I've used any kind of silicone filler in those gaps before and the bees are still alive. Today I bought a tub of plasti-bond which the neighbour recommended. I'd not heard of it before but apparently it has multiple uses, and bonds to a variety of surfaces. I specifically wanted it to repair a bottom board the bees chewed out, but I might start using it in other little gaps and defects in the hives. I don't know if it's toxic, but if the bees start making rock hard honey I'll know they've been licking it.