Frank mentioned removing mites with a ‘large stainless steel colander’. The best colander is not just any colander. More about this later, but first, lets back up a little and I will explain my rationale for removing mites.
Getting rid or removing ALL mites from cocoons is difficult. I think the main aim is to remove the majority of mites, so that mason bees have a better chance of producing healthy offspring. Even if all mites are removed from harvested cocoons, there will be the occasional mite covered wild bee that arrives from within the local wood. These mites are spread successfully ensuring mites are always around. The best that anyone can do is to remove the majority of mites from harvested cocoons. This give mason bees a better chance in producing healthy offspring rather then mites.
Washing with water, removes adhering frass and the majority of loose mites. After washing, there are still lots of mites in amongst the threads of the cocoon.
These mites are best removed by friction. I have found the most successful way to remove these mites is to gently roll them over a METAL window screen stapled to a frame. Another way is to gently roll them around colander with a metal screen (NOT PLASTIC, NOT STAINLESS). Plastic and stainless steel do not have the abrasive quality of metal screen.
This can be done in two stages. First wash with the appropriate colander under and in running water. Second, when cocoons are dry, roll them over another screen to get the remaining mites off.