Some beekeepers have found that MAQS strips have negative effects on their bees, especially during warm weather.
Here is some useful advice from Jim Vivian-Griffiths of Dean Forest Beekeepers:
The instructions for use of MAQS strips are rather misleading, as they were written for the Canadian/US market, in which the majority of beekeepers use Langstroth hives with solid floors. The advice is to remove the entrance block to provide more ventilation.
With our colonies on mesh floors you should do the following:
1. leave your entrance block in, reduced if necessary to guard against wasps
2. put your insert board in only half way, to give plenty of ventilation
3 open the feed holes also to give ventilation
4 use only one strip of MAQS if your colony is not much stronger than a nucleus
There was also a very useful article by Wally Shaw in the Autumn 2013 issue of the WBKA Newsletter on this subject. You can read it here: Varroa_Control_by_Wally-Shaw_Autumn2013