Northern CA - Slab on grade with dowels but no key. Is there any merit in attempting to seal the joint between slab and wall? Expansive clay soil may open a slight crack at base of wall. I'm afraid ants and other insects may find the crack and squeeze through. I'm thinking of running a bead of silicone, expanding foam, or what would you use ? down center of concrete wall footprint before stacking block and pouring concrete. Here we often use tar or caulking between slab and sill plate on a stud wall. Any thoughts or recommendations are appreciated.

What's the dowel spacing?
Isn't the whole idea that the concrete grips the dowels tightly and the dowels would have to deform substantially in a short area for a "crack" to open up?

Your best bet is to make sure the interior of the forms is free of foam and other debris before the pour.

I did several projects while living in NorCal, about 1/2 had a keyway as that is what the engineer wanted, but in reality, that is old school thinking, people seem to think it acts as a lock for the bottom of the wall, which the dowels actually do, so it is not necessary, it was also though to act as a waterstop, it doesn't, if water wants in...it will find its way in. All that being said, I never had an ant problem coming through walls, it was always through the doors and windows, and I remember, once the rainy season hit, you have trails of ants.

To answer your question, if the wall has not been built and poured yet, you can run a quick cut saw 1/2" deep all around your foundation and purchase plexiglass strips, 3" +/- wide and insert into cut, hold in place with a bead of caulking, this will do both what you want and act as a waterstop as well. If you wall is poured already you could attempt to remove the bottom 1" +/- of ICF on the inside and caulk the concrete seem to your slab.

Personally I wouldn't waste my time on either, make sure the door and window flashing pans are properly caulked and sealed, along with the doors and windows themselves, but regardless you will always have an ant issue.

I've used aquaflash from dryvit several times. Helps for wind driven rain etc. You can always paint it to match the concrete if needed. Most of the time we use np1 at the seem first.

Thanks for the responses. Dowels are #5 on 18". The saw cut and plexiglas would do the trick but I was hoping for something easier. Does the dryvit aquaflash go half on the face of the foam and half on the concrete foundation? Applied after wall pour and before siding installation? Think I might start with a tall bead of sealant in center of wall and if that doesn't work I could still do the aquaflash later.