wasps? I read about Sevin dust but am unsure if that would work as a
repellent. Any suggestions would be much appreciated.

Before you spray anything, go out about sunset and watch carefully to see
how they're getting in to the attic. Once you've spotted the hole(s), you
can treat those with an appropriate insecticidal dust (Sevin used to be
labeled for this, don't know what is currently -- ask Extension in your
state.) Puff a little dust into the hole, attempting to coat the sides
of the hole. Let things stay like that for about a week (the adults
will carry the insecticide back to the nest), then go ahead and
remove the nests and caulk or screen the entrance holes.

Kay