Trout eat a large number of insects (both terrestrial and aquatic) also other fish, leeches, worms even crustaceans. Some of the most important food for trout that fly fishermen need to understand is the aquatic insects that spend the majority of their time in larval and nymph stage underneath the water in rivers and lakes.
You also need to be able to identify local insects and select a good fly fishing pattern that resembles what’s around on the river you’re fishing. Fly fishing entomology can sound intimidating, but it’s not that hard to get your head around, and you’ll quickly see the results of a bit of knowledge about the things trout would eat.
If you’re just looking for the best fly fishing flies then check out our article on that.
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What Do Rainbow Trout Eat?
If you want to know what trout eat, the first thing you need to ask yourself is: How do trout eat? Well, trout are opportunistic and often lazy fish. They don’t like using too much energy to eat, and they aren’t fussy either. Trouts would even eat snails off the riverbed! A trout’s main concern is to feed itself without expending too much energy or moving about too much. That’s all good news for you because it means you don’t need your patterns to be identical to the fish around. But if you can identify what the trout are eating , all the better.
So, what does a trout tend to eat? What are their main food sources? Well, trout food generally includes terrestrial and aquatic insects, as well as other fish, worms, and even mice! Basically, almost anything that comes their way, trouts will have a go at. Trouts do even prey on other younger trout, especially if they seem to be struggling.
So let’s dive in a little deeper  and learn more about the main trout parts of the trout diet, from mayfly to midges, caddis, and more:
Midges look a bit like mosquitoes, but they don’t bite. You can find midges down at the water all year round, and even though they’re small, midges are one of the most common foods that trout eat. Midges have four stages: the midge nymph, the midge emerger, the adult midge, and the spinner. The spinner is less important and rarely imitated to fish with.
Midge nymphs look like segmented little worms, often red in color. You can have a lot of success by fishing a midge larvae pattern in the water column.
Midge pupa or emergers have an air bubble and often form a U-shape just before they reach the surface of the water. If you want to replicate a midge emerger, try using a Mercury Midge or a Smokejumper.
The last relevant stage in the midge life cycle is the adult midge or dry. It can be tricky fishing with a midge dry pattern, so consider using a cluster pattern instead. A cluster pattern looks to the trout like a group of midge clustered together to mate, and can be much more tempting than a single midge dry – meaning you’ll catch more fish!
Mayflies are a staple in the diet of a trout. They go through four stages, from nymph to emerger to adult and then spinner. Mayflies’ color and size can vary, but you can tell a mayfly by its two vertical, transparent wings.
So, what does a mayfly nymph look like? The nymphs have two to three tails, long legs, and short antennae. There are four main types of mayfly nymphs: Burrowers, Clinger, Crawlers, and Swimmers. You can fish all types with the same pattern – you just need to alter the depth and method you fish with it. Try drawing a burrower along the riverbed, while for swimmers, you can fish them like a streamer.
It’s also useful to be able to identify a mayfly emerger, which has a shuck. The emerger does tend to be found on top of, or just under, the water.
Mayfly adults or duns have dull coloring and intact wings and float on the water surface, but they don’t tend to stay still long. Mayfly spinners are the dying mayflies that have mated and fall onto the river surface. This does make them easy prey for the waiting trout. Many patterns look similar to both the mayfly adult and the spinner and can double up as both.
There are some great patterns out there to mimic mayflies, including the Pheasant Tail, the Parachute Adams, and the Adams Dry Fly. If you’re curious about learning more about the mayfly, check out this video.
Caddisflies are one of the most common insects you’ll find in trout waters. During their lifecycle, they change from egg to larva to pupa, and then to adult caddisfly. Caddisfly larvae can be found on the stream bottom, and you’re actually more likely to find them than mayfly larvae as they’re less sensitive to water pollution. The larvae remain in this stage normally for about a year.
Although there are many different caddis species, you just need to know the two main types: cased caddis and free-ranging caddis. Cased caddis use sticks, stones, pebbles, and other debris from the riverbed to create a case around themselves, and are normally found stuck to rocks.
Free-ranging caddis look like small green worms and are the main type that trout would eat, as they are more often found in the water column. The best flies to use to imitate these caddis larvae are Green Rockworms, Green Weenies, or green Serendipities. You’ll need to stock up on sizes 12-18.
What do caddisfly behave like? Well, their behavior differs from other aquatic insects slightly. When a caddisfly emerges from its pupa, it will rise to the river surface and wait until it dries and can fly away. During this time, it’s sitting prey for hungry trout. There are some great patterns you can use to trick the trout into thinking they’ve spotted a caddis emerger, including the CDC Caddis Emerger, LaFontaine’s Sparkle Pupa, or Fox’s Caddis Poopah.
Adult caddisfly can often be identified if you see the tell-tale skittering on the water’s surface. They do this just after they hatch and also when laying eggs. Adult caddisfly tend to be gray or brown shades, but occasionally you’ll get a green or even a black caddisfly. If you see lots of adult caddisfly where you’re fishing, give an Elk-hair Caddis pattern or a Henryville Special a go, and see if you get any bites!
Stoneflies are larger insect specimens that can be a great food source. You tend to find them in and around fast-flowing, cold water. You can tell an adult stonefly by its four long, shiny wings and its lack of skill at flying.
The stonefly nymph has a flat body, two short tails, and sturdy legs which it uses to hang onto rocks in the fast-flowing water. The maturing stonefly nymph doesn’t rise to the surface in the same way that mayflies and caddisflies do. Instead, it crawls along the riverbed and climbs out onto the bank, so this stage isn’t as useful for fly fishers. If you’re looking around to see what the local bugs are doing, check under a few rocks at the edge of the stream to check for any stonefly nymphs.
The type of stoneflies varies from area to area. The most common is the salmonfly, followed by Golden Stones, Little Black Stones, and Skwalas. If you see salmonfly, your best bet is a Rogue Foam Stone in size 4-8 for the adult, or a black Kaufmann’s Stone for the salmonfly nymph.
During the summer and fall when there are fewer aquatic insects and larvae to feed on, trout tend to eat more terrestrial insects that end up in the water.
Grasshoppers are a favorite trout food, as they pack a decent amount of energy in one go. If you’re wondering, how do trout catch grasshoppers, the answer is that hoppers simply aren’t great fliers! They will often fall into the water and into the path of a waiting trout. If you spot any grasshoppers where you’re fishing, note the color and size and try to match this to your fly. You might have luck with a Joe’s Hopper, a Club Sandwich, or a Turck’s Tarantula pattern.
Ants and beetles are other terrestrial insects that brown trout love to eat. For ants, make sure you use ant patterns that have two clear body lobes. Beetle and ant flies can be hard to see out on the river, especially when a beetle rides low in the water. A good tip: Use an imitation that has a bright indicator to make it easier to spot, or alternatively, you can use a grasshopper fly as an indicator.
Other terrestrials that the browns will eat if they land in the stream include inchworms, crickets, spruce moths, and cicadas.
Crustaceans such as crayfish, sowbugs, scuds, and shrimp are another major food source for small to large trout. It’s not just bass who go crazy for crayfish!
Crayfish and other crustaceans may be harder to spot, but if they’re around, they are one of the trouts’ favorite foods! Crayfish are most likely to be found in warm, fertile, waters rather than fast-flowing streams in the mountains.
You’re most likely to find scuds swimming through the water column in lakes and tailwaters. They can be brown, green, or a rusty shade – their color tends to match that of the vegetation in the water where they live. With seven pairs of legs, antennae, and a segmented exoskeleton, they’re easy to identify. Sowbugs look a bit similar to scuds, but they would be found scuttling along the streambed in shaded areas.
If you want some tips on shrimp or scud fishing, you should check out this handy video which is filled with useful info advice. Brian Chan demonstrates some of the best patterns for scud and shrimp, plus how to fish them for the best success!
Now, the larger trout grow, the more food they need to fuel them every day. When they get to a certain size, just eating insects doesn’t seem to cut it anymore. From this stage on, they tend to eat other fish more, just like bass do.
Large trout (and even medium-sized ones who are feeling a bit greedy) will eat bullheads, sculpins, chubs, and minnows. They even eat smaller trout, particularly when the smaller fish seems to have got into trouble and is drawing attention to itself.
Sculpins can be found in most trout waters in the US. They are small fish which tend to live in rocky areas and feed off the bottom. If you want to imitate a sculpin or bullhead, try a Bow River Bugger, a Muddler Minnow, or a Zoo Cougar.
The best flies to use to imitate Clouser Minnow and the well-known Woolly Bugger.
The Wrap Up
So there you have your basic guide on what trout eat, and the things you’ll need to think about to work out what the brown trout are eating in your region. From caddis to stoneflies, terrestrials to snails, you know all about a trout’s feeding habits and how to use this info to choose the right flies. At first, it can seem confusing, but it doesn’t take long to understand this important info that will take your trout fly fishing to the next level.
We hope you’ve enjoyed this article and found it helpful! As always, feel free to drop us a comment or leave any questions below. Happy fly fishing!