Bug FAQIris Borer Moth
Macronoctua onusta · Order: Lepidoptera · Family: Noctuidae
Also called: iris borer
Source: Wikipedia
This large moth is 18 to 21 mm long, with a broad forewing that has a scalloped edge, a black streak through the reniform spot, and a distinctive postmedial line. Its larvae feed only on iris rhizomes, boring into the underground stems, while adults fly in September and October. The species is native to North America.
Source: Wikipedia
How to recognize it
Its forewing is broad with a scalloped edge, a black streak through the reniform spot, and a distinctive postmedial line.
Source: Wikipedia
Size
The moth measures about 18 to 21 mm from wing tip to tip.
Source: Wikipedia
Where it lives
It lives in North America.
Source: Wikipedia
What it eats
The caterpillars feed exclusively on iris rhizomes, boring into the underground stems. Adult moths are active in the fall, especially September and October.
Source: Wikipedia
Life cycle
Larvae develop inside iris rhizomes and later emerge as adults that fly in autumn.
Source: Wikipedia
When you see it
Adults are seen in September and October.
Source: Wikipedia
Where it comes from
Its native range is North America.
Source: Wikipedia
Where it is found in the US
Recorded in 29 of 51 states and DC. Most records: Michigan, New York, Vermont, Massachusetts, Minnesota, Ohio, Connecticut, Illinois.
Darker = more records. Counts are museum and citizen-science records, not population size.
Source: GBIF
Status: native in North America; native in Canada. Source: iNaturalist
Months it is reported in the US
Source: GBIF
More photos
Questions people ask
What does the iris borer moth look like?
It is a large brown-gray moth about 18 to 21 mm long, with a broad forewing that has a scalloped edge, a black streak through the reniform spot, and a distinctive postmedial line.
Source: Wikipedia
When are adult iris borer moths active?
Adults fly in the fall, mainly in September and October.
Source: Wikipedia
What plants do the larvae damage?
The larvae bore into the rhizomes of iris plants, feeding exclusively on those underground stems.
Source: Wikipedia
How can I tell this moth apart from similar dark brown moths?
It has a dark posterior outline, a spike from the medial reniform spot, and a postmedial line that is serrated in the front and straight in the back, features absent in similar species.
Source: Wikipedia
Same family (Noctuidae)
Ipsilon Dart MothAgrotis ipsilonDamages trees, gardens or crops
Fall Armyworm MothSpodoptera frugiperdaDamages trees, gardens or crops
Army Cutworm MothEuxoa auxiliarisDamages trees, gardens or crops
Large Yellow UnderwingNoctua pronubaDamages trees, gardens or crops
Silver ΥAutographa gamma
American DaggerAcronicta americanaCan irritate skinSources
- Wikipedia: https://en.wikipedia.org/wiki/Macronoctua_onusta
Look it up elsewhere: GBIF · iNaturalist · BugGuide · ITIS · Wikipedia
Text written 2026-10-25 from the sources above and checked sentence by sentence against them. Data fetched 2026-10-25.