At the last Open Hive Day on August 25, we focused on the different methods of sampling for Varroa mites in our colonies. I had placed sticky boards (plastic boards rubbed with vegetable shortening) under the screened bottoms of the hives to trap any of the mites that naturally fall through.
After doing a careful count, we came up with 24-hour average mite drops for each hive. Only Hives A and C had levels that were close to the threshold for treatment (50 mites on the board after 24 hours), so we did a follow-up test on those hives, just for fun.
We used another test that is relatively new to my arsenal: the "powdered sugar roll". When using this assay, you get a certain volume of bees into a mason jar with a screened lid. Then you put a couple tablespoons of powdered sugar in the jar with them, and shake and roll the jar to coat them well. After a minute or two, you can then shake the jar upside down to dump out any mites that were in the jar, as they have been dislodged by the powdered sugar. If you know how many bees you had in the jar (about 100 bees/fluid oz, or 400 bees/half cup), then you can calculate your percentage of infestation.
If you've never tried it, getting 400 bees into a quart jar really is great fun. There are several methods for accomplishing this out there. I went with shaking a few frames of bees into an upturned hive cover, and then just pouring the (totally excessive amount of) bees into the jar. Admittedly, my technique could use some refining. I used far more bees than necessary, and had bees crawling all over my arm. Still, not one sting.
Using the powdered sugar roll, we found an infestation level of 6% (6 mites per 100 bees) in Hive A, and 14% in Hive C. Published guidelines recommend 10% as a treatment threshold, so I decided I would just treat Hive C this year.
I went with using a formic acid treatment, which is an organic acid, requires just one treatment, and does not contaminate honey. Perhaps at the next Open Hive Day, we can do one more follow-up mite count to see if it was effective. See you there!