re: "if you remove one set of strips and replace with another set of strips within the same allowed treatment period, isn't this still the same one treatment?"
 
I do not think you can make that assumption.  While I am not familiar with the exact mechanism used by Hopguard, most of these products follow an exponential decay model of dispersion of the active ingredient.  That is, the dosage is strongest when you first unseal the strip (lagged slightly for the time to accumulate and spread around the brood box volume), but then the strength gets weaker over time.  At some point, the exposure is below the effective level and you pull the strips out.  By pulling the strip early and replacing it with a new strip, you will reset the decay curve which means that you are applying a dosage above what the manufacturer intended.  Worse, you will do it in an environment that has already accumulated the chemical in the surrounding volume so your dosage may now be considerably above.
 
I've attempted two ASCII-graphs below to illustrate the difference.  The first shows a hypothetical pattern for single-strips, the second  shows how the pattern might bump up again after replacement.  Even though the total time of exposure is the same, the area under the curve is very different.  (Apologies in advance if the graphs don't present well in email.)

 
|     x   x
|x                x
|                       x
|                              x  
|                                       x
|                                                     x
|
-----------------------------------------------------------
                                x   x
|     x   x               x               x
|x                x    x                          x
|                       x
| 
|
|
|
-----------------------------------------------------------
 
I do not know whether that increase would be significant or not.  It may harm your bees or it may be completely safe.  What I can say is that is definitely will not be the same total dose of active ingredient as leaving the original strips in for the full treatment period.

Mike Rossander

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