>> At least one study found that fallen mites tended to die soon

> I agree with this. This is probably the explanation as to why screened bottoms are not worth much, too. Mites that let go and fall off the cluster are no doubt old and infirm. A healthy mite hangs on tight, and isn't likely to cascade all the way down through the cluster without grabbing on to anything. To suppose that healthy mites can't crawl a meter or two, is also implausible. 

This argument is true as far as it goes and in the main.  

What we are considering here, though, is borderline and special cases. I was hoping not to have to write an essay, but maybe I do.  

These things are not black and white and may also not be universal, so the most interesting and potentially useful ideas may well be revealed by examining the details, especially where reports conflict.

As I said before, a small influence can add up over time, especially when conditions are unfavourable for the mite, such as after winter when mites are older and when the area around the cluster is cool or frozen -- or when the3 mite drops too far.  

Several small influences can combine to cause stronger effects.

Mites do have to shift positions sometimes and when they do, their grip is less tenacious.  Bees have been known to chew mites, however the why, when and how are a bit obscure.   Bees are known to eject mites from cells.  Again, this is not a constant and easily explainable universal activity.  

There are other factors which may well cause mites to fall out of the cluster, including grooming, chemicals, stimuli, changes in the condition of the host bee, etc...

Screened bottom boards have been found to be effective in some situations and not in others.  Location and management methods seem to be factors in whether they are useful from what I recall.

Rather than declaring one study wrong and the other right, would it not be more helpful to say they could both be right and then try to understand how different conditions may be responsible fr the discrepancy?

Seems to me that is how we will discover the multiple ways in which mites are naturally vulnerable so that we can exploit these weaknesses to our advantage.

             ***********************************************
The BEE-L mailing list is powered by L-Soft's renowned
LISTSERV(R) list management software.  For more information, go to:
http://www.lsoft.com/LISTSERV-powered.html

Guidelines for posting to BEE-L can be found at:
http://honeybeeworld.com/bee-l/guidelines.htm