Module 2 · Lesson 2
Basic Trout Food and Insects
Learn the major food sources trout rely on and the basic insect groups every fly angler should recognize.
What You’ll Learn
- The major food sources trout eat in rivers and streams.
- The four aquatic insect groups beginners should recognize first.
- The basic life stages of mayflies, caddisflies, stoneflies, and midges.
- Why insect life stage matters when choosing a fly.
- How terrestrials, baitfish, crustaceans, worms, and eggs fit into a trout’s diet.
- How to observe the water and identify what trout may be feeding on.
- Why exact insect identification is less important than size, shape, behavior, and location in the water column.
Quick Answer
Trout are opportunistic feeders. They eat aquatic insects, land-based insects, baitfish, crustaceans, worms, eggs, and other available food.
For a beginning fly angler, the four most important aquatic insect groups to recognize are:
- Mayflies
- Caddisflies
- Stoneflies
- Midges
You do not need to identify every species. Start by recognizing the general insect group, approximate size, life stage, and where the food is in the water.
Main Answer
Trout Eat More Than Insects
Fly fishing is often associated with matching aquatic insects, but trout eat a much wider range of food.
Depending on the river, season, water temperature, and size of the fish, trout may feed on tiny midge larvae one moment and chase a baitfish the next.
Mayflies, Caddis, Stoneflies and Midges
These insects spend part or most of their lives in the water and make up a major portion of a trout’s diet in many rivers.
Ants, Beetles and Grasshoppers
Land-based insects can become important food when they fall or are blown into the river.
Baitfish, Leeches and Crayfish
These larger food sources can be especially important to bigger trout and are commonly imitated with streamers.
Scuds, Sowbugs, Worms and Eggs
Many trout eat these food sources when they are naturally available in the river or lake system.
Why Aquatic Insects Matter So Much
Aquatic insects live in and around the river, which makes them a reliable food source. Many species spend months or even years underwater before becoming winged adults.
This means trout often encounter immature insects much more frequently than adult insects on the surface.
One of the most important beginner lessons: if you do not see fish feeding on the surface, there is a good chance they are feeding below it.
Mayflies
 - Guelph, Ontario 01.jpg)
Mayfly photo: Ryan Hodnett, CC BY-SA 4.0, via Wikimedia Commons. View source and license
Mayflies are one of the most recognizable and important trout-stream insects.
Their basic life cycle can be simplified as:
The nymph lives underwater. When it is ready to emerge, it rises toward the surface. The newly emerged winged adult is commonly called a dun. After mating, the adult returns as a spinner to lay eggs.
What Mayflies Look Like
Adult mayflies typically have upright wings and two or three tail filaments. Their body shape is relatively slender.
Mayfly nymphs vary widely, but many have distinct tails and a tapered body.
Caddisflies
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Caddisfly photo: U.S. Fish & Wildlife Service Southeast Region / Kevin Moulton, CC BY 2.0, via Wikimedia Commons. View source and license
Caddisflies are another major trout food source. Their life cycle is different from a mayfly’s.
Many caddis larvae live along the bottom of the river. Some build protective cases from small rocks, sticks, or plant material, while others remain free-living.
When they mature, they become pupae and move toward the surface. Adult caddisflies have tent-shaped wings that lie over the body.
Why Caddis Can Be Exciting to Fish
Adult caddisflies often move more actively than mayflies. Instead of sitting quietly on the surface, they may skitter, flutter, or bounce across the water.
This activity can produce aggressive rises from trout.
Stoneflies
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Stonefly photo: Bob Henricks, CC BY-SA 2.0, via Wikimedia Commons. View source and license
Stoneflies are generally associated with clean, well-oxygenated water.
Unlike mayflies and caddisflies, stonefly nymphs usually crawl toward shore, rocks, or vegetation before emerging rather than rising through the water column.
Large stoneflies can provide a substantial meal for trout. Golden stones and salmonflies are well-known examples.
Midges
Midge photo: Judy Gallagher, CC BY 2.0, via Wikimedia Commons. View source and license
Midges are tiny insects that can be extremely important to trout, especially during colder months and on tailwaters.
Midge larvae and pupae are often very small, which is why trout anglers frequently use tiny flies when imitating them.
Do not underestimate small food. A trout may feed steadily on tiny midges because thousands of them are available, even when a larger meal would seem more efficient.
Emergers
An emerger is an insect transitioning from its underwater stage toward adulthood.
This stage is important because the insect is often vulnerable and may struggle near the surface. Trout can become highly focused on emergers during a hatch.
If fish appear to be feeding at the surface but are not clearly taking adult insects, they may be eating emergers just beneath the surface film.
Terrestrials
Terrestrials are land-based insects rather than aquatic insects.
Common trout-food terrestrials include:
- Ants.
- Beetles.
- Grasshoppers.
- Crickets.
- Cicadas.
Terrestrials can become especially important during summer and early fall when land-based insect activity is high.
Scuds and Sowbugs
 (Unidentified gammarid shrimp) - (imago), Lent, the Netherlands.jpg)
Scud photo: Wikimedia Commons contributor, via Wikimedia Commons. View source and license
Scuds and sowbugs are small crustaceans rather than insects.
They can be important food sources in spring creeks, tailwaters, lakes, and other waters with abundant aquatic vegetation.
Worms and Eggs
Aquatic worms can become available when flows disturb the riverbed or when high water moves them into the current.
Trout eggs can also become an important seasonal food source when fish are spawning.
Anglers should avoid disturbing actively spawning fish or walking through visible spawning beds, often called redds.
Baitfish and Other Large Prey
Trout—especially larger trout—may feed on minnows, sculpins, young fish, leeches, crayfish, and other substantial prey.
These food sources are commonly imitated with streamers.
You Do Not Need to Become an Entomologist
Learning insect names can be useful, but a beginner does not need to identify every species.
Start with four questions:
- What general type of food is present?
- How big is it?
- Where is it in the water column?
- How is it moving?
Size, life stage, location, and behavior are often more important than knowing the exact scientific name of the insect.
How to Investigate What Trout Are Eating
Before selecting a fly, take a moment to observe the river.
- Look at the surface for adult insects.
- Watch for rising fish.
- Look at rocks near the water for nymphs or insect cases.
- Check vegetation along the riverbank for terrestrials.
- Notice baitfish activity in shallow water.
- Pay attention to the size and color of the insects you see.
Experienced anglers constantly collect these clues and use them to narrow down their fly selection.
Match the Food Before the Fly Pattern
Instead of asking, “What exact fly should I use?” begin with:
“What are the trout most likely eating right now?”
Once you understand the food source and life stage, choosing an appropriate fly becomes much easier.
Frequently Asked Questions
What are the four main aquatic insects trout anglers should know?
Mayflies, caddisflies, stoneflies, and midges are the four major groups beginners should learn first.
Do I need to identify the exact species of insect?
No. For a beginner, recognizing the general insect group, approximate size, life stage, and where it is in the water is usually more useful.
Why do trout eat so many nymphs?
Many aquatic insects spend most of their lives underwater, so subsurface food is available to trout far more often than adult insects on the surface.
What is the difference between a nymph and a larva?
Mayflies and stoneflies develop through a nymph stage, while insects such as caddisflies and midges develop through larval and pupal stages before becoming adults.
What is an emerger?
An emerger is an insect transitioning from its underwater stage toward adulthood. Trout often feed heavily on insects during this vulnerable stage.
Are grasshoppers and ants important trout food?
Yes. These are terrestrials—land-based insects that can become important food when they fall or blow into the river, especially during warmer months.
Do trout really eat other fish?
Yes. Larger trout can eat minnows, sculpins, juvenile fish, and other prey. Streamers are commonly used to imitate these larger food sources.
When are midges most important?
Midges can be important throughout the year and are especially valuable during colder periods and on tailwaters where other insect activity may be limited.
What is a hatch?
A hatch is the period when aquatic insects transition toward their winged adult stage. Trout may feed on several life stages before, during, and after the visible hatch.
What should I look at first when trying to match the hatch?
Start with size and life stage. Then consider shape, color, where the insects are in the water column, and how they are moving.