Easier Mite Testing Means Healthier Bees
A Mite Load Calculator for Beekeepers Who Use Oxalic Acid Vaporization
Testing for varroa mites requires an invasive hive inspection; puts the queen in jeopardy of being drowned in alcohol, beaten to death in a sugar shaker, or crushed during hive manipulation; and takes a lot of time if you have more than a couple colonies.
Consequently, a lot of beekeepers just don't test for mites. That's why over the last few years I've been collecting data to find a correlation between a hive's level of infestation and the 24-hour mite drop that results from an oxalic acid vaporization treatment. It turns out the correlation is usually pretty accurate, or has been for me.
I invite you to use the calculator below to estimate the number of mites in your colonies. Is it perfect? Maybe not, but it has proven to be pretty darn close, and is much better than ignoring your mite loads altogether.
How it Works
STEP 1: Insert a clean sticky board under your bottom screen.
STEP 2: Perform an oxalic acid vaporization treatment. Note the time of day.
STEP 3: Remove the sticky board about 24 hours later.
STEP 4: Count the mites that dropped since your treatment.
STEP 5: Fill out the calculator below to see your estimated mite load.
Mite Load Calculator
Phoretic vs. True Mite Load
Phoretic mites are those living on adult bees, outside the capped brood cells. So the Phoretic Mite Load is similar to what you'd get from a sugar roll or alcohol wash.
Varroa destructor do their breeding inside capped brood cells, so a significant percentage of their total population can be missed by common mite tests. This is especially true during swarm season (Apr-May) when there's a lot of drone brood and the phoretic population is still depressed from the previous winter. The True Mite Load attempts to account for those mites that are ignored by the most common testing methods. The data available for these hidden mites is sketchy and difficult to apply across hives, so take this figure with a grain of salt.
(For the record, “phoretic” is not the correct term to apply to varroa mites, because it implies the mites are simply hitching a ride without doing harm to the bees. The more accurate term is “ectoparasitic,” which applies to parasites that are on the outside of their host. I’m making an effort to replace “phoretic” with “ectoparasitic” in my personal lexicon, but I understand a vast majority of beekeepers only recognize “phoretic” in relation to mites.)
Estimating Your Hive's Population
This chart gives a basic idea of how a colony's population fluctuates throughout the year in areas with cold winters. The actual population of your hive, and hives in your area, may vary. So adjust these numbers as needed based on your own observations.
Knowing When to Re-Treat Your Hive
There are various mite thresholds that are considered acceptable in the beekeeping community. Anywhere from 1% to 4% are often cited, depending on the source. As a rule, I treat until phoretic mite levels are below 2%. If mite levels are dangerously high (let's say over 10%) and there's still brood in the hive, consider treatment methods that are persistent or that penetrate the cell walls to improve the odds of the colony's survival. Oxalic acid vaporization may not be the best choice in those cases due to its inability to kill capped mites.
Traditional Mite Tests
It's still a good idea to perform a traditional mite test (sugar roll or alcohol wash) once or twice per year. This can serve as a sanity check against the calculation you get here, and it's also nice to practice those techniques occasionally so you don't forget how they're done.
For a deep-dive into varroa mites, check out the Honey Bee Health Coalition's webpage. Download their Varroa Management Guide for a primer on the efficacy of various treatment methods.