LIFE-CYCLE OF ISOPERLA-LATA (PLECOPTERA, PERLODIDAE) IN A CENTRAL WISCONSIN TROUT STREAM

Monthly qualitative samples of Isoperla lata Frison were made from Jan uary 1992 to June 1993 in Ripley Creek, a small second order trout str eam in Lincoln County Wisconsin. Additional collecting and an in-strea m hatching experiment were conducted in 1994. This species exhibited a n S1 (slow) univoltine life cycle. Emergence was synchronous and occur red in late April through early May when stream temperatures in the fi eld were approximately 9-14 degrees C and laboratory stream temperatur es were 7-17 degrees C. Laboratory longevity was 2-25 ((x) over bar = 18.2 +/- 4.51) days for males and 7-39 ((x) over bar = 21.7 +/- 5.35) days for females. Mean fecundity of dissected females was 322 +/- 122 eggs/female. Females did not deposit egg masses in the laboratory unti l being held together with males inside modified screened plastic cont ainers. Field-collected females did not have eggs. The egg shape was o void and circular in cross section. Mature eggs were light brown and m easured 371.7 +/- 12.6 mm and 260.7 +/- 10.2 mm in length and width re spectively. Eggs required a 40-46 day in-stream incubation period and first instar nymphs hatched synchronously over a two day period when s tream temperature reached 20 degrees C. Nymphal growth was nearly expo nential from June to January and then declined until emergence. The gr eatest growth increment occurred between June and October and the aver age maximum size attained occurred in February. Males and females had approximately 18 and 19 instars respectively. Nymphs were primarily ca rnivorous throughout development and fed on larval Chironomidae, Ephem eroptera, and Plecoptera.