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Michigan Gardener | June 2020 | MichiganGardener.com
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An iris borer larva after having engorged itself on your precious iris. It’s time for action.
Carefully discard soggy, hollowed-out iris rhizomes like this one. Once one pest has intruded, the plant is weakened and avenues are open for other pathogens.
These iris borer pupae will emerge as moths in late summer.
How to deal with an infestation of iris borers
pate in the soil and emerge as moths in late summer, only to return to lay eggs in the fall.
While transplanting, clean and inspect for iris borer—bearded iris’s greatest enemy. Iris borers are very common and can devastate a beautiful patch of irises during their larval feeding frenzy as they mature to adult moths. An iris borer infestation can be identified in May or June by long brown streaks running down the length of iris foliage. In July or August they can be confirmed by visual identification of the larvae or pupae itself (see photos). The brown streaks are a sign of an early stage in the borer’s life cycle. After hatching from eggs on leaf litter in spring, young larvae bore into the foliage, eating their way down to the fleshy rhizomes. There, they continue to devour the rhizomes’ insides, turning them into mush. In summer, they pu-
If you have noticed iris borer damage in your garden, consider transplanting bearded irises at least every two years so that you can regularly comb through your plants and discard damaged rhizomes. Also, you can even find and squash iris borer larvae and pupae underfoot. Only replant firm, healthy new growth rhizomes. Upon fall cleanup, create a sterile, clean environment around your irises. Cut down iris foliage to the ground and rake up all garden debris and leaves. You will likely be raking away iris borer eggs with your debris, so discard it at an off-site composting facility or burn it. Pesticides are available and require precise timing. A non-
Action steps
chemical, biological control option is predatory nematodes (microscopic worms). These can be purchased at specialty garden stores and are applied to the garden in fall or spring, depending on the variety. Ruth Atzinger studied horticulture at MSU and has gardened professionally since 2001, including work at Frederik Meijer Gardens and Gaylord Opryland. After studying theology and exploring the art and gardens of Italy, Neil Atzinger learned gardening as Ruth’s understudy. As Atzinger Gardens (renamed from Fourth & Walnut), the pair create and tend gardens around Ann Arbor and Plymouth. Visit AtzingerGardens.com or call 734-272-7321 for monthly maintenance or new garden design and installation.