Food of the New Zealand trichopterans Hydrobiosis parumbripennis McFarlane and Hydropsyche colonica McLachlan

Summary The food of the larvae of two New Zealand Trichoptera, Hydrobiosis parumbripennis and Hydropsyche colonica, was investigated between March and November 1971 at the Wainui Valley Stream, Canterbury, New Zealand. Most samples were collected from an experimental channel constructed in the stream in 1970. The first three instars of H. parumbripennis were mainly detrital feeders, whereas the two later instars were exclusively carnivorous. In contrast, H. colonica was omnivorous. The main prey taken by H. parumbripennis were larvae of a simuliid, Austrosimulium tillyardianum, Chironomidae (subfamily Orthocladiinae), and an ephemeropteran, Deleatidium sp. The first three instars appeared to select chironomids in preference to simuliids, whereas the converse was true for the final two instars. The main prey taken by H. colonica was A. tillyardianum. The size of prey and the mean number of prey found per larva of H. parumbripennis increased with each instar. Forage ratios indicated that H. parumbripennis were eating A. tillyardianum at the same relative frequency as their abundance in the fauna, but that the chironomids and Deleatidium sp. were being preferentially selected as prey. Cased caddis larvae were avoided as food items, as was the mollusc Potamopyrgus antipodarum. Although H. parumbripennis was the most important predator of A. tillyardianum in the stream, it is considered that it has little effect on the simuliid population.