The DE is a great option but I have heard it isn't effective when the soil is wet, also. I live in southern Louisiana and we have 80-90% humidity about 9 months of the year. So, I treated the soil directly below the hive with permethrin and covered it over with mulch this winter. (this is an approved method here, I don't have the directions handy, but I could look it up). I would never have considered this but our hive had tracheal mites in October and after we treated them the population was so small that the SHB were wreaking havoc on the weakened hive. I also put in a bottom board with a SHB trap:
Those two things seem to have solved our SHB problems for now. But, I am trying a different solution. As you can imagine, when you believe you are about to lose your only hive, you do a lot of research. I think one of the long term solutions that seems to be inert is to use beneficial nematodes that you put into the soil. The nematodes eat the SHB larvae and that stops the life cycle of the SHB. However, in what I have read thus far I see no means of establishing a living colony of these nematodes. So, you'd have to buy them every year. The other problem may be what they do to the existing ecosystem. Maybe they also kill a beneficial insect larvae...It seems dangerous.
So, what appears to be the best solution is Jeff Willard's Neverwet strips around the bottom board. I posted some of his videos here:
He has more videos and seems to have done his homework. He tried to get universities to look at it, but only got one response... And it doesn't seem to be well received here either...
Good luck, I hate all of the foreign pests that have killed our hives and I hope this helps.