Among the methods for monitoring varroa infestation into beehive, the use of carbon dioxide has also been suggested. This non-lethal method consists in anaesthetising honey bees by CO2 in a jar and counting the mites dislodged from the insects. But according a recent study carried out in the US, the efficacy of this method is low, resulting in inaccurate estimates of the infestation level. The researchers concluded that the most accurate methods remain washing bees with soapy water or alcohol. Instead for non-lethal methods, according an other study, the best one is the powdered sugar shaking.

Go to the scientific paper on carbon dioxide method here, and on powdered sugar here.