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Past Articles Library | Organic Pest Control | Iris Borer


Macronoctua onusta

Iris borer moths lay their eggs on the foliage of irises in late summer and early fall so that is the best time to take preventative measures. Otherwise, iris borers will tunnel into leaves and rhizomes, damaging foliage and flower buds. Damaged rhizomes are prone to diseases.



Adults are called "miller moths" with a 2 inch (5 cm) wingspan. They have dark brown forewings and yellowish hind wings. Young larvae are greenish, later becoming pink with brown heads. They are up to 1-1/2 to 2 inches (4 to 5 cm) long, with a light stripe down the back and rows of black dots along the sides.



Adults emerge in late August to early September and lay eggs for winter. Eggs overwinter on old leaves and hatch in the spring around late April to early May. Larvae enter leaves and feed for several weeks, and as summer progresses, they tunnel their way down into the rhizome. Once they are ready to pupate, they leave the rhizome to dig into the surrounding soil then pupate in the soil near the rhizomes. Adults emerge in the fall and start the cycle again. One generation per year is average.







This is the most serious pest of iris plants. Young larvae tunnel in the leaves, leaf sheaths, and buds, then bore down into the iris crowns and rhizomes as they develop. Look for tunneling through the foliage that looks waterlogged. Also, look to see if the tips of the foliage turn yellow, and then start to look dead. Lastly, if the base of the stalk has become yellow-brown and soft, and has a rotten odor, that means that borers have gotten into the rhizome and rot has set in. Soft rots usually follow larval damage. If damage is not too severe, the plant can live, if rot has not set in. Other symptoms are fewer to no flowers all summer.




1. Remove and destroy any dead iris leaves or stems in late fall, after plants have had a hard frost, to eliminate overwintering eggs.

2. Plant resistant varieties.


1.Monitor your plants carefully in the spring. If you see signs of tunneling, prune off affected foliage below damaged areas. Throw damaged foliage away.

2. If you notice rot, dig up the rhizomes to see if they are infested. If they are, you can dig out the larvae and pupae by cutting away damaged areas; then dust the rhizome with sulfur before replanting to prevent soft rot.

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