Ecology and energetics of an aquatic detritivore, Pteronarcys proteus (Plecoptera: Pteronarcyidae)

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Virginia Polytechnic Institute and State University

Life history, food habits, energetics, and production by nymphs of Pteronarcys proteus were measured. The life cycle lasted four years in an Appalachian mountain stream in southwestern Virginia. Adults emerged late May to early June, and eggs deposited did not hatch until the following spring. Nymphs grew at least 3 years with 12 male instars and 13 female instars. The nymphal diet was primarily leaf detritus, with a small percentage of moss and animal matter. Total crude lipid content of nymphs varied from 6% to 29% of dry insect weight and was dependent on age, season, and developmental state. Lipid content of nymphs in the two youngest cohorts generally declined during late summer, but increased after leaf-fall in November. A similar pattern was observed in the oldest cohort, but a significant decline in the spring prior to emergence of adults was also observed. The data indicate that P. proteus relied on lipid stores during periods of low food availability and for reproductive maturation.

The energetic parameters of growth (G), respiration (R), ingestion (I), and egestion (E) for nymphs in each of the three cohorts were measured in the laboratory. Growth rates ranged from 0.031 to 0.0037 mg/mg/day, with small nymphs growing fastest. Ingestion ranged from 5 to 40% of dry body weight per day. Respiration ranged from 330 to 980 µl O₂/g/hr. Mean AD was 13.5%, mean gross growth efficiency was 5.2%, and mean net growth efficiency was 38.7%.

Total assimilation by a population was estimated at 119 kcal m⁻², accounted for primarily by the two oldest cohorts. Annual energetics of the nymphal population were: I= 906, G= 41, R= 78, and E= 828 kcal m⁻². Annual production was 0.438 g m⁻², 3.158 g m⁻², and 4.182 g m⁻², with the youngest cohort contributing the smallest. Mean cohort densities ranged from 23.8 to 9.3 nymphs m⁻², and mean standing stock biomass ranged from 0.143 to 1.790 gm⁻². Mean relative growth rates (RGR) in the stream were greatest for smallest nymphs and ranged from 0.939 to 0.182 percent increase per day. The data indicate that growth rates of small nymphs were influenced by temperature and larger nymphs by food supply. It was estimated that P. proteus consumed 41-61% of the litterfall in the study stream.