It's probably too late in the season for Merit to do any good now. Merit is a systemic insecticide. It is taken up by the plant roots or absorbed through the foliage (depending on whether you use granules or spray) and then poisons the borer larvae as they feed on the iris. It has no action in the soil and is not a repellent.

From everything I have read, Merit and other systemic insecticides are only effective against borer larvae when they are newly hatched and/or still very small. Once the larvae have been feeding a while and gained size, the systemics are no longer lethal to them. I expect by now the larvae that hatched this spring are already too large to be killed by Merit.

Laurie