American Grannom
Brachycentrus americanus
Key Identification Characters - Larva 2
- Case made of carefully cut plant fibers laid crosswise creating a square in cross section “chimney” shaped case.
- Larvae inside case a pale green color abdomen with brown head and legs.
- Length of case up to 17mm.
Similar larvae: 2
The chimney shaped case is one of the definitive characteristics of Brachycentrus larvae. One other caddis also builds a similar case: Lepidostoma sp.. Lepidostoma, however live in slow water habitat with a fine substrate, rather than the rocky riffles used by Brachycentrus.
Key Identification Characters - Adult: 2
- Dark gray to black body and wings, though abdomen often with pale green stripe along each side.
- Antennae body length or shorter.
- Length to tip of wings 9-13mm. Males smaller than females.
Similar adults: 2
Adult caddis can be frustrating to identify and difficult to determine on the stream. Characteristics used in taxonomy books include many that require a microscope to effectively see. The large concentrated emergence and egg laying of Brachycentrus adults provides one clue to their identity. When you encounter a large number of dark, plain colored caddis, less than a half inch long chances are good it’s Brachycentrus. Specimens should be checked by a taxonomist to confirm.
Habitat & Distribution: 2
Brachycentrus americanus occurs across the entire northern half of North America. Larvae are often found in moderate to fast currents on the top of bowling-ball sized rocks. They secure their cases to the rocks with silk thread and then hold their legs up into the current to strain fine particles of food.
In Oregon B. americanus occurs in large to medium sized streams throughout the state. Large numbers live in streams like the Metolius and Crooked Rivers where they have been carefully studied.
Life Cycle - Emergence: 2
Because of the wide distribution of this species emergence periods vary, and adult activity may be spread out over several months. That said massive hatches are well known on many western streams in April and May, and as a result it is often called the Mother’s Day caddis due to the emergence occurring around Mother’s Day.
In Oregon the dominant emergence period for B. americanus is early April through May, but fall emergence (mid-September-October) can also occur. It is uncertain if this fall emergence is the same species.
Oregon Emergence Period: April-May (also Sept-Oct?)
Metolius: May-October
McKenzie: April-May.
Necanicum/Nehalem: ?
Sources and Credits
- (c) Jerry Schoen, all rights reserved, uploaded by Jerry Schoen
- (c) Jerry Schoen, some rights reserved (CC BY-SA)
