Iris Borer: Treatment & Control

The iris borer (Macronoctua onusta) is a species of moth belonging to the Noctuidae family. 

It’s found throughout North America and is a common threat to iris plants.

Macronoctua onusta moths deposit their eggs on the dead leaves in the fall, allowing the larvae to hatch in the spring and slowly bore through the plant as they grow.

Iris flowers avoiding the Iris Borer

The iris borer targets iris plants and the larvae cause significant damage to the plant and may even increase the risk of bacteria growth.

Iris is a genus of plants known for producing showy flowers with close to 300 iris species. 

They are perennials growing from creeping rhizomes.

In this article, we look at the main characteristics of the iris borer, the damage it causes along with how to identify and control infestations.

What Is the Iris Borer?

Macronoctua onusta moths are often called iris borers. 

  • The larvae are responsible for causing damage to plants.
  • The moths measure just under 1″ inch long. 
  • They are brown or dark gray with a black streak.
  • Iris borers are found throughout eastern North America, mostly in wetlands and regions with cultivated irises.
  • These pests rarely appear on the west coast. 
  • The western limit of the moth is the Mississippi River.
  • Iris borer moths are not a threat to other plant species. 
  • They specifically target iris plants when depositing their eggs.

What Damage Does the Iris Borer Cause?

  • Iris borers attack iris plants in stages. 
  • The adult moths appear in August or September.
  • After emerging from the soil, they mate and deposit eggs in groups of three to five. 
  • The eggs are deposited in the dead iris leaves and debris around the base of the plant.

NOTE: A female iris borer may deposit a thousand eggs in her lifetime.

  • The eggs spend the winter in the dead plant material, hatching at the start of spring as new leaves on the iris plant start to appear.
  • The iris borer larvae crawl from the plant debris to the new leaves, creating small pinholes in the foliage. 
  • The larvae then tunnel into the leaves throughout the rest of spring.
  • By the middle of summer, the larvae reach the soil and tunnel their way into the rhizomes of the plant. 
  • The larvae grow to 1″ to 2″ inches in length inside the rhizome.
  • Toward the end of July or early August, the larvae tunnel from the rhizome to the soil where they enter the pupate state.
  • The pupae are a couple of inches long and dark brown. 
  • The pupa stage lasts for about two to three weeks. 
  • The adult moths then emerge from the soil.
  • The moths start to mate, deposit eggs, and repeat the cycle.

Permanent Damage

  • The damage to the rhizomes is permanent. 
  • The iris borer makes small holes allowing bacteria and fungus to spread.
  • The rhizomes often become soft and mushy, developing a foul-smelling odor.
  • The rhizomes slowly rot from the inside, causing the plant to die out. 
  • If the problem is not treated, an iris borer infestation can easily spread to nearby iris plants.

How to Control Iris Borer Infestations

The best way to save a plant from iris borers is to kill the pests before they reach the rhizomes. 

This requires early identification.

Iris borers first damage the leaves after the larvae emerge from the soil and work their way to the foliage. 

The larvae are 1″ to 2″ inches long and pink, resembling small grubs.

Other signs include small holes in the leaves and a pungent-smelling fluid. 

The fluid causes tan streaks to appear on the foliage. 

The leaves and stems may also start to weaken.

The fluid and the foul smell resemble other common iris diseases, including iris scorch, bacterial soft rot, and fungal leaf spot.

Remove the infested leaves and treat the plant with an insecticide. 

If possible, apply the insecticide in the early spring.

Using insecticide in the spring is more effective, as the larvae are smaller and easier to destroy. 

The most recommended insecticides for iris borer control include products containing Spinosad, permethrin, or acephate.

  • Repeat the insecticide treatment every two weeks until the iris borers are thoroughly eradicated.
  • If symptoms of iris borer infestations remain in the summer, dig up the plant and inspect the rhizomes. 
  • Discard any plants containing the small larvae.

Reducing The Rise Of Iris Borers

  • Reduce the risk of iris borer infestations by cleaning up the dead leaves and plant debris in the fall after the moths deposit their eggs.
  • Removing the plant debris and eggs prevents new larvae from climbing into the leaves the following spring. 
  • This step is also necessary when dealing with an existing iris borer problem to stop the cycle from repeating.
  • Clean up the area around the base of the plant again in the spring to remove any eggs remaining from winter.

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