SELECTIVITY OF GEOLOGIC MATERIALS USED IN CADDISFLY (TRICHOPTERA) CASES IN NORTHERN OHIO

Caddisflies (order Trichoptera) are moth-like insects important in water-quality studies. Larvae of case-building species use available materials, such as leaf fragments, twigs, and sand grains, to construct protective cases. The purpose of this study is to determine whether caddisfly larvae are selective in using geologic materials. To test this, the genera Neophylax and Helicopsyche, which construct cases predominantly of sand-sized grains, were collected from four similar northern Ohio rivers: West Branch Mahoning River, Portage Co.; Ashtabula Creek, Ashtabula Co.; Switzer Run, Richland Co.; and Sugar Creek, Seneca Co. All four rivers contain glacial sediment, but have differing bedrock contributions—sandstone, siltstone/shale, and limestone. Stream-bed sediment samples were sieved, and weight proportions and modal compositions determined. Ten caddisfly cases from each stream were disaggregated, and optically analyzed for size, shape, and mineral composition. Each case contained on average 500 grains for Helicopsyche and 150 grains for Neophylax, with an average maximum diameter of 0.8 mm (ranging from 0.12 to 2.15 mm).

Helicopsyche in these rivers construct their cases almost entirely (>90%) of quartz grains, and Neophylax construct their cases out of >70% quartz grains, with larger “ballast stones” of average river-sediment composition. Other grains used are typically garnet, magnetite, hornblende, and rarely, a lithic grain. The percentage of quartz used is higher than that found in the stream sediment, which we analyzed to be ~55% quartz in Ashtabula and Sugar creeks, 65% quartz in Switzer Run, and 85% quartz in the West Branch Mahoning River. A third taxon of caddifly larvae found in these streams (Limnophilidae, genus undetermined -- yet to be reared for adult identification) also use >90% quartz grains.

Helicopsyche and Neophylax caddisflies in northern Ohio are highly selective despite the composition of the material naturally available. Angularity does not appear to be a factor, since both smooth spherical glacial grains and jagged discoid grains are commonly used. The number of grains and composition appears to vary over time, possibly related to environmental stresses such as sediment load, disturbance, or water chemistry.