Nematode use is very tricky in the North. that is because they will die if you have a late cold spell, and yet you have to apply them early enough in the season to get established.
A bad catch-22 for iris growers in the North!
Presby used them 1-2 years , and there were still tons of borer.
You are better off using the liquid Merit. Its supposedly safer than Cygon and other chemicals.
Laetitia

Thanks for the info. Where do you purchase liquid Merit? How often do you apply? I have some irises near my vegetable garden. Is there anything safe to use there or should I move those?

I got my Merit in a Nursery/Garden Supply Center. Its about a quart bottle, concentrated, you mix with water and spray right on and around the rhizome.
I don't remember the exact name, when I get home tonight I will check. One bottle lasts a very long time. I still have half left over from last year. I applied only once in April.
Another form is the granules, Merit Grub control for lawns, which you sprinkle on, but I don't have very much luck with it. I don't have a vegetable garden, so can't comment on how safe it is , but if you don't spray the veggies, I think that its oK. Merit is used extensively as a lawn grub control product, and with children and pets constantly walking on the grass, I don't think they would sell such a product if it were harmful.
Check the bottle just to make sure, they should list any warnings, before you buy.