Prey Selection by a Setipalpian Stonefly Nymph, Acroneuria (Calineuria) Californica Banks (Plecoptera:Perildae)

Abstract

Predation by the stonefly nymph, Acroneuria (Calineuria) californica, was studied for 1 year in a Sierra Nevada foothill stream. Estimates of instantaneous consumption were obtained by analysis of the foregut contents of A. californica nymphs collected from the study stream at monthly intervals. The bulk of the prey items present in the guts consisted of chironomid larvae, baetid mayfly nymphs, and larvae of hydropsychid caddisflies. These three groups collectively made up > 78% of all animal prey (by numbers) in the stonefly guts in each month except November. The stonefly nymphs were not strictly carnivorous. Detritus was consumed in all months, and Ulothrix, a filamentous green alga, was an important food source in the spring. Quantitative benthic samples were obtained at each collection date to determine prey availability. Electivity coefficients were calculated each month for each prey item. The electivity values, though somewhat variable, were generally positive for baetid mayfly nymphs and chironomid larvae, and negative for other prey. The most important factor determining prey selection appears to be prey availability. No great preference for any particular prey was apparent. Size considerations operate to limit the size of prey a stonefly can ingest but make larger prey more suspectible to discovery by the foraging stonefly.