The Rearing Imperative : Associating Immature and Adult Stream Insects

Purpose of the Study: Although most aquatic insects encountered in stream ecology and pollution studies are immature forms, the taxonomy of these organisms is based upon adult characteristics. It is estimated that only 30% of the North American species have had the juvenile stages correlated with the proper adult. Most of the insects found in a stream can now only be identified to the generic level. This critical problem in systematics has been recognized for many years. Biologists must be able to identify immature insects to the species level, since congeneric species can have vastly different pollution tolerances, life cycle patterns, and ecological niches. The purpose of this study was to make species-level identification of immature specimens from Copeland Creek possible and to begin the association of the other aquatic insect species found in the region of Sonoma and Marin counties, California. Procedure: The most reliable method of associating immatures with adults is to rear the juvenile insects through their metamorphosis and determine the species of adults obtained. Special emphasis was placed on the orders Trichoptera, Ephemeroptera, and Plecoptera, and the majority of the specimens were collected from Copeland Creek, Sonoma County. This site was focused upon because of its distinction as the only known, perennially fishless stream in California below the snow line. Adult aquatic insects were swept from riparian vegetation to identify additional species that are part of the stream community but were not found as immatures. This was a four-year sampling program that terminated in 1984. Findings: In this study, 92 species were successfully reared, many of them for the first time. Sixty-eight of the species were indigenous to Copeland Creek and the rest were collected from other areas in California, with a concentration on northwest coast species. One hundred and fifty-three species of stream insects were found to inhabit the portion of the Copeland Creek stream system that runs through the Fairfield Osborn Preserve. An illustrated identification key, exclusive of Diptera and Coleoptera, was constructed to the immatures of these species. In addition, four new species of caddisflies were discovered. Conclusions: The identification key and species list allow workers to easily determine the species of immature and adult aquatic insects found at the preserve. Its status as a Nature Conservancy Preserve insures its long-term viability as a research site. Rearing studies such as this one will eventually allow the construction of species-level regional keys to the immature stream insects of California.