Problem Identifier Unit Screen//v.w/ RR Template

Problem Identifier Unit Screen//v.w/ RR Template Iris Borer

Problem
Dark streaks, water-soaked spots, and possibly slits develop in new leaves in the spring to early summer. The leaf edges may be chewed and ragged. By midsummer, the foliage is wilting and discolored. The leaf bases are loose and rotted. The rhizomes (elongated underground stems) are often filled with holes and may be soft and rotted. Pink caterpillars, from 1 to 2 inches long, are feeding inside the rhizomes.

Analysis
(Macronoctua onusta)

The larva of this night-flying moth is the most destructive insect pest of iris. In the fall, the adult moth lays eggs in old leaf and flower stalks. The eggs hatch in late April or early May. The emerging larvae initially feed on the leaf surface, producing ragged leaf edges and watery feeding scars. They then bore into the inner leaf tissue, and gradually mine their way down into the rhizome, on which they feed throughout the summer. The damaged rhizome is very susceptible to bacterial soft rot. The larvae leave the rhizome, pupate in the soil, and emerge as adult moths in the fall.

Solution
Clean up and destroy plant debris in the fall or by April to eliminate overwintering borer eggs. Apply an insecticide containing lindane weekly from the time first growth starts until the beginning of June. In May and June, squeeze the leaves in the vicinity of feeding damage to kill feeding borers inside. To kill the borers in lightly infested rhizomes, poke a wire into borer holes. Destroy heavily infested plants and rhizomes.

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