I only test if I see any signs of it. When you're inspecting hives make sure to look for the signs. If you ever do get a positive result then yes, I would test every hive - considering the seriousness of AFB.When testing for American foul brood is it necessary to test every hive? I tested 1 hive, out of 11 that had negative results.
I had to learn that lesson when I moved from Utah, the second driest state, to coastal CA. Moisture is a huge issue for bees and I've experienced brood disease like symptoms due to the excess moisture helping mod and/or anything else take over the hive.Out of my 11 hives I only had 3 survive the winter. I did treat with formic pro July 30 last year. The brood that I found was kind of snotty looking or rotted. After looking at each hive though I believe they were killed by excess moisture. I left for Arizona in November of last year and when I returned last week I started checking. What I believe I had done was put a sheet of wax paper down on top of the frames to put sugar on. The ventilation holes were above the paper. Dam I never though of the moisture barrier I had created. I did have the hives tipped. All the hives were really wet. I was sick after I realize what I had done. Anyway before I realize what I had done I did a test for AFB. That was an expensive lesson not to mention all the work I created for myself.