DESCRIPTION OF THE PEST
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
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