Iris Borer
Contact: Eric Day, Manager, Insect Identification Laboratory
August 1996
Macronoctua onusta (Grote)
Plants Attacked
Iris and the native blue flag.
Description of Damage
Larvae may completely hollow-out one or
more rhizomes.
Identification
Full grown caterpillars are cylindrical, smooth,
and measure up to 1 3/4" long. Pupae are dark brown to nearly
black and quite shiny. Adult moths have a wing spread of 1 1/2-2
inches. The wings are covered with scales, purplish brown on the
front wings and yellowish on the hind wings. Eggs are not smooth
but highly sculptured. From an initial creamy color with a
greenish tinge, they become lavender in color. They are found
primarily in crevices and rough spots on dried out, brown, dead
leaves.
Life History
Adult moths may produce as many as 1000 eggs.
They are laid in the fall on brown, dried leaves and remain there
through the winter until spring. Hatching occurs in April and
larvae feed on the leaves. Some leaf-mining occurs before the
larvae work down into the lower parts of the growing plants.
They reach the crown and rhizome when about half grown. During
the summer the large larvae may completely hollow out one or more
rhizomes. By August pupation is initiated and adult moths emerge
in September and October.
Control
Removal and burning of dead leaves, dried rhizomes, and
damaged plants helps destroy large numbers of eggs. Adult moths
are active fliers and may invade healthy beds.
Until withdrawn from use, DDT was highly effective against this
pest. Recent tests have shown that several other insecticides
will control iris borer. Sprays are recommended for
application in early April. Control measures should be applied
every year. Community effort is important in reducing the number
of insects over a wide area as a means of preventing infestation.
Remarks
This insect is not conspicuous, since the adult moths
fly at night, so are seldom seen. Also larval feeding is not
apparent until considerable damage has been done. Extensive
injury to the rhizomes can occur before the plants begin to
collapse.
Frequently bacterial rot is associated with the feeding of the
larvae. The bacterium is introduced through wounds and results
in progressive foul-smelling decay of rhizomes.