Iris borer is the most serious pest of iris in Wisconsin. Iris plants attacked by iris borer will discolor, wilt, and die during late June and July. On close examination of affected plants, a pinkish-white caterpillar can be found feeding on the inside of rhizomes. Affected rhizomes also often are rotted and have a foul smell. The rotting is caused by soft rot bacteria that colonize the iris borer feeding areas. The unpleasant odor is caused by other bacteria that subsequently colonize the decayed tissue. Bearded iris is the type of iris most severely affected by iris borer. Siberian and dwarf iris typically have few problems with this insect.
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