I test for three days every week. Or in the winter, continuously, as I leave the board in as a draft preventer. I have no problem finding and counting the mites even after 30 days of built-up hive debris (mostly sugar falling off the bricks).
I use kitchen oil (olive, corn, etc.) It's seems to keep any mites from avoiding being counted and it washes off readily before the next round.
When counting I use a wet, smushed-end toothpick to lift each mite and set it aside in a clean corner of the board. When I can't find any more mites, then I go back to the collection spot and group them in 5's and discard any stray bits of non-mite debris I accidentally picked up. Counting the mites without removing them when you see them, or trying to count a partial area of the board makes it tough to get an accurate count. Lifting each mite and setting it aside means your search pattern can be relaxed and you don't have to worry about double counting. I've never had to count more than 100 mites in a test (and that was a 30-day period in the middle of the winter when I couldn't get in my hives at all), and seldom more than 40-50, even for heavy-mite periods, so it's not too tedious.
In the summer some mites won't be dark brown or black, you will see lighter ones.
I use a magnifying glass and a flashlight when searching.
I wish I had a microscope, or at least a hand lens, strong enough to see if the bees have done any damage to the mites. (You go, girls!)
Take a large paper clip and unbend it enough to expose a point. Stick it through the board about 3/8ths of an inch in from the end about halfway along the short end. Work it around until you have a little tab made of the paperclip sticking out, rebend as needed. This will make extracting the board easy even with gloved fingers, or in the dark.
Set your SBB up so the slot is open at the back of the hive so you can insert and remove the boards without being in the flight paths or exciting the guard bees outfront. That way you can do the testing whenever it's convenient for your schedule, even during the evening.
Be sure to use the wooden entrance barrier, or make up a screened one so bees and hive moths can't get in to access the sticky surface. Don't want any mites hitchhiking away, nor do you want any robber bees to get that far under the SBB. Makes everybody cranky when they are banging on the floor trying to get in, even if they can't!
If you see a sudden high spike or complete absence (when you are testing weekly), immediately replace the board and do several consecutive 24 hrs counts to confirm the numbers. I'm not entirely sure what the out-of-bounds numbers mean, but they may be anomalies due to changes in temperatures, bees reproduction rates, etc. And the spikes are why infrequent testing is not very useful. What you are looking for is a read on the changing numbers. An for that regular stickies are quite useful.
And write down your counts some where. You won't remember them if you are doing them often enough to be useful.
The thresholds are some what regional, and definitely seasonal. I use the NY and Ontario ones, but that won't work for you in TX.
Don't confuse the numbers in the averaged 24 hr drop with the often-quoted percentages derived from roll testing methods. Those are completely different scales.
Also there's valuable information on the state of affairs in your hives to be gleaned from looking at the pattern and type of debris on the board before you disturb it by rooting around to do the counts. From the patterns of the debris, you will learn to see when the cells are being opened, or capped, what pollen is being brought in and where your brood cluster is. I am still studying my boards to extract all I can from them.
Also, I try to run my tests during a period when I am not messing around in the hives so as not to skew the numbers.
Enj.
