People like to say that varroa mites on honeybees are a lot like ticks on a dog, but when you compare the relative sizes, you'll see that a varroa mite is more like a blood-sucking squirrel latched onto your dog's back.  Given the size of the mites, it's not surprising that a heavy varroa mite infestation can weaken a hive so much that it dies over the winter.

Most beekeepers treat their hives with insecticidal strips during the fall and winter, but the chemical control method has obvious problems.  You have to be extremely careful not to eat any of the honey that was in the hive during the treatment period, which makes life difficult the next spring if the bees didn't consume all of their winter stores.  Beekeepers who throw in chemicals every year without testing to see whether their hives need it also start to run up against pesticide-resistant mites --- bad news.  Finally, the organic gardener in me has to wonder what such a heavy dose of insecticide does to the honeybees.  Luckily, there are alternatives.

We use quite a bit of passive management designed to reduce varroa mite populations in the hive.  Foundationless frames and screened bottom boards both help cut down on varroa mite infestations, and the latter also allows us to monitor how many varroa mites are actually present so that we don't put chemicals in a hive that isn't very heavily infested.  With winter looming, I figured I'd better check the mite levels in our three hives.


Homemade varoa mite test sheets

You can buy varroa mite test sheets ("sticky boards") from bee supply stores, but I'm too cheap so I've experimented until I figured out an easy way to make the sheets at home.  Just cut a piece of cardboard to 13" by 20", tape down white scrap paper on one side, and smear on petroleum jelly (vaseline) until it covers the entire surface of the paper.  If you've got more than one hive, it's best to label your various test sheets before bringing them outside in order to avoid confusion.  Slip one test sheet under the screened bottom board of each hive, then remove it three days later and take a look.

Chances are, your test sheet will be coated in debris, so you'll need to look carefully to see the round, dark brown varroa mites.  If you're industrious, you can count every mite on the sheet, but I generally just rule off three strips, each one inch wide, and count the mites in each one.  Since the screened section of the bottom board is ten inches wide, adding up the number of mites in my three strips, dividing by 3, then multiplying by 10 gives a rough estimate of total varroa mite fall during the three day period.  My three day mite counts came to 57 and 40 in my two smaller hives, and a whopping 540 in my biggest hive.


Varroa mite threshold

The hardest part of checking on varroa mites is figuring out how many mites you can have in your hive without worrying.  A quick search of the internet and my bookshelf yields up numbers ranging from 50 mites per day to 200 mites per day as the treatment threshold.  For a three day mite count like mine, that means I can have somewhere between 150 and 600 mites on my test sheets without taking action.

The reason the threshold figures vary so much is that you'll get widely variable mite fall numbers from the same hive when you test during different parts of the year even if the percentage of bees infested by mites stays the same.  Since the typical hive has few bees in it during early spring, few mites will fall to the ground.  The same hive in the middle of summer may have ten times as many bees present (or more), so you'd expect to see ten times as many mites.  With that information in mind, it's not all that surprising that the hive we bulked up with early double deeps has many more varroa mites than the hives which began the year with a single brood box.

A North Carolina beekeeping document suggests a way to deal with this inherent problem in the sticky board test method.  They tell you to estimate how many adult bees are present in the hive by counting how many frames are completely coated on both sides with bees during your inspection.  A medium frame thus coated will hold about 1,250 bees and a deep frame will hold about 2,000 bees.  If your sticky board count shows more than 2 mites per thousand bees per day in mid-August or more than 4 mites per thousand bees per day in September, you should find a way to reduce the mite population.  Unfortunately, I hadn't read this the last time I opened the hive, so I don't have any data available except my gut reaction that one of my hives has many more bees than the others.

Clearly, I don't need to worry about two of my hives at all since they averaged 13 and 19 mites fallen per day.  My biggest hive, though, has ten times as many mites even though I estimate it only has perhaps two or three times as many bees in the hive.  I could treat that hive, but I had a colony that was similarly on the edge last fall and it made it through the winter with flying colors, so I'm going to take my chances.  As I turn into a more experienced beekeeper (and have more data from my own hives), I'll feel more confident about which varroa mite levels are no big deal and which ones require drastic action.

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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.



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