Always been somewhat skeptical of the role this may play in mite  
growth.  Certainly there are experiences and experiments where there  
is no other explanation.  Have not played with the numbers much, but  
it seems like the cumulative mite load from immigration (drifters or  
robbers?) is small compared to that produced by exponential growth  
through reproduction of resident mites.

This may be one place in which highly resistant colonies (those with  
net negative mite population growth) have an advantage over chemical  
controls.  A control application drops mite levels to very low, and  
then the numbers start growing again, with immigration adding to the  
growth.

Active defensive mechanisms against mites in theory are there all the  
time, so immigrant mites are immediately at risk.  One of the  
simplified methods of gauging Varroa Sensivite Hygiene (VSH) in  
colonies is to introduce a comb with recently sealed brood that is  
moderately to highly infested (measured right before introduction of  
the frame).  Then after a period that allows bees to attack infested  
brood at the time when they seem to be able to detect infestation, the  
resulting infestation of the comb is evaluated and compared with the  
infestation when it was introduced.  Very good colonies routinely  
decrease the infestation by 70 - 90 %.

If this happens in every round of reproduction, simple math explains  
how these colonies can drop resident mite levels to negligible, and  
can even deal with small, and possibly large numbers of immigrating  
mites.

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