> For me, seeing a hive go from having numerous easily visible phoretic mites, to having none visible, back to having mites visible when the first brood emerges is convincing evidence that a majority of mites invade the first cycle of brood in a spring or summer split.

I'm thinking that the assumption here is that we can normally see a representative sampling of the total number of phoretic mites, just by looking.

If that were true, we would not go to the bother of doing ether rolls or alcohol washes when we need to count mites.  We would just look at a frame of bees and count the visible mites on bees covering a given area, multiply by a constant, and be done.

It is obviously not that simple, though.  Phoretic mites are very hard to spot and it is even harder to make a meaningful estimate from what we can see..

I am guessing that visible mites are most likely when a mite has just emerged with a new young bee or when mites have detected eligible cells for breeding  and are searching for a cell.  That is a tranitional state.  Normally, phoretic varroa are much less visible.

Whar we actually are able to see easily is typically just mite traffic -- the mites in transit between the phoretic state and cells.  If there is no brood, then there is little traffic.

Most phoretic mites are hard to see because they are not riding on bees in postions where where they are exposed and obvious.   

The mites we do see are those which have not yet found a spot where they can hide between the plates or are on their way to a cell and are typically riding totally exposed on top of the head, the thorax, or the abdomen of the bee.   Such positions make a mite far too vulnerable to a fall or grooming and is just a necessary intermediate state beteen the two safer conditions.

When we immerse 300 bees in alcohol, taken from a frame where we see no obvious phoretic mites on visual inspection, we often find 40 or more mites in the alcohol after shaking.

So, once again, perhaps what we think we see is deceptive.

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