I doubt the big ones are borer. Borer will not hatch until you have 2 consecutive days of 70 degree weather. And then they are tiny almost microscopic, living in the foliage. they will not get to the soil until they grow bigger...and that takes at least a month or longer.
As for the 1/4 inch white ones in the rotted rhizome, they are definitely not borer...I see them once in a while in a rotted rhizome. I think they are in the maggot family, but they only eat decaying matter and are good guys...because they devour the dead tissue and turn it into good nutrients for the soil.
As for applying Merit, I do it in April; I only use the liquid. Don't remember the proportions; it is on the container. I think half or 1/4 cup to a gallon? Not sure and not home right now. I spray the stuff right on the rhizome,and any emerging growth and make sure it is completely covered with wet. Maybe that is about a teaspoon to a tablespoon depending on the size of the rhizome. I try to get about 1/2 inch around the rhizome sprayed too. I don't think there is harm doing it twice a year since borer do hatch in batches. But after they are somewhat developed it doesn't help much. If you want to do it twice I would try early april then early may.
Laetitia

Has anyone had success with using beneficial nematatodes? We have a Sunday morning gardening show on a local radio station called the Organic Gardeners. They like to solve gardening problems organically as often as possible. Their suggestion for problems with iris borers was using a solution of beneficial nematatodes on and around the tubers. Just wondered if anyone has tried this method.

Linda