Elgon
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The Beeshaker I got from Michael Palmer.
Measure the varroa level

The beeshaker
A method of montoring the varroa lavel I have used is alcohol wash using a beeshaker. It gives an enough good and quick base for decision to treat or not.
Make your own beeshaker
Find two suitable straight plastic jars with plastic screw lids, 3.5-5 deciliter (12-17 oz). You can find them in the grocery store filled with peanut butter. You empty them in one way or the other.:)

You saw circular holes in the lids, about 60 mm (circa 2.3 inch) in diameter. Leave a small rim. The rim should cover a circular netting (mesh) with about 3 mm (0.12 inch - 1/8") square holes.

Glue the two lids together with the netting in between and the threads of the lids outward. You can also melt the lids together with a soldering iron.
Now you can screw one jar on one lid, and the other jar on the other lid. Thus you get the two jars fastened together with the lids in between.
Using the beeshaker
Unscrew one of the jars from the beeshaker. Fill it with about 2 deciliter (7 oz) technical alcohol (at least 70% alcohol, ethanol).
Shake one comb of bees into a plastic box, enough big so the bees will fall into it and not on the outside. Then shake the bees into one corner.
Take a 1 dl measure and fill it with bees from the box. Pour the bees into the jar with alcohol.
Screw the other jar with the two lids tight on the jar with alcohol and bees. Shake the rest of the bees in the box back into the hive, or in front of the hive.
Shake the beeshaker gently for 45-60 seconds with the bees and alcohol in the bottom jar.
Then turn the beeshaker around. The alcohol will fall through the mesh into the jar that has now become the bottom one. The bees will stay above the mesh.

Alive mites that were sitting on the bees died of the alcohol (as did the bees). They fall also through the mesh (about 90% of them). Also some dirt come along with the mites. You discern mites by the smooth edges of them and the rough edges of dirt.

Lift the beeshaker over your head and look at the bottom. There you will find the mites that were sitting on the bees.
The number of bees in one dl (deciliter) of bees is about 300.
Shake the bees from a comb (or two) close to the brood. Just above the center of the queen excluder, or one comb from the brood from either side of the upper brood combs in the brood nest. Then you get the most representative amount of phoretic mites and avoid including the queen.
2 varroa/300 bees = 0,6 varroa/100 bees = 0,6 % – < 3 %
19 varroa/300 bees = 6,3 varroa/100 bees = 6,3 % – > 3 %
If you find 9 mites at the bottom of the beeshaker the varroa level is monitored to 9/300 = 3/100 = 3%.
When you are done you can save the alcohol to a next round monitoring by pouring the alcohol/mite/dirt mixture through a coffee strainer. You may fill up with some technical alcohol to reach two dl.

Be sure to clean the bottle from eventual mites that may have stuck to the sides of the jar.
If you want you can shake the same bees you just did a second time to get the eventual 10% mites that may have been left among the dead bees above the mesh.

Before monitoring another colony for the varroa level throw away the dead bees in a bucket.
After getting used to the procedure you will end up with doing the monitoring in about 4 minutes.
You can watch a short video how to use the beeshaker. Speaker voice in Swedish.
Is it worth killing 300 bees
If you do nothing to help the bees to fight mites, many thousands of bees will die.

During a heavy flow about a thousand bees per day will never come back to the hive after working flowers for nectar.

Bees die for a lot of reasons helping the bee colony to survive and thrive.

Worker bees live in average 6 weeks during the working season. It's especially feeding larvae that makes them age. During a resting season such as winter or a dry season when there is no or little brood a worker bee can live 6 months (for example a long winter).

Brood can also function as a means of catching parasites or work as food buffer. Thus they mostly die for the benefit of the colony.
In late autumn most of the drones are driven out of the colony to die to lessen stresses on the colony during the coming season with scarcity of food.

During winter half of the bees population can die within a variation that can be called normal. Some bees in the outer parts of cluster die when or after insulating the inner parts. Some of the bees in the inner parts of cluster die after a period of creating warmth. They become worn out by this work.
The EasyCheck variety of the Beeshaker you don't have to turn around. You can buy it from bee equipement dealers.
EasyCheck – an alternative beeshaker
There is a commercial variety of a beeshaker that works a little differently and a little more effective. It's made by the French company Véto-pharma:
Almost the whole 100% of the mites on the bees will be collected at the bottom and be counted. You shake EasyCheck even gentler than the original beeshaker and even better, you swirl it instead of shake it up and down. The mites fall through the holes in the "mesh" as they drop from the bees and not only at the end of the procedure.
Alternative to a beeshaker – BeeScanning app
If you don't want to sacrifice 300 bees you can use the mobile app BeeScanning. You download the app to your Android phone or iPhone. I suggest you take pictures from 3 broodframes, as many pictures of each framside to you cover most of the bees on them. Follow the instructions on
Monitoring the varroa level is free, with no costs. The app can do more and these abilities cost a small sum.
Where to take the bees for measureing
Does it matter where you take the bees? Yes.

Randy Oliver in California has investigated the distribution of mites on the bees, so called phoretic mites.

In the broodnest the mite level can vary quite a bit on different brood combs. Very far away from the broodnest the varroa level is lower than average. If you choose to take a sample from brood combs, make it from a couple of them so the resulting measure will be kind of an average.

Close to the brood but not a brood comb is often the most representative place for the average varroa level in a bee colony. No sample will show exactly the average. What's important is to get a good enough correct value to get an enough good figure of the varroa level to act upon.

When taking a sample for the beeshaker the quickest and easiest is to shake the center comb(s) in the first super above the queen excluder. Or the second comb from the first brood comb in the top brood box. (The very outer one may not have enough bees.) If you take the sample from there it's least likely you will include the queen. That's what you don't want to do.
Resistance is growing
3% varroa level as a threshold for treating with thymol is working well. When bees are not very resistant a full treatment per season will give a total amount of thymol that will be greater than when resistance is higher.

A spring treatment often means that one pad with 5 grams of thymol is used. It is replaced with a second pad 10 days later, or 7 days later. If seven days interval is used it will be a third thymol pad after 7 + 7 days. Only one pad at a time is used in spring to minimize bad effect on brood rearing.
Closing in towards the end (if not earlier) of the season with still brood in the hive, it will be time to check the varroa level again. If the varroa level tells you so it will be a treatment of 2 thymol pads replaced after 10 days (if it's hot with 7 days or even 5 days interval with also smaller pads in size).
The total amount of thymol can vary between 30-40 grams for colonies not very resistant.

When colonies are treated 30-40 grams a year the microbiome may be so much affected that some resistance factors may be overshadowed by virus effects. But if the threshold of 3% is followed no colonies will be big reinvasion sources. As soon as some colonies show better mite hunter traits and other resistance traits, of course you replace queens in colonies which you have needed to use the most amount of thymol on. Different types of resistance traits are needed for long time sustainability.

When resistance is growing the beeshaker will be even more important to find out when to decrease tretaments and amounts of thymol.
Soon enough a full seasonable dose will be 20 grams instead of 30-40 grams (still for a few).

When you reach the situation where a considerable number of hives only need one 5 gram-pad in spring, and they after that get a new queen and don't need any treatment in late summer you have passed a good part of the road to resistance.
Now the hard board outside the entrance is more important. It may well be that spring varroa level is very low in some colonies, after treatment has been low or nothing the previous year. You may end up at the later part of the season that you don't even see any odd DWV-bees outside some of these colonies. It may be so that the microbiome has regained strength, if you have treated only 5 grams or so in the season(s) previous to the present one. So even if the varroa level should have passed 3% somewhat and for a short time due to some reinvasion, the colony may handle that well.
At some point, some year, you may start leaving the late summer monitoring and for some colonies just watch the hard board. Especially if they are placed not so close to other bees. The coming spring will tell you if you should have waited another year or two.