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Nasty Caterpillar 'Borers' are Worst Enemy of Iris


Q. I have written you before with much success, and am hoping you can help me this time. I have iris plants all throughout my gardens, in front and back. I thought that this spring's wet weather was to blame for the buds not opening on some, as the stalks looked brown and very wet. I have since come to discover that these plants are "infected" with what appears to be an iris borer worm. The Ortho Problem Solver book helped me identify it, but now I don't know what to do, because I refuse to use the chemicals they recommend. (McGrath here: Thank you for that decision, Gayle!) Any suggestions you can provide would be much appreciated. Thank you in advance from a broken hearted Iris-lover,
    ---Gayle in Buckingham, PA
P.S.: There is a wonderful lady near Upper Black Eddy, PA who has the most lovely (and huge) iris garden. When it blooms, she sends a notecard to those who have signed up and you can visit, pick out the irises you love, and she will graciously dig some up—for a price, of course. The flowers are perfect and she happily shares her growing knowledge. I haven't been there for a few years, but with this problem, I think I'll have to visit her next year (I believe the name is 'River Valley Iris Garden'.) Oh, and here's a photo of some of my beauties, now gone but not forgotten. Thank you for all of your help.

A.Thank YOU for your kind words and the fabulous photo of that beautiful iris variety—we'll post it at our website and with this Question of the Week.

We were able to follow your clues and verify that the name of the place is indeed River Valley Iris Farm and its owner, Helen Lewis, is still offering her treasures to the public (although this year's blooming season is over). We'll answer your question as best we can, but we should also point out that masterful caretakers like Helen are probably the best source of Iris advice, and people (like you) who have a relationship with such talented growers should try and utilize their knowledge as well.

Now, the life cycle of this pest allows a lot of opportunities to thwart it. The {quote} 'borer worm' that tunnels into these pretty flowers is a caterpillar whose eggs will be laid by a night flying moth late this August or early September. Protecting the plants with floating row covers during that stretch will physically prevent the laying of next year's eggs and seems, to me, to be the best possible response to this problem. I would also strongly advise not leaving outdoor lights on overnight; they can attract these and similar pests to their prey.

The eggs laid this fall will not hatch until next Spring, so promptly removing all the dead leaves and other iris trash in late August should dispose of any already-laid eggs and give new moths fewer places to attack. Or just lift your iris rhizomes out of the soil in early August, store them indoors and replant them after Halloween. That way, they won't be around when the moth is.

If no protective tricks are employed, the caterpillars will hatch out in the Spring and can be observed crawling on the plant for a week or so before they tunnel into the leaves. Here's another opportunity to stop them before they can damage the plants: beginning in April and continuing through May, spray the plants with the organic caterpillar killer Bt (specifically the BTK strain of Bt, available under brand names like Dipel, Thuracide and Green Step). Non-toxic to everything else, Bt (a naturally occurring soil organism) only kills caterpillars that chew on the sprayed leaves; this will stop the pests at their first bite without harming you or anything else.

Miss that opportunity and things get ugly fast. Undeterred caterpillars then enter the leaves, creating feeding channels that look a lot like leafminer damage and that, yes, make the leaves of the iris look wet. This is pretty much your last chance—prune off all the affected leaves and throw them in the trash. Your iris might not bloom that year, but it also might not die, which is a pretty fair trade.

Miss that chance, and the borers move down to the rhizome—the big fleshy root of the iris plant—and quickly grow fat as they chew into it. So if you see plants that look wet and are beginning to wilt, pull them out of the soil and look for the borer holes. If the pests haven't moved into the rhizome yet, rip off and trash all the infested leaves. If you DO see their entrance holes, shoot beneficial nematodes into the holes with a garden syringe; the microscopic predators will consume the caterpillars inside. Or stick a thin wire into the holes to kill the caterpillars.

Granted, those rhizomes may be toast by then, but at least you'll get even.