Those borers sound like a nightmare. This article helped me understand why it's so hard to be rid of them.

Here is a link that might be useful: borers' life cycle

I'm so sorry you've had this problem. I feel very fortunate that I've never had iris borers (knock on wood!). I do try to clean up my iris beds in late fall & early spring, but as many iris as I have, I would have a problem. I hope your iris recover quickly.

I just replanted, actually in larger beds. It is just that the plans are smaller and some won't bloom next year. I even gave away some to friends.

I think I could have saved more rhizomes had I dug them in spring or early summer. But I would not have new beds for the roots.

Do you folks know if the Iris borers are the same buggers as the pumpkin vine borers? I also lost most of my kabocha pumpkins due to the vine borers. Regular haloween pumpkins did ok even with the borer damage.

Not the same critter!

Both are gross fat buggers.

I only saw one iris borer after I dug. Most of them already escaped.

I killed a lot of the pumpkin vine borers. Helped a little, but not enough.

I figure the same chemical, Merit, can be used on the vine borers too?

If you use Merit for squash vine borers, realize that it will be in the squash you want to eat as well. It is a systemmic pesticide that gets into the plant tissue - that is why it is so harmful to bees. Most of the systemmics have warnings for you not to apply it around food crops, but it seems just as harmful to apply it around flowers, since they are visited by things we want in the yard. I grow daylillies in areas close to where I grow iris, and it is not unusual for me to pick daylilly blossoms to eat in the yard or add to a salad. If I use a systemmic to eliminate iris borers, there is a possibility it could get into my daylillies. I also have some fruit trees in areas where I have some iris growing - I'm not really interested in getting that stuff into my fruit trees - something that could be devastating to bees and to me. Like I said, the nematodes work just as well or better, and it seems at a similar price point, so why choose something so harmful. And please do not use Merit on things you want to eat. Check out this list of facts about Merit:

http://www.flora.org/healthyottawa/merit-pesticide-insecticide-grub.htm

where can I buy these nematodes, drmbear? I also have problems with borers and rose midges.

Just search "Beneficial nematodes iris borer" and look for some good sources. Some of the articles about how this works provide sources as well. I'm buying along about April.

Where are you managing to get affordable nematodes? I've looked into them for years, but by the time they get here, the price is going to be well over $50. The killer is shipping, since they have to be sent overnight. And it certainly doesn't help that all the sources are on the West Coast.

Arbico Organics seems to have reasonable prices - the shipping for Garden size to VA is a little better than $16.

http://www.arbico-organics.com/

SIZES & COVERAGE RATES
1/2 Garden Size: $16.00
5 million Nematodes -
Treats approx. 1,600 sq. ft.
Garden Size: $29.95
10 million Nematodes -
Treats approx. 3,200 sq. ft.
Farm Size: $68.75
50 million Nematodes -
Treats approx. 1 acre
Small Ranch Size: $198.00
250 million Nematodes -
Treats approx. 5 acres
Large Ranch Size: $379.00
500 million Nematodes -
Treats approx. 10 acres
Grow Organics seems a little higher:

http://www.groworganic.com/

They are also for sale on Amazon and eBay.

I also found:

http://thebeneficialinsectco.com/

http://www.gardensalive.com/ has reasonable prices and shipping at about $10.

http://www.gardeners.com/ has reasonable prices, though they seem to be out right about now (okay since I don't want any until April).

Close to when I am ready to buy in April, I will build a simple spreadsheet to compare prices, quantities, shipping, etc., making sure I am getting the correct recommended types, Steinernema carpocapsae and Heterorhabditis bacteriophora, and find the right deal. I had been growing iris for almost 30 years without a single borer problem until the last year or so, so I am thinking with some good management and cleanup/trimming in the fall, and a treatment at the critical time with nematodes I should be okay for a few years, particularly since I dig and divide about every three years - another opportunity to treat.