Predator‐Prey Interactions between Stoneflies and Mayflies: Behavioral Observations

Field experiments were conducted at two streams to determine the mechanisms by which mayfly prey detect and avoid stonefly predators, and to quantify the effect of stonefly predators upon mayfly behavior, distribution, and activity levels. Direct observations of the interactions between stoneflies and mayflies were made at Otter Creek, Sauk County, Wisconsin, USA and the East River, Gunnison County, Colorado, USA using plexiglass boxes with screen ends and an artificial substrate. Ephemerella subvaria and Stenonema fuscum, Otter Creek, avoided the stonefly predator Acroneuria lycorias given only noncontact chemical stimuli. Ephemerella infrequens and Baetis bicaudatus, East River, Responded to long—range chemical stimuli from Megarcys signata and Kogotus modestus, East River. Baetis phoebus and Heptagenia hebe, Otter Creek, and Cinygmula sp., East River, did not respond to stonefly predators given long—range chemical cues along. Differences among responses of different mayflies may be due to differential predation pressure or effectiveness of predator evasion tactics. None of the species of mayflies tested responded to the presence of stonefly predators given only visual stimuli. The Ephemerella species assumed a "scorpion"—like display posture upon encounter with the stoneflies. This posture may increase the apparent size of the mayflies which are then rejected by the tactile predators. The Baetis species characteristically swam or drifted in response to contact with the stonefly predators. Predator evasion could partially explain the high incidence of this genus of mayflies reported in stream drift. The remaining mayflies, of the family Heptageniidae, showed an effective crawling evasion upon contact with stonefly predators. Ephemerella infrequens and B. bicaudatus differentially responded to contact with three different stonefly predators in the East River. Both species of mayflies evaded contact with Pteronarcella badia, a large omnivorous stonefly which morphologically resembles M. signata, significantly less frequently than they evaded M. signata and K. modestus upon contact. The mechanism by which this differentiation occurs could be chemotactile. Cinygmula sp. avoided all stoneflies equally regardless of size or relative threat of consumption. Tactile stimuli are probably responsible for this behavior.