Flight activity of insects along a mountain stream: is directional flight adaptive?

Abstract

1. Flight activity of Trichoptera, Plecoptera and Ephemeroptera was studied by sticky trapping for 12 months at five sites along a New Zealand mountain stream. Over 19 000 insects were captured by the traps, which were located in forest and grassland reaches, including a reach with intermittent flow.2. Most species occurred predominantly in forest or grassland, although some were trapped throughout the stream. Longitudinal distributions of adults and their larvae were strongly correlated.3. Flight periods of 24 caddisflies, three mayflies and four stoneflies ranged from 2 to 12 months. Six species were trapped in all months and 17 (55%) in more than 5 months.4. The most abundant forest‐dwelling caddisfly species were over‐represented on the downstream sides of sticky traps located in, and immediately below, forest indicating a majority was flying upstream. Upstream flight compensates for downstream drift of larvae and should maximize the likelihood that forest‐dwelling species will locate preferred habitat for egg, larval and/or adult development. Unlike the caddisflies, the stonefly Spaniocerca zelandica was over‐represented on the upstream sides of traps, suggesting that some adults may float or fly downstream following emergence.5. In contrast to forest‐dwelling species, only one common caddisfly (Oxyethira albiceps) was over‐represented on the downstream sides of traps at grassland sites. Unlike the forest‐dwelling species, most species taken at the downstream sites probably came from a variety of sources, including a nearby stream.