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How to identify it
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Iris leaf diseases | Bacterial soft rot, iris scorch and fungal leaf spot also brown leaf tips, so check for caterpillars and tunnels before blaming borers.[1] |
Where it lives
Minnesota Extension calls it the most destructive insect of iris; in Minnesota the caterpillars reach the rhizome by July.[1]
Biology and life cycle
Eggs overwinter on old iris leaves and debris at the base of the plants and hatch in early spring.[1]
Tiny caterpillars climb the new leaves, tunnel down and reach the rhizome by midsummer.[1]
In late July or August they pupate in the soil, and moths emerge in late summer or fall to lay eggs on old iris plants.[1]
Damage and signs
Health risk
It's a plant pest only; the Minnesota fact sheet notes nothing about people.[1]
Control
Prevention
Siberian iris tolerate borer attack better than other types.[1]
Sanitation
After a hard frost, remove and destroy old iris leaves, stems and nearby debris to kill the overwintering eggs.[1]
Traps and physical control
In spring, crush caterpillars inside the leaves or remove damaged leaves; in July, dig plants with dying leaves and discard infested rhizomes.[1]
Biological control
Steinernema or Heterorhabditis nematodes, applied to moist soil as leaves near full size, can reduce borer numbers.[1]
Pesticides
Where damage has occurred before, acephate or spinosad applied as eggs hatch, when leaves are 4 to 6 inches tall, can help; follow the label.[1]
More from University of Maryland Extension
It attacks bearded, Japanese and blue flag iris and Belamcanda; the moths fly at night from August through early October and lay eggs at the base of old iris leaves and debris.[2]
Common questions
- Why are my iris leaf tips turning brown?
- Iris borers are one cause, but bacterial soft rot, scorch and leaf spot look similar. Look for caterpillars and tunnels in leaves or rhizomes.[1]
- When is the best time to clean up iris beds?
- Any time after a hard frost, when moths have stopped laying eggs.[1]
- Do I need to spray every year?
- No. If your iris haven't been affected before, no pesticide is needed; just watch for early signs.[1]
Sources
- University of Minnesota Extension, “Iris borers (Yard and garden insects).” extension.umn.edu/yard-and-garden-insects/iris-borers (accessed 2026-10-06)
- University of Maryland Extension, “Iris Borer.” extension.umd.edu/resource/iris-borer (accessed 2026-10-15)
- iNaturalist, “Macronoctua onusta taxon page (order and family).” www.inaturalist.org/taxa/224312 (accessed 2026-10-06)