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Insect Indicators

Lift a creekbed rock and you may find a "shredder," or "collector." These important sounding job titles refer to the kind of work two types of aquatic insects perform in the riparian food web. Stonefly nymphs and some caddisflies are "shredders," biting and cutting up plant material into tiny pieces of detritus. The "collectors," usually caddisfly larvae, then gather up the detritus, acting as a living water filtration system.

The presence of such insects can also help citizens and scientists working to assess the health of our creeks do their job. If you look carefully and don't find these insects, the creek could be polluted. Indeed recent U.C. Berkeley studies conducted on two forks of Strawberry Creek indicated that the north fork, which flows through an urbanized landscape, supports less insects that are pollution sensitive than the south fork, which was recently restored and drains a less developed area.

U.C. biologist Scott Fetherston offers several tips on how to use aquatic insects to assess water and habitat quality. "Don't look for the more pollution tolerant taxa because they can befound in both clean or impacted habitats," he says. "Look for pollution sensitive taxa like certain stoneflies, mayflies and caddisflies that can't survive in polluted streams."

Stonefly nymphs -- often brilliantly patterned critters with two legs that measure less than an inch big in size -- can be found clinging to the undersides of stones, where the current is weaker. Their bodies are flattened to allow the current to flow smoothly over them. Caddisfly larvae -- wormlike critters even smaller than the stonefly nymph -- can often be found inside protective casesthey make from sand and pebbles and attach to rocks with an adhesive secretion.

Fetherston says aquatic insect surveys have become a popular method for evaluating water and habitat quality in streams, especially for citizen monitoring groups, because they require less technical expertise and fewer dollars to conduct than chemical tests. There's one other major plus. "People really like to work with insects," he says.

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