Aquatic Insects in Iowa’s Driftless Area

Aquatic insects are abundant in Iowa’s Driftless area streams and rivers. Some species hide under rocks, some burrow in the mud and others seek shelter among water weeds. They are a critical food source for most fish. Let’s start by talking about these lovely Mayflies.

Mayflies are included in the Insect Order Ephemeroptera

Adult Mayflies truly are ephemeral, living for only a single day or so after emerging.

Unlike most adult insects, Mayflies transform from young Duns to sexual Spinners before dying

Trout feeding season for Mayflies opens in early spring when some similar pint-sized species descriptively referred to as Blue-winged Olives (BWOs) appear along area cold-water streams

Blue-winged Olive nymphs (Baetis sp.) are classified as swimmers. Commonly called Small Minnow Mayflies, they’re streamlined and quick and dart around like mini-minnows.Like most Mayfly larvae, they graze on algae and rock slime.

Here’s a good-looking buggy Baetis nymph for trout fishing

Blue-winged Olive nymphs like to live among the dense vegetation in shallow weedy runs. PT4

Blue-winged Olive duns, a buggy red-eyed male here, emerge early in the spring, April in Iowa, often on dreary cold damp 400 days. Look for another flush in autumn.

Blue-winged Olive dry fly pattern. As the old saying goes “Don’t leave home without them”

March Brown (Maccaffertium vicarium) nymphs are classified as clingers. Commonly called Flathead Mayflies. They lie flush on the undersides of rocks in running water.

March Brown nymphs are synonymous with Pheasant-tail nymphs

March Brown nymphs inhabit fast rocky runs.

Drift your Pheasant-tail nymph downstream as shown. PT4

March Brown duns have mottled brown bodies. They hatch out in May during midday. Noted May 26

March Brown dry fly. A good pattern to have on hand in the spring.

Miniature Eastern Sulfur (Ephemerella dorothea) nymphs are classified as crawlers, just like their slightly larger Great Eastern Sulfur cousins. Commonly called Spiny Crawlers.

Tie your favorite Pheasant-tail type fly a little smaller for the Miniature Eastern Sulfur.

Miniature Eastern Sulfur nymphs live in the leafy debris beside deep quiet pools. Work them slow. PT1

Miniature Eastern Sulfur duns pop up in pools during May & June.

They are fragile mayflies with creamy yellow bodies and three trailing tails.

Females drop their eggs like little bombs over water

Miniature Eastern Sulfur dry fly pattern.Also called the Little Sulfur Dun or Pale Evening Dun.

This monster Giant Michigan Mayfly (Hexagenia limbata) larvae is nearly two inches long. Those toothbrush-like bristles are breathing gills.

Giant Michigan Mayfly nymphs are classified as common burrowers. Note the elephant-like tusks to help them tunnel down.

Burrowing mayfly nymphs dig out hollow U-shaped burrows in the substrate of their choice, where they can sit in safety and filter microscopic food from the water passing by.

Giant Michigan Mayfly nymphs prefer to burrow into marl, a harder silty clay substrate sometimes found in slow-flowing pools. PT4

This dried-out stream bottom is made of marl. Notice how the holes don’t cave in.

Here’s a Giant Michigan Mayfly nymph that highlights legs and gills

Giant Michigan Mayfly duns are huge. Mating swarms arrive in June.

The Giant Michigan Mayfly hatch is legendary on the Au Sable & Pere Marquette rivers in that state. Around here they’re called Fishflies.

This “snowstorm” around the streetlight is a swarming Michigan Hex hatch along the Mississippi River.

Among aquatic insects, Caddisflies are included in the Order Tricoptera. Many Caddis larvae hide in homemade cases.

Little Black Sedge (Chimarra) larvae are pumpkin-colored.

They are classified as Finger-net makers, trapping tiny food particles in a very finely-woven mesh net

Look for them to be full size by March.

Identifying aquatic insects gets nit-picky. Caddis larvae in the Genus Chimarra have a little notch cut into their forehead between the pincers.

Even though they look like dangerous predators, Little Black Sedge larvae are collectors and chew up leaf bits and such that get caught in their nets.

Little Black Sedge larvae are on the small size, as seen on this dime.

Little Black Sedge larvae are fairly finicky and live in cold spring- fed streams, weaving their finger-nets under the front sides of sizable rocks in fast riffles. PT3

Little Black Sedge larvae net open for business.

Little Black Sedges emerge during daylight hours at Hendrickson mayfly hatch time, later April extending even into June. They lay their eggs on the water surface while sitting still.

The Leadwing Coachman is a good dry fly to try for the Little Black Sedge.

The Green Sedge or Green Rock Worm (Rhycophilia) is classified as a free-living larvae, which means it doesn’t manufacture a safe shelter like most Caddisflies. They do spin a safety rope that keeps them from drifting away from bigger rocks where they hunt for smaller bugs to eat.Noted 2-19-13 Bohemian Creek.

Green Rock Worms live in rocky riffles given good current flow. PT1

The Green Rock Worm pupates and emerges as the Green Sedge, even though the wings are grey. Adults show up along the stream in May, extending into June. Afternoon hatches are common. Females dive to the bottom where they lay their eggs on rocks.

The Grannom(Brachycentrus) larvae is classified as a Humpless Case-maker.

Grannom larvae construct ornamental four-sided cylinders for their mobile homes.

Grannom larvae fasten their cases to rocks so they aren’t swept away. They can rappel from rock to rock on silk ropes they spin. Grannom can be quite abundant in Driftless streams

Grannom larvae thrive in fertile streams which are full of their favorite food – algae. PT1

Grannom adults are dark brown. Look for them to on the water in May and June, emerging during the day and then laying their eggs on the surface before expiring.

Little Tan Short-horn Sedge (Glossosoma) larvae are classified as Saddle Case-makers. They construct little turtle-like shells for their portable shelters that they carry on their backs while scraping detritus off rock surfaces.

Little Tan Short-horn Sedge larvae often leave their cases and drift free in the current. If not eaten by a trout, they’ll build a bigger home somewhere downstream.

Little Tan Short-horn Sedge larvae live in the gravelly sections of refrigerated trout waters, blending right into the small colorful stones. PT0

Little Tan Short-horn Sedge adults start emerging in late April and continue into June.Caddisflies (and Stoneflies) are colloquially called “Downwings” by flyfishers.

White Miller (Nectopsyche) larvae are classified as Longhorned Case-makers, some species making stone homes and other out of woody plant matter.

White Miller larvae shred plants apart for their food.

White Miller larvae live in slower-flowing weedy streams. PT4

White Miller adults are unmistakable, looking like white moths since they only emerge after dark. Look for them to be bouncing around on the water surface on midsummer nights.

Stoneflies are in the aquatic insect Order called Plecoptera. As a rule, Stonefly nymphs require cool clear pollution-free streams to survive. Look for them to be crawling around under the bottom debris eating up dead leaves

Little Black Stone (Allocapnia) larvae mature in late winter. Even then, they are easy to overlook. In contrast to Mayflies which have typically have three tails as immature nymphs, Stonefly nymphs only have two.

This woodland trout stream has a good supply of drowned leaves for hungry Little Black Stone larvae to eat. PT1

The legendary Prince Nymph is a dead ringer for most Stonefly larvae.

Little Black Stoneflies are tough critters, coming out of the water in March when snow still often covers the stream bank.

Little Black Stoneflies are aptly named. Notice how the wings are folded flat over the back, rather than tent-shaped like a Caddisfly.

Golden Stonefly (Perlesta) larvae are a cluster of several related colorful forms. Some species look and act like little aquatic tigers.

This Golden Stone (Agnetina) larvae is boldly decorated in bumble-bee colors

Golden Stonefly larvae are lightning quick predators and terrorize Caddisfly larvae in swifter stream sections under rocks. PT1

Golden Stonefly adults are aptly named.

What are called Salmonfly (Pteronarcys) larvae out West look like some science fiction movie monsters here in the Midwest. Notice those fluffy gills sticking out around their legs.

Pteronarcys nymphs are slow and lazy and like to eat dead leaves in rubble-bottomed rivers. PT0

Iowa’s adult Salmonflies look very similar to their famous western cousins. Too bad they don’t hatch out in the huge numbers seen on Rocky Mountain freestone streams.

Alderflies and Dobsonflies are included in the aquatic insect Order Megaloptera. Sometimes referred to as “Fishflies”, these bugs are big but not very common and as such the fish seldom see them. We seldom see them either as they emerge along the shore after dark.

Alderfly (Sialis sp.) larvae look similar to Stonefly larvae, but see how the gills stick out like little spears all along their abdomen.

Alderfly larvae are predators on smaller Mayfly and Caddisfly nymphs.

When full-grown, they crawl out on shore and pupate in a cocoon over winter before coming out after dark next June.

Look for Alderfly larvae to be prowling around in slower stream sections where there’s lots of bottom debris. PT4

Alderfly adults superficially resemble Caddisflies with the tent-type wings, but they’re a much more laid-back bug. They seldom fly, preferring to crawl on the ground. Search for them about summer solstice time.

Dobsonfly (Corydalus cornutus) larvae are the original “ugly bug”. Often called Hellgrammites or Go-Devils, they look rather daunting when encountered. It goes without saying that they are predators on just about any other insect in the stream.

Dobsonfly larvae are big, up to 3” long. Compare with Golden Stonefly larvae. They crawl to shore and pupate underground over winter.

Dobsonfly larvae terrorize their prey among sizable stones in streams with good current flow. Mayflies and Stoneflies beware. PT0

Look for Dobsonfly adults to hatch out at the end of June. They’re about as big as a small bird with a 5” wingspan.

Adult Dobsonflies can barely fly and only live for a couple days. Pick one up if you dare.

Damselflies and Dragonflies are included in the Order Odonata. While most species are more likely to be found flying around lakes or muddy marshes, a few seek shelter around our streams. This Green Darner dragonfly has been called the Devil’s Darning Needle and was said to sew the ears of bad boys shut in an earlier era. Green Darners are a calm water species.

Damselflies are the dainty members of the Odonata order. Around here, the Broad-winged Damselflies(Calopterygidae) are commonly found in moving waters. They have skinny bodies that look like walking sticks under water.

This Ebony Jewelwing (Calopteryx maculata)larvae lives among the water weeds where it hunts for tiny meaty Mayflies.

Ebony Jewelwing larvae as well as adults like quiet shady stream sections. PT5

Ebony Jewelwing adults may be one of the easiest aquatic insects to identify. Note the black wings and iridescent blue-green body.

Female Ebony Jewelwings lay their eggs under water on aquatic vegetation. See the white dots on their wings, which the males lack. That flying guy is guarding his wives from possible attack, at least from above.

Dragonfly nymphs are more robust than Damselflies and have flattened fat abdomens. These voracious creatures can even catch and devour creek minnows.

That lower lip, called a labium, can shoot out like frog’s tongue to snare passing prey. Dragonfly nymphs are often referred to as naiads.

When mature, after spending up to two years underwater, Dragonfly naiads crawl out of the water and up on a rock or weed stem, where they change into the flying form.

Around here, most stream Dragonflies belong to the Gomphidae Family.

Stream Dragonfly (Gomphidae) larvae and adults are likely to be collected or spotted along creeks and rivers with scattered rocks and shallow debris-covered bottoms. PT1

This Midland Clubtail (Gomphus fraternus) feeds on Deerflies, Midges and Mosquitoes. Notice the swollen tail-end.

Midland Clubtails like to bask in the sun on big rocks along the creek. Look for them to be buzzing about in June.

True flies are found in the insect Order Diptera, meaning they have only two wings. Several families of True Flies are found in and around water, and at least one is a major player for fly fishers. This Midge looks a lot like a Mosquito but lacks the bloodsucking sticker.

Midge (Chironomidae) larvae are called bloodworms because many are bright red. They only measure a few millimeters in length. They live in the mud on the stream bottom.

Bloodworm close-up showing the oxygen-carrying hemoglobin coursing through the larvae

Midge larvae live by the millions in the soft mud along shallow weedy stream runs. Midge larvae are one of the brave few that emerge all winter long on relatively warm days. PT6

Midge larvae are real easy to imitate. Just wrap a little red or black or green thread around a tiny hook and go to town.

Most adult Midges are either green or black if seen up close, though they’re no bigger and often mistaken for mosquitoes. They whine but they don’t bite.

Midges can be mesmerizing when we see their swarms dancing up and down along the stream bank in the evening light.

That little Griffith’s Gnat can be deadly when a midge hatch is on, as the bitty fly still looks like a whole cluster of floating Midges to greedy trout.

This innocent-looking little bowling-pin shaped larvae will one day turn into the dreaded Blackfly (Simuliidae). They attach to rock surfaces with that sticky suction cup at their butt.

Blackfly larvae can be incredibly abundant in streams given good oxygen content. As filter feeders, they glean algae and diatoms from the stream.

Blackfly larvae love to live in fast broken water. PT6

Adult Blackflies are sometimes called Buffalo Midges because of that shoulder hump. They can bleed you dry in June and July.

There’s a couple troublesome true flies in the Family Tabanidae. This little white larvae, pointed at both ends, is an immature Deer Fly (Chrysops sp.)

Deer Fly larvae live in quieter stream segments where they hunt for midges and such in the bottom muck. They pupate in the damp mud beside the stream in autumn & overwinter there. PT6

Deer Flies are about the size of a standard House Fly, but they have black wings broken by transparent bars, giving them a striped look when they land on your skin to inflict a painful bite.

Deer Flies have a habit of landing on high points, like our heads, so wrapping some sticky double-sided tape to your hat band will simply stick it to those persistent pests.

June and July are the peak swarming months.

This tubular white larvae is a bit bigger than a Deer Fly larvae, but belongs to another troublesome cousin called the Horse Fly (Tabanus sp.).

Horse Fly larvae live in shallow muddy stream bottoms where they eat small snails & other animal matter amongst the debris. They take two years underwater to mature. Pupate on land PT6

Adult Horseflies are big biters with bright green eyes. Once bit, you never forget.

This juicy-looking ringed grub will one day turn into a Crane Fly (Family Tipulidae).

Crane Fly larvae live in the mud along stream shores, either in or out of the water, where they feed on fungi and plant debris.

Crane Fly larvae crawl out of the water and pupate in the soft mud on the stream bank in autumn, emerging the following spring or summer. PT3

Crane Fly larvae are called Leatherjackets by some fishers since their tough skin helps keep them on the hook. Crane fly larvae are big enough to interest hungry Trout or Smallmouth Bass in mid-summer. Noted 2-19-13 Bohemian Creek.

Crane Fly adults often creep people out since they look like super-duper mosquitoes.

Crane Flies have very long fragile legs that often break off if caught.

Crane flies do not bite. In fact, they don’t even eat anything.

Watch for different species all summer long.

Though Crane Flies are most often found hiding in the dense vegetation along stream banks, they sometimes show up on our window screens back home.

Aquatic Beetle larvae, belonging to the Insect Order Coleoptera, can be confused with Caddisfly larvae.

This Riffle Beetle (Family Elmidae) larvae lives among bottom debris under rocks and feeds on organic detritus, meaning any disgusting junk on the stream floor.

Riffle Beetle larvae require lots of dissolved oxygen in the water to survive, and concentrate around rapids and riffles where that requirement has the best chance of being met. PT4

Riffle Beetles have long legs for a beetle and can resemble a slow-moving spider crawling over a rock in the creek.

Riffle Beetles eat diatoms and algae on rock surfaces.

Riffle Beetles are not very big, smaller than a pea.Compare with Caddis & Mayfly larvae

Riffle BeetleCaddis

Mayfly

This scary little larvae will one day turn into a Whirligig Beetle (Family Gyrinidae). They swim snake-like through the water and prey on water mites and other slower bugs.

Whirligig Beetle larvae cruise around in quieter river stretches with lots of rich organic matter around.

Whirligig Beetle larvae pupate in cocoons attached to submerged plant stems near shore. PT5

Adult Whirligig Beetles look like shiny little black stones floating on the water surface. They are also predaceous, and feast on snails, Midge larvae or other soft-bodied invertebrates.

Whirligig Beetles look like little motorboats tooling around in calm shallow spots along the stream. They can dive underwater for extended periods to find food or hide from danger, carrying along an oxygen tank in the form of an air bubble attached to their body.

Whirligig Beetles will often congregate in flotillas that spin wildly about like bumper cars

True Bugs, such as this menacing Giant Water Bug (Family Belostomatidae), belong to the Insect Order Hemiptera.

Nymphs look just like the adults, only smaller as they molt into older forms.

Giant Water Bugs are affectionately known as Eastern Toe-biters.

Small fish are included in their varied animal diet

Giant Water Bugs swim or crawl around in quieter waters while they hunt for food. PT6

Adult Giant Water Bugs can fly and sometimes show up around porch lights at night, hence another common given name “Electric Light Bug”. They can live for years and prey on tadpoles, small Frogs and even Salamanders. They fit comfortably in your hand.

Another big mean bug out there in the slower moving river waters is the stick-like Water Scorpion (Family Nepidae). Water Scorpions sit still in the water weeds and wait for Stoneflies or other aquatic insects to wander within range of those specialized front legs that work like long pincers.

Water Scorpions can row themselves through the water in slow-motion but they’re more likely to be found hiding in flooded grass close to shore. PT5Water Scorpions can grow to over two inches long and live at least a year.

Water Striders (Family Gerridae) are pretty cool True Bugs that are often confused as being water Spiders.But they only have six legs

Water Striders are also called Pond Skaters or my personal favorite – Jesus Bugs – since they can walk on water.

Notice how their legs never break through the fluid surface film.

Water Striders glide over still pools where they catch and devour mosquito larvae suspended under that busy surface film. PT6

True Bugs can be told from Beetles by looking at their backs. This Water Boatmen (Family Corixidae) has leathery wing covers rather than a hard shell. Notice how their paddle-like middle legs oar them through the water

Water Boatmen swim around where water weeds give them cover as they catch and eat algae.PT9

Look close and you can see that this supposed Water Boatmen is actually a Common Backswimmer (Family Notonectidae). They strangely swim around upside down and look like little torpedoes under water, but instead of rocketing around they move in herky-jerky motions.

Common Backswimmers have a lot of white on their backs, which would be seen from the stream bottom, so they blend right in with the bright surrounding sky. They eat animal matter. PT8

These last two aquatic invertebrates are not insects but they look like bugs and trout definitely relish them.

Freshwater Shrimp (Gammarus)are Arthropods In the Order Amphipoda.

Freshwater Shrimp are commonlyknown as Scuds or Sideswimmers, since they havea different way of swimming too

Freshwater Shrimp often swim on their sides. Most species are orange though some are pink and others are green. Trout that feed heavily on Gammarus have bright orange flesh, thanks to all that carotene contained in the scud’s carapace. Active all year.

Scuds require lots of water weeds to hide in and hunt for food. Being omnivorous, they munch on those water plants as well as smaller Mayfly larvae. Scuds can be super-abundant in such situations, and trout gorge on them, but good luck getting a lure in there. PT6

Scud patterns should be thought of as a must-have fly, tied up in orange and green for sure.

Aquatic Sowbugs (Asellus) are Arthropods in the Order Isopoda. They feed on algae, plant parts and even other immature invertebrate.Aquatic Sowbugs are commonly called Cressbugs.

Aquatic Sowbugs build up their biggest numbers in quieter waters with heavy weed growth. PT8

Aquatic Sowbugs are cousins to the terrestrial Cellar Sowbugs we all know as Pillbugs or Roly-polys.

Aquatic Sowbugs are seldom seen in the same numbers as Freshwater Shrimp, and they crawl around on the bottom or rocks rather than swim, so they’re not quite such a big target for feeding trout to hit.

Active all seasons.