Yeah, you're mostly right. Sometimes we just try to fix the problem at hand without even looking at the basic issue. We don't know what's down there that needs to be killed, whether there are any openings that could just start the problem again, etc. Scattering boric acid at first sounded smart to me, but what if you have to later crawl around in the space? I don't know how that would affect people and I don't know how long it would be effective as a pesticide. So i don't know if that's a safe option.
Could just do Diatomaceous Earth over the ground and any area that you crawl through or possibly contact which you can buy food grade and actually eat it, so 100% non toxic.
Then just use the Boric Acid along the walls and floor joist.
Even though Boric Acid is poisonous it isn't REALLY poisonous to humans, as in just getting a small amount on your skin should be no problem, just make sure you do not inhale a large amount or get in your eye.
It IS rather poisonous for a dog though, so the dog could never go into the crawlspace.
http://npic.orst.edu/factsheets/borictech.html#toxbox
http://npic.orst.edu/factsheets/degen.html#cancer
These are effective with any sort of bug that has an exoskeleton.
Probably your best bet to control any sort of bug and mice problem in a crawlspace is to encapsulate it, once you get rid of all/most moisture and the fiberglas insulation in the floorboards, cover the ground with thick plastic then all you need is just a small amount of Boric acid UNDER the plastic and the polyiso foam insulation on the walls and you should be as critter free as a crawlspace will ever be. Just getting rid of the high moisture/humidity gets rids of about 85% of your bug problem alone.
You also get the added benefit of no mold or mildew, smells, no place for mice to harbor, less humidity in the house, and up to a 18% savings in energy cost, plus the crawlspace then becomes a storage space.
I know ours went from a total nightmare that was literally covered with spiders, camel crickets, silverfish, Ants, and a few of those large waterbug type roaches.
When I first went under there I swear the ground appeared to be crawling there were so many spiders and camel crickets, the insulation was wringing wet and hanging down, there were actually 3 little rivers that flowed through.
In the summer the entire space was dripping wet, so much so that standing in the center it sounded as though it was RAINING.
It was hot as hell, so humid it was unbearable and looked like a freaking horror movie.
Today I could sleep under there comfortably and store cardboard boxes with zero ill affects, all I see now bug wise is a spider once in awhile but I don't know what they are living on.
Also no more mice because of getting rid of the fiberglas insulation.
I have noticed about a 12 to 17% decrease in electric bills in the Summer, ($20-$50 savings each month) kinda hard to judge exactly because the power company keeps raising the rates and it seems the hotter it gets the more I save. So early summer late spring, early fall maybe save 8-12% and mid summer more like 13-18%
Not sure if I save much in the winter, I would say maybe 4-8% maybe, BUT it is now in the 60s in the crawlspace in the winter when its in the single digits outside and it used to get well below freezing under there.
Our hot water heater and all our pipes are way better off, plus our downstairs hardwood floors are warmer instead of like ice.
Our gas bill has never gone over $100 anyway and usually about the highest in like February is only around $90 so monetary savings in the winter is minimal like maybe $4-$8 a month at most, but the electric bill savings in the summer and rest of year really makes up for it.
Its just nice having a really large conditioned and bone dry and almost bug free space now even if it didn't save a dime it would be worth it.
The roughly $200+ savings each year is just the cherry on top that will 100% pay for itself in about 7 years.