Why do we want to measure this in dry weight? Water is also a resource, one that takes a good amount of work to supply to a beehive.
Is this more of a situation where...
- We believe that differences in water allocation are significant to the question, but we also believe that all bees receive allocations proportional to their dry weight;
- We believe that differences in water allocation are not significant to the question, because there is effectively unlimited water available and every bee can have as much as they want without affecting any other bees; or
- We believe that differences in water allocation are significant to the question, but we're measuring something else because we don't know how to measure the water allocation?
So my guess is that including it would increase variance and error without offering any benefit.