What's the best chemical treatment for iris-boring bugs?

Iris borers are worms that eat thru the rhizomes. Your best bet is Malithion. Always had good luck with it. You can see then the larvae is starting down the stock. They chew downward toward the rhizome and turn then leaf yellow. Remove that leave or use you fingers to squeeze that area to kill the larvae before it reaches the rhizome. That is where the damage is done. About every 5 years I would dig out all of my iris and soak the rhizomes in malithion in a large bucket. Cut off all dead pieces and any piece that has been chewed up and dispose of. Replant. Them back and separate them with a little distance between.

Spinosad is a safe spray that is a bio-insecticide. It should be applied when the iris growth is just 6 to 8 inches high in early spring.

You can get this at amazon.