Caddisfly larvae signal clean water in the islands

October 5, 2026

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Pollution-sensitive caddisfly larvae from multiple genera were observed emerging or active from late August through September 30 in the San Juan Islands, including , Onocosmoecus, and Limnephilus species.^1, 2 Late-summer caddisfly emergence indicates clean, well-oxygenated water conditions supporting aquatic insect development, as these mayflies and caddisflies require good dissolved oxygen and low pollution to complete their larval stages.¹ Their presence in September aligns with the fall emergence period when larvae pupate and adults emerge to breed.²

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