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The Varroa Mite Treatment Decision

April 9, 2011

There is no single answer as to when to take arms against Varroa and subject the mites and the bees to an onslaught of chemicals.  But in our area, now is the time that decision has to be made.  Temperatures are warming to the point where Formic acid and Thymol miticides are becoming effective, and it’s nearly time to place honey supers on the hives, so we have to treat now or not until the fall after the honey comes off.

For a number of years after mites came into the beekeeping world, like many others, I used Apistan strips to control for mites.  I never liked doing it, and when word came out that the mites were becoming resistant to Apistan, I was ready to try other treatments, or perhaps no treatment at all.  The hottest thing to try at the time was a screened bottom board so that the mites could naturally fall though and out of the hive.  I made a couple of these screened boards in 2002, and put them on two of my colonies after honey harvest that year.  The screens are set up with a sheet of corrugated plastic as  a drop board.  (recycled political campaign signs work for this)  The nice thing about the drop boards is that they provide a very easy way to check on the colonies without opening them.  Besides mite levels, you can quickly ascertain the size of the cluster by the size of the fresh debris field, and spot problems like wax moth predation from the nature of the debris.

So in 2002, excited about this new tool, I regularly monitor the mite drop rate in the two colonies I made screened boards for.  Here is what I found that year:

Of the two colonies, Hive 1 was the larger.  There were a few things I learned from this exercise.  First, counting mites is tedious!  Most interesting was the decrease in the drop rate in February.  I now attribute this to the beginning of major brood production.  Mites that were restricted to just living on bees in the overwintering colony could now move into cozy brood cells capped away and protected.  Lo and behold, especially in Hive 2, a few weeks later when the brood started hatching, out came the mites and the drop rate soared.

After the last data point on the graph, I treated Hive 2 with Apistan (the last time I’ve used hard chemicals). My notes indicate that the mite drop was about 3500 mites in 10 hours, and I kept seeing high drops for another couple of weeks as the hatching brood released more mites.

In the exponential race that is spring build up, a strong colony with a fertile queen can outgrow the parasites.  I think that’s what happened with Hive 1. I never treated Hive 1 that year, and it went on to produce a good crop of honey, but the mites never went away.

I don’t count mites any more, but I still use the drop boards as a primary monitor.  I’m reluctant to treat unless I really need to.  The soft chemicals are harder to use – take longer to kill mites, and don’t kill them as throughly as the hard chemicals did.

The ultimate solution is no treatment at all, and let natural selection do its job.  Commercial beekeepers who have taken the leap to go off the chemicals cold turkey lose most of their colonies.  But some survive to become the progenitors of a resistant strain.  Most beekeepers – including me – are unwilling to accept the almost certain loss of their bees if they don’t treat.  For now, this ensures that there are plenty of non-resistant bees out there. Most of our bees rely on the kindness of strangers to put chemicals in their hives on occasion.

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