Science Center Objects

Aquatic insects live in the water as larvae most of their lives, then emerge onto land for a brief period as winged adults. Sampling these emerged adults on land is therefore a useful tool for understanding the condition of the aquatic insect population that is in the water, particularly in large rivers where sampling the larvae on the river bed is impractical. Our group uses a variety of methods for collecting these emergent insects, which we sample principally in the Colorado River in Glen, Marble, and Grand Canyons and also in the Little Colorado River.

Background & Importance

Sticky Trap for Aquatic Insects (close-up) Showing Captured Insects

Sticky trap for aquatic insects (close-up) showing captured insects. (Credit: Eric Kortenhoeven, USGS. Public domain.)

Aquatic insects have a terrestrial, winged adult life stage in which they leave the water and fly onto land in order to find a mate and reproduce. Sampling insects at this terrestrial, adult life stage, rather than the more traditional larval, aquatic life stage, allows us to understand aquatic insect population patterns in ecosystems, such as large rivers, where sampling the aquatic larvae directly is unsafe or impractical. Our group samples these emergent adult insects primarily using sticky traps, a method we developed in-house. In the Little Colorado River, we are using these samples to understand the patterns of aquatic insect abundance throughout a river segment that is critically important to an endangered fish, the humpback chub (Gila cypha). Additionally, we collect samples monthly from Lees Ferry on the Colorado River downstream of Glen Canyon Dam to better understand patterns of food availability for recreationally-important rainbow trout (Oncorhynchus mykiss). We also deploy traps throughout the Colorado River in Grand Canyon to better understand patterns of insect movement in and out of tributaries, and to describe how insect abundance varies throughout this > 250 mile stretch of river and in response to operations from Glen Canyon Dam.

Sticky Trap for Insects Located on the Banks of a River, at the Confluence of the Litter Colorado River and the Colorado River

Sticky trap for insects located at the confluence of the Litter Colorado River and the Colorado River in AZ. (Credit: Eric Kortenhoeven, Public domain.)

General Methods

We use a motorboat equipped with a large boom to sample drift in the Colorado River. A reel on a winch is mounted to this boom, with a net and 75-pound weight attached. The net is raised and lowered throughout the water column of the river, at approximately mid-channel, for approximately five minutes. The resulting sample is taken back to the lab, where any collected invertebrates are counted and identified.

Important Results

Sticky trap-based results from the Little Colorado River have underscored how aquatic insect abundance may be driving spatial patterns of humpback chub density and growth that are observed by cooperators at the US Fish and Wildlife Service, Arizona Game and Fish Department, and USGS. In Lees Ferry, seasonal patterns of emergent adult aquatic insects also correlate well with observed patterns of rainbow trout growth, with peaks in spring, and very low densities in winter. Thus, monitoring of insect abundance is a useful, early indicator of ecosystem health that can foretell future changes in fish population health and abundance.    

An adult caddisfly

An adult caddisfly (Hydropsyche oslari) on researher's finger from the Little Colorado River, Grand Canyon, Arizona, 26 June 2014. (Credit: Eric Kortenhoeven, USGS. Public domain.)