Mayfly locomotory responses to endoparasitic infection and predator presence: the effects on predator encounter rate

SUMMARY. 1. The effects of predaceous stoneflies and endoparasilism by mermithid nematodes on the locomotory behaviour of individual mayflies were studied in the laboratory. The effects of predator‐ and parasite‐induced changes in activity on predator encounter, attack and capture rates were also examined. 2. In response to predator presence non‐parasitized mayflies decreased activity, while parasitized mayflies increased activity, in comparison to a control group. As a result, the faster‐moving parasitized individuals encountered stonefly predators much more frequently. The frequency of attacks per encounter did not change. 3. In view of this and other studies, two predator‐avoidance strategies based on mayfly locomotory and defensive capabilities are proposed: (a) Mayflies that are able effectively to flee a predator's attack and those able to employ a behavioural defence may decrease their mean velocity when a predator is detected, thereby reducing the likelihood of an encounter while allowing foraging to continue; (b) Slower‐moving or otherwise defenceless mayflies may immediately move away from the area in which a predator is perceived in order to avoid an encounter. 4. In the absence of a predator, parasitized mayflies demonstrated greatly increased activity. This probably reflects increased foraging in response to the nutritional deficit imposed by the endoparasite. A decrease in upstream movement by parasitized mayflies may also indicate a parasite‐imposed energy stress.