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Iris Borer

Susceptible Plants
Bearded iris (Iris germanica)

Description & Symptoms
Translucent streaks appear in young leaves as borers tunnel through leaf tissue. By summer, affected leaves turn yellow and wilt. Borers spread a bacterium that infects the fleshy iris rhizome, causing a soft, spongy rot with a foul odor.

Timing & Life Cycle
Iris borers are the larval stage of an adult moth. The grayish moth emerges in August and September and begins laying eggs on old iris leaves or other debris. The eggs overwinter on the debris and hatch in April and early May when new leaves are 5 to 6 inches high. The larvae enter the leaves a few inches above the ground and eat irregular tunnels in leaf tissue for 10 days to two weeks. They then bore into the rhizome and and continue to feed until mid-July. In late July to early August, they exit into the soil. There, they pupate and soon emerge as moths, ready to lay their eggs for the next year.

The soft rot spread by the larvae is the most serious injury caused by borers. Untreated, borers can heavily infest iris plantings, destroying many plants.

Treatment & Solutions
Sanitation is critical to successful control of borers. Cleaning up and destroying all plant debris in the fall will get rid of eggs laid during the summer. Leaving the rhizomes partially exposed when planting can help kill bacteria on the surface by exposing them to sun. In the spring, inspect young leaves for streaking and pinch the streaked area between your fingers to crush the larvae. Rotting rhizomes should be destroyed. An insecticide treatment of young iris leaves in the spring may be warranted. 

For more information about iris borers, call the Plant Information hotline at (847) 835-0972.