Native Delaware: Stoneflies

UD entomologist discusses stoneflies, official state macroinvertebrate

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3:11 p.m., June 24, 2013--On a recent morning, Ingrid Feustel and Ed Trommelen donned rubber boots, grabbed nets and collection buckets, and splashed their way into a tributary of the Red Clay Creek in search of stoneflies.

The duo are spending the summer conducting a macroinvertebrate survey as Stream Watch interns for the Delaware Nature Society.

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As Feustel set out sampling jars, Trommelen kicked up pebbles with the toe of his boot, then dragged the net over the churned-up water. “I’ve got some stoneflies,” said Trommelen, after a glance inside the net.

Most people would probably only notice leaves, twigs and pebbles but Trommelen quickly identified the stoneflies, even though they’re itty-bitty (one-half to 1 1/2 inches long as adults and even smaller as juveniles.)

The interns collected stoneflies throughout the morning, making note of how many were in each sample, as well as any other macroinvertebrates -- such as mayflies, caddisflies and worms -- that they found. While it’s certainly fun to splash around a sun-dappled creek on a June day, why would the Delaware Nature Society care how many stoneflies are in local creeks?

“Stoneflies serve as the canary in the coal mine in terms of assessing the water quality of streams and creeks,” said Brian Kunkel, an entomologist with University of Delaware Cooperative Extension. “It makes sense to focus on stoneflies. Stoneflies need running water that contains lots of oxygen. Scientists can tell if a stream is polluted or not based on whether stoneflies are present.” 

In fact, the stonefly is such an excellent indicator of water quality that it was chosen to be the official state macroinvertebrate. The May 4, 2005, proclamation that made it official says that the designation of the stonefly was a means “whereby Delaware state government could recognize the importance of excellent water quality and the vital role played by healthy aquatic ecosystems in Delaware.”

Delaware actually has three official insects – the lady bug, the tiger swallowtail (which is the state butterfly) and the stonefly. The distinguishing feature of a macroinvertebrate is that it doesn’t have a backbone.

Stoneflies aren’t just an indicator of pure water; they also help to keep it that way. Most species are herbivores; they eat decaying vegetation and algae, said Kunkel.

There are 38 known species of stoneflies that call Delaware home. Nineteen live exclusively in the Piedmont, in the northern one-fourth of New Castle County; eight live in the Coastal Plain, which makes up the other three-fourths of the state; and 11 can be found in both the Piedmont and the Coastal Plain.

Adult stoneflies are weak fliers and like to hang out on stones (hence their name) and also on logs. If they lose their grip and fall into the water they may get eaten by trout, which prefer mayflies but will eat stoneflies if that’s the only food around. 

Even if it doesn’t become dinner for a trout, an adult stonefly only lives about three to four weeks. Nymphs, on the other hand, which make their home in the water, can exist for one to three years in their immature state, says Kunkel.

Good places to search for stoneflies, says Kunkel, are the White Clay and Red Clay creeks in New Castle County, the Murderkill River in Kent County, and Woodenhawk, a tributary of the Marshyhope Creek, in Sussex County.

Delaware Nature Society has been conducting a Stream Watch program for 20-plus years, says Ginger North, the society’s associate director for natural resources conservation. During that time the Red Clay Creek has shown improvement in its water quality. 

“Twenty years ago, the Red Clay was classified as a waterway that was severely impaired,” says North. “It only supported black fly larvae and other macroinvertebrates that can tolerate polluted water. But today, stoneflies can be found in the Red Clay’s tributaries.”

Article by Margo McDonough

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