Tuesday, July 31, 2012

Iris Borer - Bacterial Soft Rot Control

Today, I noticed the Iris Borer (Macronocutua onusta) among some Bearded Iris (Iris germanica). Yellow to brown leaves and drooping foliage showed evidence of their presence. Unfortunately, this borer is known to eat all forms of Iris (Siberian, Japanese, and Bearded), but the Bearded Iris' large rhizomes generally can tolerate their devastation. Yet, today I was proactive in trying to rid the pest for good.

Here, the front leaves were flopping forward.
Hollow centres towards the base of rhizome.

I began to look further, inside the the centre of the plant to see blades of iris leaves with this streaking and damage.  I pulled on these and they came free easily, revealing a hollow centre to each segment.

Taking off the streaked leaves you can clearly see where small larvae have made their way into the base of each blade segment. They borrow between the blades of Iris leaves, tunneling down to the rhizome.

When you see this tunneling, please remove the entire leafy section, and break off the adjoining rhizome.

Pink bodied, with a brown shiny head.
Here I cut the rhizome root in half, and you see the tunnel and pithy excrement of the larvae. 

At this time of year (late July), you will see in the rhizome, a pink wiggler of a larvae. Quite large - spanning about 1 to 1.5 inches in length. These larvae hollow out the rhizome and eat the innards causing significant damage and Bacterial Soft Rot. This then can transfer to neighbouring rhizomes, making the entire plant collapse given time.

 The rot can become quite noticable, making the foliage droop or collapse as I saw today.

Here is the larvae as I removed it from the rhizome hide-away:

Remove and squish all these larvae, to prevent them from becoming adult moths. The larvae move to the rhizomes to feed and to pupate, becoming adult brown moths.

Adult moths will lay eggs in late August to September at the base of the Iris to start the whole process over again.

Control:  The best control is a thorough tidy up at this time of year. Remove any streaking foliage, and dying leaf blades. Examine any blades and rhizomes for tunnels.

Divide your Iris every 2nd to 3rd year. This should be done in late August - September. By doing this, you take notice of the rhizomes and also gives the plant room to grow more abundantly.

In spring, when the foliage reaches about 6 inches tall, the eggs of the Iris Borer will have hatched and tiny larvae will begin to bore through the blades. You'll see pin-pricked sized holes with some streaking. Squishing tunnels or removing the damaged leaf blades will reduce their numbers significantly.

Thoroughly keep your iris free of debris in August/September. This will deter the adult moth from laying eggs at the base of the plants. In late fall, clean up all the dead and dying leafy blades from frost damage. This will help expose overwintering eggs that you may have missed. Destroy and don't compost any of the foliage.

Keep an eye on your iris throughout the growing season for signs of damage.

Hope that helps!


  1. Oh dear, exactly my situation! I have removed most of the ruined foliage but left the rhizomes alone...the heat is too much for me. I will put serious effort into my irises over the next couple of weeks! thanks for the fall headsup!

  2. Thank you, Heidi- I knewI had borers but was unable to pay attentio over the past 2 years. I'm in Eastern Pennsylvania and still have a few iris trying to grow but most rhizomes are gone. I just removed copletely the small patch I have left & found several (about 7) large white larvae. Destroyed those. I am going to replant those left in a "test patch." Thanks for the info on thelifecycle.


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