I have always used Bayer 2 in 1 Rose and Flower on my irises in early spring. Last year they changed the formula from imidacloprid to acephate which does not curtail the borers. You have to check the label to make sure. If you are very observant, you can spot the telltale signs of the borer travel and smash them in between the fans before they get to the rhizome. Fertilome has a tree and shrub drench that is effective in killing the borers but is also systemic. I still have lots of bees on my other plants so do not think the irises are their primary source of pollen or if so, they are still buzzing around every year.
Some folks use some sort of nematodes but I am not as familiar with that process. Borers can devastate an iris bed if allowed to go unchecked.
