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Problem: Iris borer - Macronocutua onusta
Plants Affected: Iris
Description: Overwintering eggs hatch in April and young larvae start to cause damage by feeding on leaf surfaces causing scars. As the larvae grow, they begin to bore into the leaves and start mining downward toward the base of the leaf. Look for small pinholes in leaves, slits, or young leaves notched or with ragged edges. The larvae move toward the base of the plant resulting in a slimy appearance near ground level. Small piles of “sawdust” (frass) may appear near the base of the iris. This is also the time that the rhizomes are hollowed out by the maturing larvae. Larvae are about half-grown (1/2 to 3/4 inch long) when they first enter the rhizomes. Mature larvae are have whitish to pinkish bodies with dark brown heads and are about 2 inches long. Pupation starts in August and adults emerge in September and October. The pupal stage normally lasts two to three weeks with the pupa found about 2 inches deep in the soil. The adults are seldom seen because they are nocturnal. Eggs are laid on brown, dried leaves. Damage by the iris borer is often associated with a disease known as bacterial rot. Wounded rhizomes are easily attacked by this bacterium which results in a foul-smelling decay of the rhizomes.
Recommendations: Removal and burning of dead leaves in the fall will eliminate a number of the iris borer eggs. Larvae can also be killed by hand in June bye squeezing infested leaves in the vicinity of the injury. During division, rotted and heavily infested rhizomes should be discarded. Borers in lightly infested rhizomes can be killed by poking them with a piece of wire.
Chemical control can also be effective. Start spraying when growth first starts until
about June 1. Normally, two applications are needed. Recommended insecticides
include malathion and imidacloprid.
References:
1. Iris Borer Control (Nebguide G73-12-A), University of Nebraska Cooperative Extension
2. Iris Borer, Virginia Cooperative Extension
Last Update: 05/01/2003
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