Iris Borer

Iris Borer Control

Iris Borer, Macronoctua onusta

Iris borers are the most damaging pest of irises. They will attack all varieties of irises. The adult moth has dusky brown front wings and lighter yellow-brown rear wings with a wingspan of up to 2 inches. The moths emerge August and September. The females lay eggs in the fall in dead, brown leaves. The larvae appear in April or May. They are pinkish in color and about 2 inches long.

The tiny larvae make pinprick sized holes into new leaves, tunneling into the leaf to feed. As the larvae grow, they feed behind leaf sheaths and in stems and flower buds, gradually moving downward into the rhizomes. They typically reach the rhizomes by midsummer. The feeding causes streaks on leaves that appear tan or water-soaked. The larvae devour the rhizomes; just one iris borer can eat several rhizomes in a summer.

The iris borer can also cause bacterial soft rot which makes infected rhizomes slimy, soft, and foul-smelling.

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