Growth rates and prey selection of two congeneric predatory caddisflies (Trichoptera: Rhyacophilidae)

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Abstract
Rhyacophila fuscula (Walker) and R. melita Ross inhabit cool southern Ontario streams, often together. In laboratory experiments, R. fuscula and R. melita showed significant weight gains in 2 weeks on four common prey taxa occurring in the field (Diptera: Chironomidae and Simuliidae; Ephemeroptera: Baetidae; and Trichoptera: Hydropsychidae). Rhyacophila melita grew significantly less than R. fuscula on three prey, but significantly better on Chironomidae. The prey eliciting highest growth rates from R. fuscula (Prosimulium, Baetis) and R. melita (Chironomidae) are most abundant in the habitats occupied by each of these predators. Prosimulium and Baetis are common on the surface of rocks in headwater regions where R. fuscula are at high densities, and chironomids are the most abundant prey organism in the marl (travertine) microhabitat of R. melita farther downstream. Where the two predators co-occur, they may compete for foraging areas in the spring, when prey and microhabitat resources are reduced by flood scour.