Calories in Common Aquatic Insects (Trout Food Sources)
Midge Larva / Pupa (Chironomidae)
- Calories: ~0.02–0.06 calories each
- Very small, but extremely abundant
- High protein-to-fat ratio
- A primary winter food source for trout
- Use a small Perdigon or Brassie
Mayfly Nymph
- Calories: ~0.10–0.30 calories each
- One of the most important trout foods year-round
- Moderate energy return with minimal effort
- Use an appropriately sized Mr. Rapidan Bead Head , Jigged Head Hares Ear or Blowtorch (If the water is high, you need to increase the weight to get down. The larger of these flies have larger beads) If the water is low/ slow go with a lighter mayfly nymph such as a Murray’s Sulphur Nymph
Caddisfly Larva
- Calories: ~0.20–0.50 calories each
- Higher fat content than mayflies
- Especially valuable in colder water
- Use a #16 Caddis Larva , Caddis Pupae , Mr. Rapidan Soft Hackle
Stonefly Nymph
- Calories: ~0.50–1.20 calories each
- Large-bodied and energy-dense
- A single stonefly nymph can equal many midges
- Use a #16 Miracle Stonefly Nymph
Scuds / Sowbugs
- Calories: ~0.30–0.80 calories each
- Extremely high energy relative to size
- One of the most calorie-rich winter foods in tailwaters and spring creeks
- Use a Cress Bug or Murray’s Shrimp
Why Calories Matter to Trout (Especially in Winter)
In cold water (less than 40ºF) trout metabolism slows, and energy efficiency becomes critical. Trout prefer foods that offer the best calorie return for the least effort. That’s why winter trout often key in on:
- Small but abundant prey (midges)
- Slow-moving, bottom-dwelling insects (mayfly nymphs, caddis larvae)
- Occasional large, high-calorie meals (stoneflies)
Rather than chasing surface hatches, like you will see in water temperatures above the low 40º’s, trout feed opportunistically on benthic drift, selecting prey that drifts close to their holding position.
By definition; The benthic zone is the ecological region at the lowest level of a stream, or any body of water for that matter, including the sediment surface and some sub-surface layers.
What This Means for Anglers
- Smaller flies like a Perdigon or Brassie match the most common winter calories fish consume
- Heavier nymphs imitate calorie-rich bottom food
- Slow, near-bottom presentations align with how trout feed efficiently and opportunistically
- Break out your fluorocarbon tippet material, multi sized split shot and hand warmers
Midge larvae and pupae may not offer many calories individually, but they are present year-round and require very little effort for a trout to eat. In winter, when water temperatures drop below 40ºF and a trout’s metabolism slows, midges become one of the most important food sources available. Trout can feed efficiently without moving far from their holding water.
Insects like mayfly and caddisfly larvae fall right in the middle as far as bio availability in cold water. They provide a reasonable caloric return and are common in most trout streams. When these insects become dislodged from the bottom and drift naturally in the current, trout will feed on them with very little movement. This is why the correct leader and properly weighted nymphs that drift near the stream bottom are so effective.
Larger insects such as stonefly nymphs offer a higher caloric reward, but they are less common and often harder for trout to capture. Trout will eat them when the opportunity presents itself, but they won’t work hard to find them—especially in cold water.
Understanding the caloric value of aquatic insects has helped me choose better flies and fish them more effectively. At Murray’s Fly Shop, we stock proven nymphs, midges, and bead-head patterns that imitate these natural food sources and get down where the trout are feeding.
If you understand what trout eat—and why—they become much easier to catch.
(Caloric information taken from study recorded by Charlie Fox and Joe Brooks in conjunction with Penn State University c.1967)