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Dear Frank and Jean:

What you have is the dreaded Iris Borer!  Neither mothballs nor garlic will 
have any affect on them.  At this time the best thing to do is to dig 
infested parts of the iris clumps; pull or dig out the borers that you can 
find; wash rhizomes with a strong spray from the hose to clean out the mess 
left by the borers; cut away any mushy parts and soak in a bleach solution (1 
part bleach to 9 parts water) for about 20 minutes.  Trim back about 2/3 of 
the leaves.  Let the rhizomes dry in filtered shade for a day or two before 
planting again in fresh soil.  If you find shiny brown pupae in the soil 
under the clumps you dug, destroy them before they hatch into moths.

In the spring is when you take action to prevent the borer infestation.  
Early in the spring before active growth begins, clean up and discard all 
dead iris foliage and foliage from other nearby plants because this is where 
the Iris Borer Moth lays its eggs. There are several things that can be used 
to kill the borers emerging from the eggs.  One is spraying with a Lindane 
solution every week or so from the end of March to early June.  This is a 
contact spray that only kills the borers that it touches.  Systemic sprays 
such as Cygon 2E are more effective, but still have to be reapplied about 
three times during April and May.  Some new products being tried with some 
success are Merit and Marathon which are also systemics.  We tried a granular 
form of Marathon that you spread on the soil around the iris plants and water 
it in.  We only used it once at the beginning of the growing season and still 
got some borers.  We have heard that it should be applied a second time about 
6 to 8 weeks after the first application for the best performance.  Another 
thing that some people are using is beneficial nematodes.

If you can find your way to the Iris-talk archives, you will find many 
messages about this problem.

Dorothy Willott in Northern Ohio, Zones 5/6

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