Park City Area Fishing Report
For current river flow charts CLICK HERE
5/27/26
Understanding River Bugs: A Beginner’s Guide to the Insects Trout Eat
By Mayfly Guide Service
If you’ve ever gone fly fishing with a guide, you’ve probably heard someone say:
“The fish are eating PMDs.”
“There’s a caddis hatch coming off.”
“Match the hatch.”
And if you’re new to fly fishing, you may have wondered:
What does any of that even mean?
The truth is, understanding river bugs — also called aquatic insects — is one of the most important parts of fly fishing. But it doesn’t have to be complicated.
In this guide, we’re going to break down the life cycle of river bugs in a simple way that actually makes sense, especially if you’re brand new to fly fishing.
Why River Bugs Matter
Trout spend most of their lives eating bugs.
Not minnows.
Not giant prey.
Mostly bugs.
The majority of a trout’s diet comes from aquatic insects that live in rivers and streams. These bugs hatch, drift, swim, crawl, and fly around the river every single day.
Fly fishing is simply our attempt to imitate those bugs well enough to fool a trout.
Once you understand:
The Basic Life Cycle of River Bugs
Most river insects go through four major stages of life:
Different bugs go through slightly different versions of this process, but the general idea stays the same.
Stage 1: Eggs
Everything starts with eggs.
Adult insects land on or near the water and lay eggs in the river. Those eggs sink to the bottom and eventually hatch into immature insects.
At this stage, trout usually are not feeding heavily on them because the eggs are tiny and difficult to notice.
Stage 2: Nymphs and Larvae
This is where most bugs spend the majority of their lives.
Depending on the insect, they’re called:
Most trout feed underwater most of the time.
That’s why nymph fishing is usually the most productive way to catch fish.
When anglers fish with:
Stage 3: The Emerger Stage
Eventually, the bugs need to become adults.
To do that, they rise from the bottom of the river toward the surface.
This stage is called the emergence.
And honestly, it’s one of the most vulnerable moments in a bug’s life.
They struggle.
They drift helplessly.
They get stuck in the current.
Trout know this.
During a hatch, fish often feed aggressively on emerging insects because they’re easy targets.
This is why emerger flies can work incredibly well.
Sometimes trout aren’t eating bugs floating neatly on top of the water — they’re eating the insects trapped just below the surface.
Stage 4: Adult Bugs
Once the insects reach the surface, they hatch into adults and fly away.
This is the stage most people picture when they think about fly fishing:
And yes — this is the exciting part everybody loves.
Watching a trout rise and eat your fly on the surface never gets old.
The Three Most Important River Bugs
You do not need to memorize hundreds of insects to become a good fly angler.
Start with these three:
MayfliesMayflies are the classic fly fishing insect.
They usually have:
Common mayfly imitations include:
Caddisflies
Caddis are everywhere in western rivers.
As larvae, they look worm-like underwater.
As adults, they resemble small moths fluttering above the water.
Caddis hatches can create some of the most exciting dry fly fishing of the year.
Midges
Midges are tiny insects that hatch year-round.
Even though they’re small, trout eat enormous numbers of them — especially during colder months.
If you’ve ever heard guides talking about:
“Matching the Hatch”
This phrase gets thrown around constantly in fly fishing.
All it really means is:
Try to imitate the bugs trout are currently eating.
That could mean:
If trout are eating emerging insects, you fish emergers.
If trout are rising to adults on the surface, you fish dry flies.
It’s really just observation.
You Don’t Need to Be an Entomologist
One of the biggest misconceptions in fly fishing is that you need to become a bug scientist to catch trout.
You don’t.
You simply need to understand:
The rest comes with time spent on the river.
Final Thoughts
The next time you’re standing in a river, flip over a rock and take a look underneath.
That little world crawling around below the surface is what drives almost everything in fly fishing.
Understanding river bugs helps you:
Because once you understand the bugs, the whole river starts to make a lot more sense.
We’ll see you on the water.
Understanding River Bugs: A Beginner’s Guide to the Insects Trout Eat
By Mayfly Guide Service
If you’ve ever gone fly fishing with a guide, you’ve probably heard someone say:
“The fish are eating PMDs.”
“There’s a caddis hatch coming off.”
“Match the hatch.”
And if you’re new to fly fishing, you may have wondered:
What does any of that even mean?
The truth is, understanding river bugs — also called aquatic insects — is one of the most important parts of fly fishing. But it doesn’t have to be complicated.
In this guide, we’re going to break down the life cycle of river bugs in a simple way that actually makes sense, especially if you’re brand new to fly fishing.
Why River Bugs Matter
Trout spend most of their lives eating bugs.
Not minnows.
Not giant prey.
Mostly bugs.
The majority of a trout’s diet comes from aquatic insects that live in rivers and streams. These bugs hatch, drift, swim, crawl, and fly around the river every single day.
Fly fishing is simply our attempt to imitate those bugs well enough to fool a trout.
Once you understand:
- what bugs trout eat
- where bugs live
- and how bugs move through the water
The Basic Life Cycle of River Bugs
Most river insects go through four major stages of life:
- Egg
- Larva or Nymph
- Emerger
- Adult
Different bugs go through slightly different versions of this process, but the general idea stays the same.
Stage 1: Eggs
Everything starts with eggs.
Adult insects land on or near the water and lay eggs in the river. Those eggs sink to the bottom and eventually hatch into immature insects.
At this stage, trout usually are not feeding heavily on them because the eggs are tiny and difficult to notice.
Stage 2: Nymphs and Larvae
This is where most bugs spend the majority of their lives.
Depending on the insect, they’re called:
- Nymphs (like mayflies and stoneflies)
- Larvae (like caddisflies and midges)
- under rocks
- in gravel
- in weeds
- in slower seams and riffles
Most trout feed underwater most of the time.
That’s why nymph fishing is usually the most productive way to catch fish.
When anglers fish with:
- pheasant tails
- zebra midges
- hare’s ears
- perdigons
Stage 3: The Emerger Stage
Eventually, the bugs need to become adults.
To do that, they rise from the bottom of the river toward the surface.
This stage is called the emergence.
And honestly, it’s one of the most vulnerable moments in a bug’s life.
They struggle.
They drift helplessly.
They get stuck in the current.
Trout know this.
During a hatch, fish often feed aggressively on emerging insects because they’re easy targets.
This is why emerger flies can work incredibly well.
Sometimes trout aren’t eating bugs floating neatly on top of the water — they’re eating the insects trapped just below the surface.
Stage 4: Adult Bugs
Once the insects reach the surface, they hatch into adults and fly away.
This is the stage most people picture when they think about fly fishing:
- trout rising
- dry flies floating
- fish eating on top
- Mayflies
- Caddisflies
- Stoneflies
- Midges
- Some float calmly
- Some skitter across the water
- Some struggle to take off
And yes — this is the exciting part everybody loves.
Watching a trout rise and eat your fly on the surface never gets old.
The Three Most Important River Bugs
You do not need to memorize hundreds of insects to become a good fly angler.
Start with these three:
MayfliesMayflies are the classic fly fishing insect.
They usually have:
- upright wings
- skinny bodies
- long tails
Common mayfly imitations include:
- PMDs
- Blue Winged Olives
- Pale Evening Duns
Caddisflies
Caddis are everywhere in western rivers.
As larvae, they look worm-like underwater.
As adults, they resemble small moths fluttering above the water.
Caddis hatches can create some of the most exciting dry fly fishing of the year.
Midges
Midges are tiny insects that hatch year-round.
Even though they’re small, trout eat enormous numbers of them — especially during colder months.
If you’ve ever heard guides talking about:
- zebra midges
- chironomids
- tiny black flies
“Matching the Hatch”
This phrase gets thrown around constantly in fly fishing.
All it really means is:
Try to imitate the bugs trout are currently eating.
That could mean:
- matching the size
- color
- shape
- or stage of the insect
If trout are eating emerging insects, you fish emergers.
If trout are rising to adults on the surface, you fish dry flies.
It’s really just observation.
You Don’t Need to Be an Entomologist
One of the biggest misconceptions in fly fishing is that you need to become a bug scientist to catch trout.
You don’t.
You simply need to understand:
- trout eat bugs
- bugs live in stages
- and trout feed differently during each stage
The rest comes with time spent on the river.
Final Thoughts
The next time you’re standing in a river, flip over a rock and take a look underneath.
That little world crawling around below the surface is what drives almost everything in fly fishing.
Understanding river bugs helps you:
- choose better flies
- understand trout behavior
- fish more confidently
- and appreciate rivers on a deeper level
Because once you understand the bugs, the whole river starts to make a lot more sense.
We’ll see you on the water.
