Welsh River Insect Species

On The Menu – Welsh River Insect Species

The following insect species provide an important food resource for trout, grayling and coarse fish on Welsh rivers. A sign of a clean river environment, these invertebrates are found on many waters in Wales. Should you see them you know the river is in good health!

We hope this visual guide will help you ‘match the hatch’ and catch more fish.

Up-wing fly species:

Large Dark Olive – Baetis rhodani

These emerge throughout the winter even on the coldest days, then into spring and again from late summer through autumn. In the winter and early spring, look out for these either side of mid-day into the early afternoon.

March Brown – Rhithrogena germanica

A large fly usually seen from March to mid-April; often several brief hatches each day from mid-morning onwards, when hatches may coincide with those of Grannom. Given the potential brevity of such hatches, be prepared with a rod already set up with an appropriate imitation! Historically, and still, the iconic fly of the River Usk, March browns are found on many other rivers in Wales. To avoid possible but unnecessary confusion of identity, see Large Brook Duns below.

Large Brook Dun – Ecdyonurus torrentis

Usually seen from May onwards, and from then well into the summer and early autumn. An upwing, somewhat larger than the March Brown, but often and unnecessarily confused with it. Both have heavily veined wings but only the March Brown has dark oval marks on each femur, and only the Large Brook Dun (LBD) has yellow pigmentation on the leading edges of its wings. Like Yellow May Duns, LBDs tend to trickle hatch and may often be seen drifting down river margins as well as in more open water.

Yellow May Dun – Heptagenia sulfurea

An unmistakable, beautiful, sulphur-yellow up-wing with an orange thorax, usually seen from early May onwards. Often seen from mid-morning but can emerge earlier in the day. A trickle-hatching species that can still be seen until early October. It has often been said that “fish don’t like Yellow May Duns” but this is simply not true. Duns may often be seen passing unmolested whilst fish rise to something unseen – such fish are likely to be feeding on Yellow May emergers (YMEs). Unlike most Ephemeroptera, Yellow Mays are almost, but not quite, unique in that they can shed the nymphal shuck sub-surface before rising to the surface as true emergers, rather than classic Ephemeroptera spp. nymphs which undergo eclosion at the surface. Fish love YMEs!

Olive Upright – Rhithrogena semicolorata

A major species generally seen from May and into June but can be earlier. Like Yellow May Duns, a medium to large-sized fly, but with an olive body, clear upright wings and unmistakeable dark oval patches (like March Browns) on the femurs. Often seen in and downstream of riffles, from where they hatch. Fish love them!

Iron Blue Dun – Baetis muticus

This fly has a reputation for emerging on cool or rainy days in late spring or early summer, when you may be foxed by trying to represent other more obvious or more likely flies; then you may find these little inky dark flies stuck in the film. It is certainly worth having an imitation on hand.

The “classic” mayfly – Ephemera danica

Green Drake mayflies are the iconic species of southern English chalk streams but are being seen increasingly on Welsh rivers, as result of increased siltation. This is a species regularly seen, especially during May and June, but later as well. They are notably prevalent on the river Monnow and its tributaries, on the Lugg and Arrow, on the Llynfi in Powys and increasingly on the Usk. Mayfly are also found in the middle Wye and tributaries such as the Eddw, and on streams in the Vale of Glamorgan such as the Thaw.

Blue winged olive – Ephemerella ignata

As nymphs, these are classified as Moss Creepers. They live on mossy stones and are not agile darters, but laboured swimmers with a distinctive, slow, rocking style of movement. They metamorphose into a small to medium sized fly which can be easily distinguished from Baetis spp. by having three and not two tails. Emergence can be during daylight hours but is more characteristically associated with evening and dusk. As with all Ephemeroptera up-wing species, spent spinners of all sizes make easy prey, and those of the BWO.

Smaller upwings

As late-spring merges into summer, so smaller olives (Baetis spp), pale wateries (Baetis fuscatus) and spurwings (small: Centroptilum luteolum and large: Procleon pennulatum) predominate as the large hatches of the bigger up-wings diminish, though these smaller up-wings will have already been around for a month or so. Pale wateries are a major food item throughout the summer and into the autumn, when grayling love them!

Caddis Fly Species:

Grannom – Brachycentrus subnubilis

The first sedge of the year, appearing during April and whose emergence is triggered by sunshine, often early in the day. Don’t confuse blizzards of Grannom seen drifting upstream with a hatch, as these are generally pregnant females, which emerged a day or two earlier, returning to lay eggs. Fish feed on Grannom cripples, spent adults, pupae and emergers, from which you will see adults popping out of the surface film accompanied by rising fish.

Sedges – Trichopetera spp

Whilst Grannom constitute one of the earliest food items of the year, there are many other species of sedges / caddis which, as cased or caseless caddis larvae, are on the menu all year round. After pupation and emergence, skittering sedges with their roof-shaped wings, along with spent spinners, become a classic item on the piscatorial menu in summer evenings.

Mini-caddis – Agapetus spp

Worth looking out for anywhere, especially in the margins. Agapetus ochiripes is the species found on the Usk around Abergavenny. Their homes are those millions of small, gravel-encrusted dwellings on the edges of almost every marginal rock in the river. Inside each is a small ~5mm larva which, after pupation, becomes a mobile, pharate adult which swims to the bank, or onto an above-surface prominence (including your boots and waders!), where it becomes a small, grey-winged sedge. Fish, and often the bigger ones, will be found sipping in the margins, early morning and late evening in May, June and into July, on “in-transit” pharate adults.

Other species:

Midges – Chironomids

Somewhere on our rivers throughout the year and significant during summer months. Big fish, often in long slow glides, can sip on these small (3-5mm) but abundant items of food.

Stoneflies – Plecoptera spp

These are species of well-oxygenated and so faster-flowing, more rocky waters which live their sub-surface lives as frequently moulting nymphs, which are primarily nocturnal. At maturity, after the final moult and with developing wings, they crawl out of water onto rocks to subsequently emerge as an adult stonefly. They are not particularly strong flyers and the females return to the water, dipping to lay their eggs. Commonly seen species are willow fly, needle flies and yellow sallies.

Freshwater shrimp – Gammarus spp

Abundant shrimp populations are a sign of a healthy river, offering sub-surface food all year round. They are often greyish-green and sometimes have a small, red parasitic spot in the middle. Like caddis larvae, they were the key families for which weighted patterns were developed and upon which Czech nymphing was founded.

Terrestrial insects:

Black gnat Usually begin to appear in numbers from May onwards and provide staple food throughout the spring and summer. Not just Bibio species, as the Usk has Hilara maura as well!

Other terrestrials

Wind-blown or dropping from trees, terrestrials contribute to fish-food on the river and become more significant as the season progresses, as insects of all species proliferate and end up on the water. e.g. hawthorns in April and May, bees, wasps, flies, daddy long legs, moths, butterflies and associated caterpillars, aphids, grasshoppers, beetles of all shapes and sizes, and of course ants! For rivers, it is worth having representations of at least some of these species especially hawthorns, beetles, caterpillars, aphids and ants.

In many places in Wales, if you are lucky enough to be on a river on during an ant hatch (often around mid-July), you will rue not having any ant patterns to hand, because every fish in the river will be sipping ants, spent with open wings. The moment that ant-fall stops, so fish stop rising as well.

A small net to scoop bugs off the surface, available from all aquarium shops, can be invaluable, and routine inspection of bank-side vegetation and cobwebs can usually be very instructive. Watch what the birds are doing as well. Tight lines and happy bug hunting!

This review, for insects on Welsh freestone rivers, was initially prepared by Gwent Angling Society for its 2018 Open Day, providing information for fishing the club’s waters on the Usk, Wye, Sirhowy, and the Llynfi in Powys, as well as when venturing further afield in the Wye and Usk catchments.

The club’s website www.gwentanglingsociety.co.uk holds much more information on insects and artificial patterns to “Match the Hatch”.

Acknowledgements: photographs were provided by Gwent AS members, Lee Evans and Dave Collins, and by the late Dr Michael Wade, to whose family Gwent AS is indebted for allowing the club to use his wonderful photographs, and by Paul Procter, Tim Hughes, Ceri Thomas and WUF.

Words: Dave Collins



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