Life cycle and production <i>of Isoperla signata</i> (Banks) in a Central Wisconsin trout stream

Abstract

The life history and production of the stonefly Isoperla signata (Banks) was studied in a central Wisconsin trout stream for one year. Adults exhibited a 2 week spontaneous emergence beginning in early May when mean daily stream temperatures were approximately 10°C. Emergence was completed and no larvae remained in the stream when mean daily stream temperatures reached 18°C. Ova were oval in shape, round in cross section and measured 250μm (ira long by 195 μm wide. Ova exhibited a differential hatch which lasted approximately 51 days and underwent a mean incubation period of approximately 36 days. Larvae exhibited an S2 (slow) univoltine life cycle and were present in the stream for approximately 11 months. The cohort production interval (CPI) was 329 days. Size frequency distributions indicated ca. 14 instars for both males and females. Size of male and female larvae were similar until the 9th instar, but females always weighed more than males. The mean cohort standing crop and biomass were 119 N m‐2 and 708 mg m‐2 respectively. Total annual production calculated from the size frequency (Hynes) method was 3.0 gm‐2 (30 kg/ha/yr) wet weight and the annual turnover ratio (P/B) was 3.56. Larvae were related to small gravel substrate from June ‐ March and to organic debris in April and May. Larvae were predominately herbivore ‐ detritivores in early and middle instars and omnivores in later instars.