help iris borer

kmudpuppyJune 24, 2006

i have 10,000 iris in my garden, the leaves starting turning brown and drying up. i took some to the university and they said that it was from the late hard frost that we had. they suggested that i just cut back the brown spots as i was doing that this week i found this web site and the real culprit of my problem the iris borer. can any one help with suggestions on what to do. everything i have read on here says dig them up and do the bleach treatment. i don't know that i can do all of them that fast to save them. does anyone have any suggestions on what i can do to halt the borers and kill them as i work through the beds and do the bleach thing? does anyone know everything else that i have read also says to move the beds is there any kind of treatment so i don't have to move all of these beds? any suggestions or ideas would be appreciated.

thanks kim

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njiris(z 5 NJ)

Hi Kim:
With so many iris I think it would be dificult if not impossible to dig, bleach, replant 10,000 iris. This is probably only practical with a small garden.
Have you actually seen the borers? How many of your iris are infested? Are you just assuming because they have brown tips they have borer?
I never do the dig, bleach, replant, method unless it is a single iris which is heavily infested and it is obvious the borer is in the rhizome not the leaf. I have about 1,000 plants at this point, and it is impossible to have a borer free environment, but I am very vigilant with borer control, and I keep borer to a minimum.
In the early spring I do a heavy cleanup of dead leaves. Then by early April I spray each and every rhizome with Merit (not the granules). This will limit but not eliminate borer.
At this time of year (June-July) I search all iris for sign of borer, which is: chewed, weepy leaves; particularly accompanied by a vertical chew mark across a leaf close to the fan base. When finding this I carefully open the leaf and dispense with the borer, which may be quite small, maybe 1/2 inch or larger.
If it is obvious the borer has eaten to the rhizome, I try to pluck it out with an eyebrow tweezer. If I cannot get him, I will try to dump bleach water into the hole in the rhizome, and if I'm lucky the bugger will come out to see what is going on. Only as a last resort will I pull the rhizome and treat with the bleach method above.
If you follow that method you will probably do better than to try to dig and replant all the iris.
Good luck, and try the Merit next spring.

    Bookmark   June 24, 2006 at 12:14PM
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Dig up all the irises rinse them and soak the completely,then keep the plants completely under water with some dish detergent (sometimes I also add vinegar for aphids) for 24 hours.
Then replant in an open field with no shade . The moths that spread the eggs of the borer like some shade .
DO NOT REPLANT A SINGLE TINY ROOT WITHOUT DROWNING IT COMPLETELY SUBMERSED FOR 24 HOURS . The iris borers camophlage extreamly well in the roots .
After you have rinsed the old plants and replanted them you can pour the dish detergent water on the old bed you will see a lot of bores you did not find .They can wipe you out in 3 weeks by the way and a borer can grow as large as a man's finger. So get busy !

    Bookmark   June 24, 2006 at 1:56PM
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thanks for the advice yes they are in almost every plant. i was treating with fungicide and herbicide, that is what the university told me to do they said there were no signs of bugs and the brown spots must be from a late frost or fungus or something so when i saw more turning brown i figured it was the samething i called them back and they said to cut the fan down to stop the spread if it was from the frost that is when i saw these nasty fat (very fat)things that had to have been munching for weeks. so again at this point with a bad infestation would you dig and replant or would you do what you suggested earlier just pour bleach down them?

thanks kim

    Bookmark   June 25, 2006 at 12:35AM
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njiris(z 5 NJ)

The objective is to get the borers OUT!. One way or another. They are now immune to pesticide. If you poured bleach water (not straight bleach)on them, you would still have to make sure you removed every borer from each clump.Bleach water doesn't even kill the borer, just kind of stuns him. I have seen a borer swim around a bowl filled with bleach water for a few hours! Sometimes it is not possible to remove the borers while the clump is still in the groumd. If there is one hole and you can pour the bleach ontop of it, you can watch to see if the borer comes out. If he doesn't you have to go in after him. If there is substantial rhizome rot, multi holes, multi borers per clump, it may be that you must remove the whole clump, soak, cut away dead stuff and replace. Logistically I just don't know if it is physically possible for you to remove 10,000 iris plants and do the drastic treatment, that is why I suggested the conservative treatment.
You have to decide for yourself if you can dig, cut, soak, treat soil and replant all your iris. It may be that you do triage: determine the worst infected areas, dig those and do the conservative treatment on the others.
You would then get a good number, but not all.
You could then consider in the fall (after borer lay their eggs) to do a burn. I know this sounds scary, but people swear by it: In October/November cut down all foliage remaining. Take a blowtorch to the iris beds and scortch and burn all remaining dead foliage and debris. This will kill the eggs and you will have few borer next year, and the iris don't seem to mind being toasted.

    Bookmark   June 25, 2006 at 12:28PM
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myohhmy(z5 MICH)

I got the darn buggers this year too. I felt really devistated by it until I read your post. I have 100's of irises but thank god not 1000! Wow your yard must be incredible in bloom!!!!!!!!

I just got one bed dug up today. I will do all my reblooming iris with this drastic method but boy I don't have time or engergy to do them all.

Do the darn bugs travel from bed to bed? Or are some years just heavily infested? My mom's bed even has them this year but she hardly has any iris (although I gave her what she has).

Also I heard in the spring you MUST put the pesticide down only during a certain time to prevent the eggs from hatching. Is that true?

Also in digging up the iris I have seen lots of white dots is that the eggs for next year? If so will things get worse next year?

Once you get the borror are you prone to keep getting them? if so yikes I will be switching over to daylillies. this is so fustrating!

thanks for your help...

    Bookmark   June 26, 2006 at 12:19AM
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njiris(z 5 NJ)

To answer your questions: Yes, borer seem to be able to go from bed to bed. Mostly because the egg laying moths fly from bed to bed. Do borers themselves travel? Looking at the buggers, it is heard to believe that they do, but yes, I have seen them show up in a newly planted bed....I have seen pupae in the ground 8 feet away from the nearest iris. But also, just like any other pest, there are good years and bad years.
You MUST put pesticide down in the Spring at an early enough time for the plant to ingest it before the buggers get too big and are immune. They are only vulnerable when very very young. Pesticide (Merit) does not stop hatching, but it will kill them once they start munching. They start at a near microscopic size, so theoretically they will die before you even see any damage. I put it down in April. Early April or late March is best.
The 'white dots' you see are not borer eggs. They are probably aphid eggs. The borer eggs are very difficult to see, and are cemented at the base of the fan. If you were lucky enough to see them, they would not be there until after the borer goes thru its cycle: worm turns into Pupa; pupa turns into moth; moth flies around, mates then lays eggs. That doesn't happen until Sept-Oct in my region.
I think it is impossible to eradicate borer completely, and even if you did, the iris are still vulnerable from other borer who might be in your neighborhood. Just like almost any other insect pest, you cannot eradicate them all.
The best we can hope for is control, not elimination. With my 1,000 iris, 2 years ago with poor control(ignorance) there were about 150 borer; last year after good control, I found only about 1/2 dozen borers and I consider that excellent. This year I have already found about 2 dozen, and I attribute it to one of the following:
1) I used the half bottle of Merit that was leftover from 2005 - might it have been weaker than year oldl Merit?
2) I also could not apply till very late April
3) I did not get every single bed - I left the newly planted beds alone - and of course, they turned up there!
So all and all, I would never give up iris because of the borer. But it is a constant battle of vigilance if you want to grow alot of iris. If you do no borer control, and realize you have a problem when the iris are looking sick the borer is now big problem. My iris never get to this point, because I am constantly looking for the early signs of them.

    Bookmark   June 26, 2006 at 9:53AM
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northerner_on(Z5A ONCanada)

This is my first visit to this forum and I came searching for specific advice on Iris borer. I have noted the advice in this thread. Over the past 10 years, I have had 2 occurrences, and each time I dug up, used the bleach treatment and planted in another location. About two weeks ago, I noticed evidence of borer, and this time, I did find, and heartily squished the little buggers. In some cases, I removed the leaf, in others I removed part of the tuber. I thought I was home free but I saw more evidence today, including some evidence of tuber rot, and it seems to be spreading to my little patch of Iris pallida that I BOUGHT and that bloomed for the first time this spring!! My clump is only about 25 plants( I had such a lovely and long show this spring), and I was planning to dig them again tomorrow, but I do not have another location to put them. They are in full sun, and I feel they need to be to prevent the infection. (1) AM I RIGHT ABOUT THIS?
This is really maddening. I have a small clump in my backyard that is quite shaded - so shaded that last year the daylilies among which they are planted had rust (also a wet summer here)and they are perfect. Please help, I can't imagine my garden without them.

    Bookmark   July 2, 2006 at 3:39AM
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Neither Merit nor any other chemical or soap treatment will eliminate borers at this time of year because they are too large and resistent. In order for Merit to be effective, it must be applied early in the spring so the borer larvae are newly hatched, tiny, and still vulnerable to pesticide.

In order to manage borers effectively, you need to understand their life cycle. Borer eggs are laid on iris foliage in the fall and overwinter there. In the spring, egg hatching is triggered when temps reach 70 degrees. The hatchling borer larva bores into a green iris leaf, where it will start to feed and continue to feed as it works its way down the interior of the leaf to the rhizome over a matter of weeks, getting larger and more resistent to pesticides as it feeds.

Eventually it reaches the rhizome where it continues to feed until mid-summer. If it runs out of food in the original rhizome, it may migrate to another nearby rhizome through the soil and continue to feed. When it is ready to pupate, it leaves the rhizome and creates a small, dark brown capsule near the iris and within the top 4" of soil in which to pupate for the rest of the summer.

In late summer-early fall, the adult borer moth emerges from the pupal capsule and the soil and lays eggs on iris foliage to begin the cycle all over again. Because the adult form of the iris borer is a moth, they can easily fly from one garden to the next to lay eggs wherever they please. If you have neighbors with irises that are untreated for borers, chances are that you will have borer populations every year in your own garden. The link below will show you pics of the various stages of borer development.

The only way to interrupt the cycle at this time of year is to dig the irises out of the ground, wash and do a bleach solution soak of the rhizomes, and inspect them for signs of borer activity. If you find holes in the rhizomes, either cut the borer out of the hole with a sharp knife or poke around in the hole with something pointed to try to kill the larva inside the rz.

If the larvae have already left the rzs to pupate in the surrounding soil, the damage to your irises is already done for this year, so now you just have to treat for secondary bacterial infections in your plants. Don't worry about the pupae in your soil. You can take care of them this fall and again early next spring with a thorough garden clean up to remove the old iris foliage with next year's borer eggs. Early next spring, apply an appropriate pesticide like Merit before temps reach 70 degrees so the plants can uptake the chemical and be toxic to the hatchling borers as soon as they begin to feed.

Don't give up. I know this all seems very discouraging right now, but borers really can be managed with relative ease through the timely application of Merit early in the spring. For now, just do what you have to in order to treat your infection and hope that enough of your rhizomes survive to be able to rebuild their clump.


Here is a link that might be useful: Iris Borer Pictorial

    Bookmark   July 2, 2006 at 8:45AM
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Does anyone know the range of the borer? I have never seen a borer in NE Mississippi. Is that because the soil gets too warm for them in the summer?

    Bookmark   July 2, 2006 at 2:59PM
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Iris borers are primarily a midwestern and eastern pest. I'm not sure how far south or west they range, but their presence is limited or non-existent in those areas.


    Bookmark   July 2, 2006 at 3:16PM
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Thanks Laurie, I sure hope they never discover Mississippi. Some of the people that post on this board really have a lot of trouble from those critters. One reason I found it so easy to get addicted to iris was the fact that they are so easy to grow in NE Mississippi. I am sure if the borers ever make it here they will like it and stay. It seems everybody else does.

    Bookmark   July 2, 2006 at 8:51PM
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northerner_on(Z5A ONCanada)

God bless you, laurief, your detailed explanation of the life-cycle of the critter was just what I needed! I will save it in my Gardening Stuff file for future reference. Now I know what I have to do and will proceed with the will break my heart but now that I am informed, I will probably never have to go there again. Thank you so much.

    Bookmark   July 4, 2006 at 4:30AM
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northerner_on(Z5A ONCanada)

I don't know if anyone is still following this thread, but since I realize I have to treat my Irises with Merit next spring I decided to investigate this product. It is listed at $98.00 for 2 oz.!! Is this the right thing or is this a version for commercial use? It is a Bayer product according to the article I read. Is there another 'version' for home use, or is this what everyoe else is using? This is a little steep for me if I have to do it every year. Does this product have to be diluted and how long is it viable? njiris mentioned something about having used 'left-over' Merit and wondering if it was ineffective or under-effective. Please let me know.

    Bookmark   July 5, 2006 at 2:10AM
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Merit comes in both liquid and granular form. Laetitia uses the liquid. I use the granular. I can't remember how much the granular cost last time I purchased it (I purchased enough the first year to last me 3-4 yrs, so it's been a while since I've bought any), but it wasn't terribly expensive. I use it at a rate of 1 tsp per iris clump applied once in early spring just as irises are breaking dormancy and beginning active growth. I believe the name of the product I use is Bayer Advanced Lawn Season-Long Grub Control. I do remember having to order it from my feedstore because I couldn't find it at any of the local nurseries or garden centers.

The two most important aspects of application of granular Merit are the timing (as stated above) and making sure you immediately water it in very well. If you don't water it in, it'll just sit on the surface of the soil and never be taken up by the irises' roots.


    Bookmark   July 5, 2006 at 7:25AM
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I too recently found the iris borer in my iris bed. My beds looked so poor, I began digging them up and found it. Intutively, I trimmed the foilage and soaked in a tub of bleach water. I thought that the 20 minute soak I gave them last night may not be enough so my current batch is soaking overnight. I guess that may be a mistake :(

Thanks for the detailed explanation. I wondered what those brown capsules that I found in my soil earlier this spring were. I only wish I would have read this earlier.

I have hundreds of iris. But only 3 cultivators. They keep multiplying!! Only one type, my favorite purple ones, are affected. I'll be digging & soaking for a few days.

Thanks again for sharing the information.


    Bookmark   July 6, 2006 at 12:16AM
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northerner_on(Z5A ONCanada)

Thank you, Laurie. I have located the granular product at a more reasonable price - $23.00 for 14 lbs. Will have to try to find it here in Canada. It seems to also control grubs and with our recent infestation it should be readily available.

    Bookmark   July 6, 2006 at 2:08AM
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Northerner, if you do manage to find it in Canada, please let me know. I have a Canadian friend and irisarian who has been looking for Merit for years and does not believe it is available or approved for use in Canada.

For the record, Merit has been doing a wonderful job of controlling aphids on my irises, as well.


    Bookmark   July 6, 2006 at 8:24AM
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bobharem(5 mich)

WOW wealth of info on this. I too have iris for years and no problems, until now. I thought they were called larve, but corrected to borers and yes there all mine. I dug them up and wash them out and look for hole or mush spots. I have a coffee stick stur from vender work great to pop the suckers out of iris. Now I learn to soak them in water bleach over night.

Q Should I cut the fan down and replant them now or in fall?

Q I am planting the iris in same spot. Should I treat the soil now before planting and with what? Spring, yes with Merits.

pics show the worm in the iris too. Is this common?

Thank for the help

Here is a link that might be useful:

    Bookmark   July 13, 2006 at 12:39PM
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you should: drown the plants completely under water for 24 hours with some dish detergent in it . (2) replant in a different ,(very sunny)location in your yard . dump the dish water in the old bed but not if you used bleach or borax.

    Bookmark   July 17, 2006 at 12:41PM
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Just updating this old thread because I used the info. here to deal with an infestation of borers in my plants, and wanted to add that if you are going to do the soaking treatment, definitely soak 24 hours as stated above - don't cheat! I soaked about 30 rhizomes in a 10% bleach solution for 12 hours. After a short time in the water, two fat borers came out of two plants. over the course of a couple of hours, a couple more came out, then no more. I knew there were borers in the other plants, because I could see their holes. I left them soaking over night and figured after 12 hours, any that were still in there were surely drowned. So, I took them out and laid them out to dry. The next morning I came out to check on them, and there were two borers poking their ugly heads out, still alive. They didn't seem well, but they were most certainly not dead. So, a word of warning - 12 hours in bleach is not enough to kill these persistent buggers!

    Bookmark   July 27, 2013 at 4:18PM
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Thank you to everyone who provided helpful information. I was cleaning out my iris bed today, wondering why some had died. I found my first borer and googled to find out what it was. After reading the posts on here, I dug out all the iris (luckily I only have about 30 or 40) and am soaking them in dish water right now. I used a small margarine tub to keep every borer I found so they wouldn't get away. Turns out they HATE sunlight and by the time I was done many were dead and turning black.

I soaked all, even those seemingly unaffected, to help control the infestation. Over the years I had seen rotten rhizomes and just thought it was bad planting or too much water. I had no idea about borers so all the information I found on here was great. I'll be sure to mark my calendar to put on Bayer Season Long Grub Control in the spring as well. After reading all the posts and re-examining my iris stalks, I now know what to look for. THANK YOU!!!

    Bookmark   August 3, 2014 at 3:02PM
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Is anyone really concerned about using a systemic pesticide like Merit, considering how bad it is for bees? I really want to deal with the borers, but not at the expense of bees. What else can be done to keep these things from the plants?

    Bookmark   September 3, 2014 at 10:36PM
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njiris(z 5 NJ)

Haven't been here for a long time, but found this old thread where I had posted. In response to the last poster, yes there IS an alternative to Merit. Its called nematodes! You need to purchase the nematodes specifically bred to go after borer. You may need to get them from a mail order place such as BioLogic, but you must specify you want the kind that kills borer .Iits the steinenema carpocapsae variety. They are not cheap. I paid 79.00 for my 1000 iris. You mix the powdery semi dry nematodes with water and spray thoroughly. Directions are included. It works and does not poison the soil.But you must follow the directions as the nematodes are live organisms

    Bookmark   September 10, 2014 at 12:38PM
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That sounds like a far more advantageous proposition without the drawbacks of using systemics anywhere in the environment if we don't want to be harming bees. I found that ARBICO Organics has very reasonable prices for the nematodes. Now I just need to do more study on the lifecycle to find the very best time to apply for going after the iris borers. Of course the nematodes are also good for cutworms and Japanese beetles, so there really are no drawbacks. The ideal would be getting enough going in your soil so they keep these things away permanently, but I'm not sure that's entirely possible. I wonder how many years you could go once soil is thoroughly treated(say 2 or 3 treatments over 6 months), before there are no longer enough nematodes left in the soil to give you protection. So my two questions - when is the best time to treat to affect the iris borer lifecycle, and how many years before needing full retreatment?

    Bookmark   September 11, 2014 at 2:57PM
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