspring. If you continue to use the same control time after time it gradually becomes less effective and eventually useless.
The best way is to use a variety of controls, different ones each year and at different times of the year. Naturally the bee keeper has to be careful that residues from these treatments do not get into any honey they may be harvesting.
One lower level method, when mites are not too much of a problem, is to dust the bees with fine powdered sugar. The sugar gets under the mites legs and prevents them from gripping on to the bees. They fall through the mesh floor and die. We tried this in August. With the wasp year we have had in 2011, all it seemed to do was attract more of those. Difficult to say that it had a positive effect on the hive mite population and perhaps any benefit was offset by the extra wasp attention.
With rising mite levels this autumn, we have used a product called Api Life Var. This is a foam like strip impregnated with thymol, eucalyptus,camphor and menthol. Very smelly, but in a nice way. These strips are placed, according to dosage details on the pack, above the brood frames. The oils fill the chamber and this kills the mites.
How effective they are is the difficult thing to assess. The active ingredients are temperature dependant so somewhere between 18°C and 30°C seems to be best. In the UK at the end of summer it seems reasonable to have this range for part of the day but less so at night. This variability makes it hard to know whether it has knocked down most of the mites. You can only monitor the drop rate during the treatment and then repeat afterwards to see if there has been a decline in numbers.
It’s not until you use these treatments that you realise how many mites are in the hives. Early monitoring in June suggested we had a low level and that treatment wasn’t necessary. By the end of August mite numbers were increasing (16 per day on average) and required some positive action. During the treatment the daily drop rate increased to 62 per day in week 1, 79 per day in week 2 and down to 36 per day in the final week. This suggests the treatment certainly reduced numbers as the kill rate fell off.
We’ll give the bees a rest for a week then monitor the natural daily drop rate to see if there needs to be further treatment.