Trout feeding patterns

For the fly angler who is seriously trying to maximize his/her catch rate and success, having a selection of beetle patterns in their fly box is a necessity. Insects which live on land and are fed on by trout only when they incidentally fall into the water are known as "terrestrials" to fly anglers, and they're very important in during the summer months. It may seem counterintuitive that a trout would select a terrestrial insect over an aquatic insect which shares its habitat. However, many studies have shown that this is just the case. The results of many studies have shown that, when identifying the food found in the stomachs of trout or grayling, a significant proportion of their diet consists of land-bred insects that have been blown or have falling off overhanging vegetation onto the water. In oligarchic (nutrient poor) lakes in Britain and Ireland, over 90% of surface foods taken are land bred. The answer to this question lies in the trout’s prey search image. Trout have often been described as opportunistic predators, feeding on any type of invertebrate drift prey which passes them in the current. However, as trout live in a very demanding habitat, they must also ensure that they maximize their net energy gain. They do this by selecting the most abundant prey which is available and most easily acquired at any one time. By selecting the most abundant prey they can acquire the search image required to locate that prey type, whilst ensuring that they expend as little energy as possible in its acquisition. As most anglers should know, this is why matching the hatch is very important. Most river anglers will be aware of the usually hatches that occur during the season (Baetis rhodaniSerretella ignitaCaenis spp., Plecoptera [Stoneflies], Trichoptera [Caddis] etc.). Fly selection during these hatches should resemble these species in as far as possible. However, these hatches are usually short lived, except for certain species like Baetis rhodani which can have 5 cycles in a year. One must keep in mind that unlike hatches of mayflies and sedges, beetles and other terrestrial prey items are usually available to trout all year round. As mentioned earlier, terrestrial prey (such as beetles, terrestrial flies) can contribute more to a trout’s diet during certain periods of the year. This is because they provide the greatest net energy gain during these periods. During summer months, especially when evening hatches dominate a trout’s diet, beetles must be considered by the trout as a relatively abundant surface floating prey.

Terrestrial prey items

When considering terrestrials, the first species most anglers will think of are beetles. Other prey items such as worms, terrestrial flies, and other small invertebrates are also taken as prey by trout during certain periods. For example, trout have been known to gorge themselves on annelid worms during periods of high rainfall and spate conditions.

Terrestrial fishing provides a very good opportunity to catch trout during the summer months when the main mayfly and sedge hatches are confined to early morning or late evening. The best terrestrial fishing usually occurs during midday, as this is the period when most animals are active. The main causes of bankside terrestrial invertebrates falling in are rain, wind and a lack of attention when moving around. These terrestrials that end up on the water's surface near the banks provide a valuable prey item for trout during the summer months.

To provide a description of the entire terrestrial prey items available to trout would be a rather large undertaking. Instead I will give a quick mention to the most common types found in autopsies and the most appropriate fishing techniques which should be used for each insect.                                        

or Thrip: a tiny (2.5mm) black insect with two pairs of feather-like wings. These usually occur in vast numbers in hot, thundery conditions. I’m sure I am not alone when I say that most anglers have been pestered by these flies at some stage during the summer season. When they do occur, fishing a single small (size 20-22) black dry fly can provide some good results. However, like Caenis spp., their small size can often make it difficult to entice a trout to your fly in favour of the many other real flies available.

Green grasshopper: up to 20mm, commonly found in riparian grassland in late summer-autumn, and where abundant often leaps accidentally into water. Fishing a variety of hoppers can often provide very enjoyable sport. My favourite hoppers would be claret/olive/black hoppers (usually very effective on lakes during late summer and autumn); foam hoppers (such as the Tan-Black Chernobyl Hopper pictured on the right) can also be a very effective fly when fished on the surface creating a disturbance.

Green shield bug: length 13mm. Found mainly in late summer and autumn having falling from overhanging vegetation. The very similar, but copper-bronze, Gorse bug is commonly found in the stomachs of Irish trout in late summer. These insects usually require a more imitative pattern. I find a size 12-14 pattern with a green body and a lightly dressed black hackle is probably the most effective imitation. As these insects fall in from the banks, the angler should remember that this is where the trout are most likely to be searching for the insect. Therefore, this imitation should be fished dry in close to the banks and beside vegetation to provide the best results. 

Hawthorn fly: a 12mm long black dipteran with long trailing hind legs, commonly on the water in late April. The similar Heather fly has a red femur and is blown from heather in late summer. The Hawthorn is a fly famous with many river and lake fly angler alike. Their emergence (usually one of the first more worth while terrestrial hatches) is usually waited upon with considerable excitement as it can provide some very effective fishing. I usually fish the hawthorn during this period but other flies such as the Coch-y-bondu or the black & peacock spider can also be effective imitations. These can be fished wet or dry. Very often, fished in a team of wets is a very rewarding method, both on river and lake.

Dung fly: length 8-10mm. Most anglers will associate this fly will cow pats on pasture land during the summer months. Where they occur they usually occur in vast numbers. Fishing a dung fly imitation is probably most effective when fished on windy days when the wind would be the only cause of blowing the natural flies into the water. I usually fish these dry with a relatively long leader, however, they can also be fished sub surface in a team of wets. As the real fly is quite large one can afford to fish a size 14-16. The fly I usually fish can be seen to the right.

Heather, Willow and Oak beetles: these are all around 4-7mm in size and usually occur on heather moor, willows of Salix spp., and oak Quercus woodland respectively. These are quite varied and often only vary in colour. I find fishing a size 14-16 foam beetle of one of three colours olive, black and golden olive (depending on the colour of the most abundant beetle in question), is usually the best selection. On rivers, I always fish a single beetle dry close to the banks. After the end of the evening rise, this fly can also be very effective in creating a disturbance on the surface which can often entice a big trot unto take. On lakes, this is a very good fly to fish close to the shoreline. I usually fish a foam beetle on the top dropper followed by two wets beneath it. My theory is that the trout’s attention is caught by the rippling beetle and quite often you will find that the second dropper or point fly takes the fish. This is a very exciting technique to use.

Wood ant: length c.10m. On warm, windy days in summer and autumn, mating swarms of this and other ants are often blown onto the water. Fishing dry ants during a summer hatch in July/August can be a very effective method also. I usually find black or red ants the most effective but other colours also work. The most important factor to consider when fishing an ant, either dry or wet, is that the body should be distinctively segregated with a distinct head and abdomen separated by a skinny body in between.


As can be seen there are many forms of terrestrial prey items available to trout, especially during the summer months. For the angler that just isn’t satisfied with waiting until the evening rise then imitating the most available terrestrial invertebrate is a must. These can provide some very good results during periods when the conventional angler would not usually wet a line.