Indicator Insects: Stoneflies and Mayflies

Insect identification is part of being a Penn State Master Gardener volunteer. I always enjoy receiving an insect that is not a pest requiring management. Frequently, the contributor is surprised when I say, "You are so fortunate to have a high-quality stream nearby! Have you explored it?" Yes, the mere presence of some insects can give us information about the environment. It's music to my ears to hear, "What's with that buggy swarm I drove through? I just washed my car, and now it’s covered with millions of dead bugs!" WooHoo! Another indicator of excellent water quality!

Generally, we consider insects terrestrial, sharing the land with us. However, there are many incredible insects called benthic macroinvertebrates. Benthic refers to bottom-dwelling where they live, in this case, in water. Macro means large, the opposite of micro. These creatures are big enough to see without additional magnification, like a lens or microscope. Invertebrate means they don't have a backbone. Instead, their support comes from an external structure, the exoskeleton. There is a world of creatures below the surface of lakes, ponds, rivers, and streams.

Stoneflies are members of the insect order Plecoptera, which means braided wing. These long, slender insects fold their four narrow wings flat over their backs. The prominent veins make them appear braided. Harmless, most stoneflies do not feed as adults. Others may feed on nectar, pollen, and perhaps some streamside vegetation. Their time on land is brief, only a few days or, at most, a few weeks. Most of their life is spent in a fast-moving stream with plenty of rocks and stones, hence their name. Unlike many familiar insects, like butterflies, they do not undergo complete metamorphosis: egg, larva, pupa, adult. Stoneflies undergo incomplete metamorphosis: egg, nymph, and adult. Because the nymphs live in the water, they are correctly called naiads.

In its stream environment, a few stonefly nymphs may be predators. However, most are referred to as shredders because they nibble on leaves that have fallen into the stream from nearby trees and shrubs. They prefer leaves from native plants. The nymphs resemble their adult parents in that their bodies are long and slender, with six sprawling legs. The antennae are slender and moderately long. Their mid-section holds stubby wing pads on the upper side, with gills on the underside. Two long tails called cerci extend from the tips of their abdomens. Like all insects, stoneflies have an exoskeleton that requires shedding as they grow. Some stonefly species may spend one year as nymphs, while others take longer to mature. All nymphs avoid hungry trout and other fish by crawling among the nooks and crannies provided by rocks. Transforming to an adult is a hazardous time for stoneflies. Some complete the final molt underwater, but many crawl to a streamside rock to emerge as adults. Here, they become prey for both aquatic and terrestrial predators.

Adult stoneflies mate, and the females deposit their fertilized eggs into the stream, beginning the cycle again. As she skips across the stream surface, dropping the eggs, the fish take advantage of her closeness and feed on the stoneflies. Anglers have learned to tie flies and mimic the movement of stoneflies across the water's surface – one predator tricking another predator!

Stoneflies are not the only indicators of stream health. Mayflies in the order Ephemeroptera share this role. Like stoneflies, mayflies undergo incomplete metamorphosis with egg, nymph, and adult stages. The nymphs, also called naiads, are aquatic, and the adults are terrestrial flying insects.

At first glance, mayfly nymphs resemble stonefly nymphs. Both have adaptations for similar environments in clear, fast-flowing, rocky streams with a high level of dissolved oxygen. Depending on the species, mayfly nymphs have six legs modified for burrowing, swimming, or crawling. Unlike stoneflies, mayflies usually possess short antennae. Their gills are along the side of their abdominal segments. The most straightforward feature that distinguishes mayfly nymphs from stonefly nymphs is their tails. Stoneflies have two cerci, while mayfly nymphs have three. Curiously, adult mayflies have only two cerci!

Like stonefly nymphs, a few mayfly nymphs are predaceous, but most feed on live or decaying plants in the water. Mayfly nymphs prefer streams or rivers, but a few live in well-oxygenated lakes and ponds. Generally, mayflies spend one year as a nymph. When conditions are perfect, with the day length and temperature just right for that species, the nymphs in the water emerge as adults simultaneously. A hatch of mayflies signals the fish to binge feed. Birds binge. Bats binge. It's a feeding frenzy!

However, adult mayflies bear no resemblance to adult stoneflies. Mayfly adults hold their two large forewings and two small hindwings straight over their backs like a triangular sail. Their tails are as long or longer than their body length. Upon emerging from a nymph, these fliers appear ghostly white and are called duns by anglers. We insect lovers call them subimagoes. They are not yet reproductive. They must undergo one more molt to be sexually mature. Usually tan or brown, the anglers call these true adult mayflies spinners. This final molt happens within a day of the stream emergence. As adults, mayflies have no mouthparts and cannot feed. Their time on land is exceptionally short and ephemeral, only a day or two, which reflects their order name, Ephemeroptera.

The adult mayflies mate in swarms, with the females laying hundreds of eggs on or in the water. You might drive your newly washed car through a fluttery swarm of mayfly adults. Please pause while rewashing your car to admire these unique insects.

So, how do our gardening practices affect these insects? These indicator species are susceptible to the climate within their streams. Avoid mowing up to the stream bank. Providing a buffer of plants along streams is one of the best things you can do to maintain or improve the status of a waterway. Trees and shrubs provide shade to keep the creek cool. Native plants offer the leaves that mayfly and stonefly nymphs feed upon. A buffer zone prevents loose soil from running off and adding silt to the stream during heavy rain events. Mayfly and stonefly nymphs cannot tolerate salt, so take care when dealing with snow and ice melt products. The buffer also helps prevent salt movement and contamination of the stream. Do you need to apply fertilizer to your lawns and gardens each season? Don't guess! Soil test! Excess nutrients from fertilizer runoff negatively impact our streams.

Explore your nearby creek. Take along a light-colored basin. Partially fill it with creek water. Gently lift a rock from the stream and place it in the basin. Look carefully for creek bugs. What kinds do you find? Stroud Water Research Center has an excellent downloadable resource for identifying macroinvertebrates. Be sure to return the rock to its original position in the creek bed. Always empty the water and its creatures back into the stream. If you enjoy investigating a stream, consider participating in FISH: First Investigation of Stream Health.

If you have a pond or water feature in your yard, you may find dragons and damsels lurking among its plants. Yes, dragonflies and their close relatives, the damselflies, have aquatic nymphs. Although stoneflies and mayflies require fast-moving waters for their nymphs, many species of dragonfly nymphs tolerate still waters well. Observe with patience to see these voracious macroinvertebrates feeding on almost any creature that is their size or smaller.

Although not all of us have a stream in our yards, Pennsylvania has a vast network of streams and rivers, with over 80,000 miles of flowing water. We gardeners must share the responsibility for the health of our streams.