Status Overview

Yellow Mayfly Potamanthus luteus is considered Endangered in the UK under IUCN guidelines for limited range and area of occupancy (Natural England, 2016). Due to its rarity and decline in numbers this insect was made a Priority Species on the UK Biodiversity Action Plan (BAP) (JNCC, 2007). The species has always been rare in Britain with very few rivers or catchments supporting a population:

UK Current sites

• Single Site  on the River Teme
• 7 km stretch of the River Severn near Kidderminster
• River Wye (only found reliably downstream of Whitney)
• Bangor-on-Dee (since 2020)

In the nineteenth century there were only two records of this species: 1835 in the “metropolitan district‟ (Stephens, 1835) and from the Thames in 1878 (McLachlan, 1878). The 2020 sightings at Bangor (further North than traditional range) suggests the possibility of spreading into North-West counties but as yet, it remains undiscovered.

Habitat

P. luteus nymphs usually live in silt trapped amongst stones on the bed of the river and can be found in side channels and pools following flood events. Larvae in the River Wye have been found in loose gravel and stones with the highest numbers  found in slower, deeper waters (Macadam et al., 2011).

Life History

This insect has one generation per year in the UK with overwintering larvae. The adults, like most mayflies, are very short lived. The peak  emergence period is July, however, adults can be found between May and October. Emergence typically takes place at dusk and the adults usually emerge from the surface of the water. Nymphs may also climb up stones or plant stems to emerge partially or even entirely out of the water (Natural England, 2016).

Recording / Identification

The larvae are usually collected via kick sampling. The adults fly at dusk and are known to be attracted to light so moth/light traps operated on the banks of a river during June and July are likely to produce specimens if the species is present. Alternatively during peak emergence the insect can simply be caught in nets as they fly over water or land in marginal vegetation.

In terms of nymphs they are very distinctively yellow with branched feathery gills. The only other mayflies with these branched feather gills are some species of genus Ephemera. However in Ephemera, gills are held back over the bod, where as the gills in P. luteus extend outwards from the body. Additionally, Ephemera have larger mandibles that extend past the head, this is not the case in the Yellow Mayfly.

Adults are large with a dull yellow-orange body and a distinctive broad yellow-brown stripe along the length of the abdomen. As mentioned they are very characteristically yellow, both in wing and body. This is only shared by two other species Heptagenia sulphurea and H. longicauda. In both these Heptagenia species there are only 2 tails where as in the Yellow mayfly there are 3.

Conservation

This species is highly dependent on good quality water so the most urgent course of action would be to clean up waterways in which it is present. Maintaining marginal vegetation is also important.

References

Natural England (2016)., A review of the status of the mayflies (Ephemeroptera) of Great Britain. https://publications.naturalengland.org.uk/publication/4635857668538368
Macadam, Craig, and Conservation Officer. “Potamanthus luteus.” (2011).
Stephens J.F. (1835) Illustrations of British Entomology, Mandibulata, 6: 53-70.
McLachlan R. (1878) Potamanthus luteus at Weybridge. Entomologist’s Monthly Magazine 15:92.
JNCC (2007).,UK BAP List of UK Priority Species 2007. https://hub.jncc.gov.uk/assets/98fb6dab-13ae-470d-884b-7816afce42d4#UKBAP-priority-terrestrial-invertebrates.pdf