I also find Michel's data of interest, although I must admit that I'm
skeptical of 100% of the mites being present under the cappings, especially
in July.

Keep in mind varroa behavior.  The mites emerge from cells when the bee
emerges.  In a normal colony, there would be no cells of 8-day larvae   (the
age at which foundress mites invade worker cells) next to emerging brood.
Very few mites just walk around on the combs (as can be readily
observed)--the vast majority emerge riding on the emerging bee, or would
crawl onto a cleaner bee.  The newly-emerged mites have a much stronger
attraction for the odor of nurse bees than for newly-emerged bees, so
typically rapidly shift rides.  And then they must wait until that nurse
happens to stick her head into the cell of an 8-day larva for the mite to
hop off and invade that cell.  The amount of time that it takes between
emergence, ride swapping, and then waiting for the right pheromonal signal
to hop off determines the phoretic (dispersal) period (relative to the
fixed 12-day reproductive period in the cell).

For a nice series of pie charts showing the proportion of mites that are in
dispersal phase relative to total mites, see Rinderer's
https://www.ars.usda.gov/ARSUserFiles/60500500/PDFFiles/301-400/386-Rinderer--Resistance%20to%20the%20Parasitic.pdf

Another excellent data set from which the dispersal phase duration can be
derived is in Harbo and Harris's excellent study:
https://www.apidologie.org/articles/apido/pdf/1999/02/Apidologie_0044-8435_1999_30_2-3_ART0008.pdf

I based my varroa model upon the work of Boot, who determined that the rate
of invasion correlated with the ratio of adult bees to 8-day-old larvae.
Feel free to download "Randy's Varroa Model" and look at column AV, which
indicates the predicted percentage of mites in the brood at any time.  I've
checked the predicted results against a number of data sets from other
researchers, and although there is variation, the model is usually right in
the ballpark.  One data set was from many thousands of cells in a number of
hives inspected by Dr Katie Lee for her doctoral dissertation, during a
time period that overlapped that of Michel's.


-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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