I attended a talk a couple of years or so ago on grubs in lawns, and it was said that, at least in some areas, milky spore does not work as well as they used to think it did for Japanese beetle larvae and that beneficial nematodes are more effective. The talk recommended Heterorhabditis bacteriophora nematodes.
The downside is that Japanese beetles are good fliers so even if you treat your own lawn for the grubs, they can still come in from other gardens unless your neighbours all do the same.
I'm not sure that the same nematodes that works for Japanese beetles would also work for iris borer without looking that up, did you confirm it? The Iris borer is a moth as opposed to a beetle so different insect order.