Skip to content
Pocket Botanist
Get the app

Guides

Iris Borer

Pest

The iris borer is the larval stage of a moth that tunnels into the foliage and rhizomes of iris plants. It is a serious pest because the feeding damage often leads to secondary bacterial rot, which can destroy the entire plant.

Iris Borer on a plant
Photo: Sheila Furlong, CC BY-SA

Symptoms by part

Leaves

Small pinprick holes or translucent streaks appear on the leaves, often accompanied by water soaked spots or jagged edges as the larvae tunnel downward.

Roots

The rhizomes become hollowed out and mushy, often emitting a foul odor due to secondary soft rot infections.

Causes

  • Overcrowded iris beds that prevent air circulation.
  • Leaving old, dead foliage on the ground over the winter where eggs can overwinter.
  • Warm, humid conditions during the early growing season that favor larval development.

Could be confused with

  • Bacterial Soft Rot · Soft rot is often a secondary symptom of borer damage, but if you find no larvae or tunnels in the rhizome, the rot may be caused by poor drainage or injury alone.
  • Iris Leaf Spot · Leaf spot creates distinct brown or gray lesions with reddish borders, whereas borer damage starts as narrow, vertical tunnels or streaks.

Treatment

  1. Cut away and destroy any leaves showing vertical streaks or pinprick holes in early spring to remove larvae before they reach the rhizome.
  2. Dig up infested rhizomes during the growing season and manually remove any pinkish larvae found inside.
  3. Trim away all mushy, rotted sections of the rhizome with a clean, sharp knife until you reach healthy, firm tissue.
  4. Replant the healthy portions of the rhizome in fresh, well draining soil after allowing the cut surfaces to callous over for a day.

Without chemicals

  • Remove and destroy all dead iris foliage in the autumn to eliminate overwintering egg sites.
  • Divide iris clumps every 3 to 5 years to prevent overcrowding and improve airflow.
  • Keep the area around the base of the plants clear of debris and weeds.

Chemical options

  • Spinosad, apply as a foliar spray to the leaves in early spring when larvae are small and before they enter the rhizome, follow the label.

Preventing it

  • Clean up all garden debris around iris beds every autumn.
  • Divide and replant iris clumps regularly to maintain plant vigor.
  • Inspect foliage closely in early spring for signs of initial larval entry.

Season

The warm growing season, starting when larvae hatch in early spring and continuing until they pupate in the soil during late summer.

Spread risk

Moderate

Severity

High

When to act

Act immediately when you see leaf streaks or signs of rot to save the rhizome. If the entire rhizome is hollowed out and mushy, remove and dispose of the plant entirely.

Think it's Iris Borer?

This guide covers the general picture. Pocket Botanist looks at your actual plant from a photo and tells you what's really going on, how serious it is, and what to try first. Identification and a health check are free.

Download on the App StoreGet it on Google Play

Get the app · Or check it from a photo now, free

Questions people ask

Can I save an iris if the rhizome is soft?
Yes, if you catch it early, you can cut away the rotted parts and replant the healthy, firm sections in clean soil.
Do I need to spray my irises every year?
Not necessarily, if you are diligent about cleaning up dead foliage in the autumn and dividing your plants, you can often manage the pest without chemicals.

Plants it affects

Back to all guides

This guide was generated with AI and is general information, not horticultural advice. Read about how we build these guides.