I have never really paid much attention to the bottoms of the BB's. I would guess that it may be similar to what I find at my screened inner covers and that would be beetles hiding out above the wire around the edges on the wood frame. The bees typically keep the beetles pushed up in that area and I'm just guessing that they will do the same on the other end of the hive.
When I pop a top on a hive that has them, most of them of course scurry to get out of the light. Some dive off in the hive while others seem to avoid entering the hive at all costs. They are quickly disposed of with the ole hive tool, all the while those that dove off into the hive start coming back up with the bees hot on their tails so that they can meet mr. hive tool themselves.
I don't have many italian bees left that resemble a true italian much anymore. Not for any reason other than how I have been breeding the bees. Most are now predominately carnie to a more even mix of the two. Some russian and russian mixes in a couple of yards. I can only say what I have observed over time with the beetles. And that is that I have consistently saw more beetles in the italian hives than in the carnie/russian hives including the mixes. But at best, I see varying degrees of rejection of the beetles across any given yard. Some hives typically have little to no beetles on one end while some have plenty. I would say that the model would fit well into the typical bell curve.
Knock on wood, and this year may prove different but I still have yet to loose a hive to these devils. Including all three races of bees.
I have noticed on a couple of yards where the bees set under a canopy of trees that there is a pretty good number of skinks that hang out under the top covers on the screens. And I see them scurry out from under the hives if I'm moving things around. I see very few beetles in those hives and I can't help but wonder if they are waiting for free meals to be chased up out of the hives? Maybe they are dining on an occasional bee? I figure if they are, they can't eat to many and everybody has to eat somewhere. Seems like an awful strange coincidence that they are where the beetles aren't though.
I know the traps you folks are talking about. Don showed them to me some years ago. I tried them out during one season and didn't see any affect, some weren't even touched. But from what I've seen in my hives I don't really see that the beetles spend much time inside the hives to get to them. I guess if you keep bees differently than I do, probably most do, and you have a ready presence of beetles inside your hives, they would probably work well.
If nothing else it helps get rid of those stinking political signs all over the road sides!!
"Success is not final, failure is not fatal: it is the courage to continue that counts." Winston Churchill