Boric acid on the soil will not do a thing for spider mites, as they do not use the soil surface in any life stage.
In fact I would be very wary of using it near seedlings at all.
If you use alcohol in a spray, don't spray to the point of runoff.
Why don't you just use a product made for spider mites, like neem oil? It's much safer for the plants than alcohol spray, which can dissolve the leaf's protective waxy cuticle.
stinkyattic Reviewed by stinkyattic on . Hi there! Haven't been around in a while! I've revamped my stealth grow cabinet (the new cab is larger & taller, which means less bending and topping the door's lopsided and doesn't close properly though grr...I'll have to figure something out when I initiate flowering), I found me a Natural Light 200 watt replacement CFL (rated at 3000 Lumens/4100K) and some KC-36 seeds. So I'm doing a test run, the seedling is just barely poking its head from the soil (and I've just noticed critters in there, they Rating: 5