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.)

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.