> I don't see late season immigration as an issue, and still question the math. 

Other people see this as a very big problem. In fact, it was documented years ago:

Honey bee colonies should be treated against varroosis in late summer to guarantee the production of healthy winter bees. However, the invasion of Varroa mites from non-treated colonies may wreck the treatment effect. To quantify the invasion rates we used a former military training area which is not accessible to other beekeepers. Four heavily Varroa infested honey bee colonies served as "donator" colonies. In distances of 1, 30, 400, 1300 and 1500 m from the donator colonies we placed two mite-free monitoring colonies each which were continuously treated with different acaricides. The "mite free status" was confirmed by the analysis of bee and brood samples. 

From August-October 2006 the number of Varroa mites on the bottom boards of the monitoring colonies were counted at weekly intervals to record the Varroa invasion rate. The average Varroa infestation rates of the adult bees in the "donator colonies" increased from 18% in August till 38% in October. Two colonies died at the beginning of October due to the high Varroa infestation. In total, 2029 mites were recorded in the bottom boards of the 10 monitoring colonies during the 8-week period. 

The average invasion rates per colony were 283 at 1 m distances from the "donator colonies", 201 at 30 m, 155 at 400 m, 122 at 1300 m and 255 at 1500 m. Under normal beekeeping conditions the density of bee colonies and, therefore, the invasion pressure would be considerably higher. Our data confirm that highly infested colonies represent a risk for treated colonies over distances up to 1.5 km. For beekeeping practices, the treatment of all colonies at the same time during the late summer period is an urgent need for treatment concepts. 

Invasion of Varroa mites into mite-free colonies at a military training area. E. Frey, P. Rosenkranz (Apicultural State Institute, University of Hohenheim, 70593 Stuttgart, Germany) 

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