You were probably just lucky not to have encountered borers in your irises before now. We're definitely in borer country here in MN. You may have received them in rhizomes from other folks in borer country, or they may have simply flown over from your neighbor's iris patch. There's really no way to know where they came from, but there are effective ways to handle them now that they're in your iris bed.

Late this fall and/or early next spring before temps reach 70 degrees (borer egg hatching temp), do a very thorough clean up of your iris bed. Remove all dead, dying, and diseased iris foliage, weeds, leaves, and other plant debris from your garden and burn or trash it. Do NOT compost. That'll get rid of most of the borer eggs before they can hatch next spring.

Early next spring just as your irises are breaking dormancy and beginning active growth, apply Merit granules around your irises (marketed as Bayer Advanced Lawn Season Long Grub Control). If applied early and watered in well, Merit is very effective at controlling borers. If you wait too long into the season, though, neither Merit nor any other systemic insecticide will be able to kill borer larvae. They need to be killed soon after hatching before they get large.

They are certainly bothersome and destructive, but borers aren't invincible.

Good luck,

Laurie

I, too, have had borer infestation in NJ. I tried Merit granules (the Bayer product) alone, and found it helped, but I was still devastated by borer. This year I followed the advice of an article in the AIS journal last Fall (Bayer and the Borer) and applied both the granules and the liquid Merit (sprayed each of 500+ rhizomes in early Spring) and that did the trick. Liquid AND granules. Together in April...just like the article said.
You could try the granules alone and see how it works. Its easy to scatter granules. Apparently it works for Laurie, but if it doesn't (or if you are either extra cautious, have a strong back or are a glutton for punishment) try the double dose approach.
I'll be doing it next Spring too.
Laetitia

THANK YOU BOTH!!!!!
I guess I've been lucky and just coasting on my good fortune at being iris borer free all these years and have not been as diligent as I should have been in cleaning up/out old, dead leaves, etc.
I will definitely be using the Merit (liquid & granules) this coming spring... I normally avoid pesticides but this is WAR!!!
I have had some of these irises (well, technically the offspring) for well over 10 years now... These are my hardy standbyes that never fail with only a minimum of care/upkeep (dividing every three years, etc.).
I was just soooooooooooooo upset and angry about these nasty borers....

Yeah, aren't they just arrogant little ....$$##!!!'s? Like they just have no respect for the rhizome...after eating their fill like the little pigs they are, they 'mine' out little balls of rhizome and spit it out just to get you annoyed.
Laetitia

ha! i often wondered what that was, njiris :)
they are fat little things! last week, i glady took a piece of hard echinacea branch and bored the borers i found. i don't think i could stand to squish them b/c they are so juicy looking. made me wish i had a chicken. i don't like hurting things that aren't sucking on me, but it was war.

One thing I've always been told is that if you are digging up irises and find borers, do not replant in the same spot for a couple of years. If you replant in the same spot, you will probably have borers in those plants the next year. No matter how much cleanup you do, it's really hard to get all the eggs - some will have ended up in the soil.

Please some body .... I have granules and liquid merit, want to tray them for borer, don't know how much to use ??? The label with instructions so complicated.
HEED HELP...