Iris Borers - Knowledgebase Question

Newport News, VA
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Question by tmparker
August 11, 1998
I have several iris beds that are not doing well, upon digging one up (to thin & clean) I find I have borers. Of a bed 3'x4' I only had 10 roots worth replanting. I am wondering if I can use Ortho' Rose Pride on iris the same as my roses?

Answer from NGA
August 11, 1998
To control the borers, it's important to know a little bit about their life cycle. The larvae feed on the iris rhizomes in mid to late summer, then leave the rhizome and live in the soil for the pupa stage. The adult moth will emerge in a few weeks and lay eggs on the iris. The eggs overwinter on the plants and hatch in April or May. Then they start feeding on the leaves, working their way back to the rhizomes again.

At this point, lifting the rhizomes and cleaning them is probably going to stop the current year's damage. Be sure to remove and/or destroy any borers you find. (Some gardeners pierce them with a piece of wire such as an unfolded paperclip.) You might also sift the top few inches of soil and remove any that have already moved to that stage.

Since bacterial rot may infect damaged rhizomes you should also cut off any damaged parts and dip the healthy pieces in a bleach and water solution prior to replanting. (Use 1 part of household chlorine bleach
and 10 parts of water.)

The next most important thing to do is to remove and destroy all the iris debris during the fall garden clean up and again in the spring. By doing this you will remove most if not all of the eggs before they can hatch and will eliminate many of next year's generation of borers.

While some chemical controls are available, they are applied in the spring while the larvae are in the leaves. Any spring larvae can also be controlled by simply squeezing the leaf between your fingers where you see feeding injury occurrring -- watch for this in May and June.

Good luck with your iris!

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