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Monday, June 25, 2012

Is Iris Borer in Southern Nevada?

Hole in iris leaf

Q. While cleaning my iris beds this morning, I found quite a surprise.  I hope you can help identify my problem before I have more damage. I put 100% white vinegar in this hole and it foamed up immediately. I hope I didn't kill my Iris - the hole is right in the middle of the plant.  Further checking revealed that I have these holes in several of my iris beds spread over 1/2 acre.

            I also found what looks like a "perfect" 1/4" round, 1 1/2" excretion, it is almost black.

I've checked on the internet and there is nothing that describes a 1" hole with mounded dirt; the round holes are perfect in the ground and on the leaf.

            Some suggest slugs - I have no type of residue and I think they would eat as they go NOT make a round circle in the middle of a leaf. Then cutter bees were mentioned but their holes don't seem to be as big as these. I will soon be leaving town and I would like to get a handle on this problem before I go. Thanks for any help you may give.

A. I hesitated to answer for a bit because I was not quite sure what is going on. My first reaction was iris borer except that this iris pest is not usually a problem here but rather in the Midwest.

Hole in the ground near the iris
            But it fits the description. They will leave roundish holes in the leaves and they also pupate in the ground so when the moth emerges they leave this fairly large, smooth hole in the ground near the iris.

            But you mention no other symptoms of iris borer which also makes me doubt it. Other symptoms would include, in the early stages, the tips of the foliage turning yellow and then brown. As the problem gets worse the base of the stalk may become yellowish-brown and mushy with a bad odor if the attack is severe. This is when rot of the rhizome has started.

            If you are digging irises to divide or move them this fall, you may notice holes in rhizomes as well. If you got any of these rhizomes from the Midwest then it is possible they may have been transported here.

            I agree that it is also possible that the hole in the leaf could be from slugs although, as you pointed out, you are not using any surface mulch which they like. The hole in the ground could be from an insect like the cicada killer, a large wasp that is yellow with black stripes, not very threatening to humans but can deliver a severe sting if provoked. Normally cicada killers will not bother you but they do have a very large hole in the ground that fits this description and the picture you sent.

            I will post both of your pictures on my blog. But first I would check to make sure it is not iris borer when you dig up some iris this fall for inspection. If you see some iris with foliage that is yellowing or dying back I would dig and look here first.

            Look for holes in the rhizome or even possibly mushy rhizomes. I would discard these and hopefully you will see no other problems. If you do not conclude it is iris borer then I would assume it is a cicada killer and not be concerned about it. Watch for slugs and put out some slug bait or stale beer for them.

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