Iris Borer of Flowering bulbs

Disease Information

Iris Borer


Iris borer (Macronoctuaonusta) is the most serious pest of iris. It is the larva (caterpillar) of iris borer moths. The mature caterpillar can reach 13/8 inches in length. It is pale yellowish-pink to pink with a brown head and has rows of black spots along its sides.

The adult moths that emerge late in summer, mate and lay eggs on old iris leaves and nearby plants. The borer overwinters (survives the winter) as eggs. In the spring, tiny caterpillars hatch out and feed on new leaves.

Symptoms and damage

a result of the feeding, the leaf margins may appear water-soaked, bleed plant sap and become ragged. The caterpillars then mine (feed in slender tunnels) inside the leaves before moving down to the rhizomes. They may feed on the outside of the rhizome or bore inside and feed until they mature. A single iris borer can ingest the entire contents of a rhizome before moving to another rhizome. It is common for bacterial and fungal root rot to develop in borer damaged rhizomes. At maturity the borer leaves the rhizome and enters the soil to pupate (transform into the nonfeeding stage where the larva changes into an adult form). Iris borer damage is generally first noticed in the fall when the damaged rhizomes are dug for transplanting in late summer.


Old stems and leaves should be removed in the fall or winter to eliminate overwintering eggs. If borer damage is noticed in the spring, hold young leaves up to the sun and if present, the borers will be readily visible. The small caterpillars can be crushed between thumb and finger or by using a pointed stick.

When rhizomes are dug for dividing, cut off and destroy heavily infested rhizomes. With rhizomes that are only minimally damaged, you can attempt to kill the maggot by trying to stab it with a needle or wire inserted through the entry hole. Again, there is always a chance of not killing the maggot and simply damaging the bulb even more.

If iris borer has been a problem in the past, rhizomes can be sprayed with an insecticide when the new foliage is 4 to 6 inches tall in the spring, and again 2 weeks later. Insecticides that will control the iris borer are acephate, bifenthrin, cyfluthrin, lambda cyhalothrin and permethrin.



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