Patty: When I lived back East and had a borer problem ( we don't have them
here in Oregon) I found that the best way to control them with spray was the
use of Cygon sprayed at 10 - 14 day intervals beginning when the fans were
about 4 - 6 inches tall and continuing right up to bloom time. In that way
it is possible to catch the little buggers when they first start working on
the leaves from the outside and still catch any that might have gotten by
somewhere in between.
Since it is impossible to control other people's gardens it is best to
practice garden cleanliness to the Nth degree. Remove all dead iris leaves
from the garden (burn them if you are permitted but DO NOT compost them) Keep
your iris beds as weed and debris free as you can. In the fall keep the
leaves out of your garden because the borer moth will lay its eggs on any
available garden litter and not just iris leaves. It probably wouldn't hurt
to give the iris another spray in late summer or early autumn.
Regards, Dave , Molalla, Oregon