Zachary Huang, Associate Professor, Michigan State University
So it is August, if your bees were strong in May and June, chances are that they have made some honey during these two months. August is a good time to check for your mite levels, especially if you have never checked them before. It is a better idea to check it once a month to know a trend, but checking it once now is better than nothing since you cannot really go back in time
There are some plants still blooming but bees would be happy if they are breaking even (i.e. bringing enough nectar that counter balances their daily consumption: which is about 7 lbs of honey per day (0.011 g/0.84 x 30,000 x 8,000/3=3 kg).
[explanation for the formula: the average consumption from my study (and others confirmed) per worker per day is 11 mg of sugar, while honey has about 16-18% of water. Assuming that a colony has 30,000 workers (medium toward strong strength, bees in 2 brood boxes), and 2 frames of brood (which can be an underestimate). I am assuming that each larva on average takes 1/3 of the energy of an adult bee. Wintering bees will consume less sugar than this, perhaps as little as 1/8 of the 11 mg per day when their brood temperature is not regulated at 95F. That translates to 1 lb of honey per day during the Michigan winter, for a very large colony].
What I am saying is that now is a good time to take your honey off, treat for mites, and let your bees to recover a bit before they take in the goldenrod honey (in about 2-3 weeks), which can be used for wintering. [you would have to have started treating in July if you want to harvest the goldenrod honey, since some medications require you to have withdrawn the chemical 40 days prior to putting on a honey super].
A good IPM (integrated pest management) practice is to “know your enemies”. This means knowing how many mites are there in the colony before you even attempt to treat. If there are not enough mites, you do not need to treat.
There are two common methods to check for how many mites are in a colony. One is a sticky trap. For this one you leave a piece of board, which has a screen material over it so bees do not interfere. Below there is a white piece of paper or cardboard, with sticky materials (sprayed Pam, Vaseline, or contact paper). Leave this trap in the colonies for 2 days and then take it out to count the mites. In Georgia and Delaware, the threshold to treat was set at 60-90 mites per day. In Michigan we should use 30 simply because of our much longer and colder winter.
Another method is using sugar dust or ether roll. They will yield the same results if done correctly. Sugar dusting has the advantage of not killing bees and also know your results right away in the field.
Sugar dusting method involves:
1. open the hive and finding a frame of open brood. Check to make sure the queen is not on this frame.
2. shake the bees into a pan
3. scoop up ½ a cup of bees (~300).
4. transfer into a mason jar, which has the solid lid replaced with hardware cloth (mesh size=8)
5. add enough sugar to coat all the bees (3-4 large spoons). Roll the jar to coat bees with sugar.
6. shake the jar inverted over a piece of white paper and count the mites on the paper. Mites will be upside up with their legs wiggling.
Yes, each black dot here is a varroa mite!
7. put the sugar-coated bees back to their hive.
The threshold for treating for mites is set in Delaware about 10-12 mites in 300 bees. Again in Michigan we should reduce this to 6.
Treating for mites:
[Zach’s pledge: if you have a colony that yields more than 40 mites per jar, please email me at email@example.com – we need a lot of mites for our studies! Yes it is possible. The highest I have seen is 152 mites per 300 bees. In general, you will need an overwintered colony to see this high a mite level].
Over the summer, you could have managed for mite population through sugar dusting, open screen bottom board, and drone brood trapping. But right now you probably have only one chemical option left (unless you want to shake your bees out inside a box, dust them all in sugar and shake the box for 10 min. And that would rid the mites off your adult bees, not those in brood).
1. The old Apistan is no longer good: chances are most mites are already resistant to it (one can do a simple test by cutting a small piece of Apistan and exposing 300 bees to it for 24 hrs and see the proportion of mites alive. This is called the Pettis field test.
2. Coumaphos: an organophosphate containing a neurotoxin. Toxic to other animals (and humans!) if at high dose. Some mites are resistant.
Photo showing a coumaphos strip being used for treating Varroa mites.
3. ApiGuard/Api Life Var: products containing Thymol, best used at 70-80 temperature. Read the label.
Miteaway II and oxalic acid are longer available for mite control. So basically for most beekeepers the option would be number 3 above.
In Michigan, I believe you would be too late if you are treating your bees in late Sept and Oct, because colonies will need undamaged bees in Oct/Nov to overwinter. It takes about 40 days for a treatment to rid of most of the mites. So the latest you can start treating is about Sept 1st.
Zachary Huang, Michigan State University