Britain's aquatic moths

When we think about aquatic insects, various groups spring to mind – caddis, dragonfly and damselfly nymphs, and water beetles among others. However, moths do not often appear on this list.

Worldwide there are several hundred moth species with aquatic larvae, and more are being discovered as research unravels the biology of previously unknown larve. Those considered truly aquatic feed on or mine aquatic vegetation, with a few species consuming diatoms from the surfaces of rocks. Some species construct portable cases similar to caddis and may produce submerged silken spinnings. Most species, especially the external plant-feeders, are in the family Pyralidae; some others are in the family Arctiidae. Truly aquatic larvae often have filamentous gills on the body and may produce a portable case as mentioned above. Also, they usually have small abdominal prolegs with crotchets (small hooks) in an oval pattern, as opposed to terrestrial forms which have well-defined, raised prolegs with crotchets in a circle (Bouchard 2009). Aquatic moth larvae usually live in still or slow-flowing waters such as ponds and marshes, with some found in streams. 

In most aquatic moth species, females swim to the bottom of the water-body to lay their eggs; a few species lay eggs on the surface (e.g. species where wingless females wait on the water surface until mated). In at least one species, the adult female moth is completely aquatic and never emerges from the water.

Larva of Parapoynx showing gills © Hugh Clifford

In Britain, four native pyralid species can be considered truly aquatic and will be covered first; the Brown China-mark (Elophila nymphaeata), the Beautiful China-mark (Nymphula stagnata), the Ringed China-mark (Parapoynx stratiotata) and the Water Veneer (Acentria ephemerella). Of these, the first three are in the subfamily Nymphulinae, while the Water Veneer is in the closely related Acentropinae. Some other species have semi-aquatic larvae such as those feeding low down in the stems of emergent vegetation and those which raft between food-plants; these will be covered secondly.

The larvae of the three nymphuline species all have aquatic larvae which live submerged in spinnings among aquatic vegetation (Goater 1986):

Elophila nymphaeata

Larvae feed on pondweeds (Potamogeton spp.), frogbit (Hydrocharis morsus-ranae) and bur-reeds (Sparganium spp.) from September to June. The initial stage is a leaf-miner, later living in a floating case made of leaf fragments. To facilitate feeding, the larva attaches the case to the underside of a leaf with silk; pupation occurs in a silk cocoon covered with further leaf fragments, attached to a plant stem just below the water surface. This is a common and widespread species in Britain, found where there are suitable pond and lake margins, as well as abundant vegetation in slow-flowing rivers and canals. Adults differ from Nymphula stagnata by being larger in size and having broader, browner wings; a smaller dark brown form is known from bogs in the New Forest and Dorset (Manley 2008).

Nymphula stagnata

Larvae feed mainly on yellow water-lily (Nuphar lutea), also on bur-reeds (Sparganium spp.) and probably other water-plants from August to May. The initial stage is a miner within the pith of the stem where it hibernates until April. Post-hibernation feeding occurs in the stem or in a chamber of leaves spun just below the water surface; pupation occurs in a white silk cocoon attached to the food plant. This is a common and widespread species in Britain, found at the margins of rivers, streams and lakes, although it is more locally distributed north of southern Scotland.

Parapoynx stratiotata

Larvae are more specialised for aquatic life than the first two species, with a profusion of branched gills, and feeding on pondweeds (Potamogeton spp.), Canadian waterweed (Elodea canadensis), hornworts (Ceratophyllum spp.) and other water-plants from July to May. The larva spins leaves together, forming an open web, and makes wriggling movements to aid gas exchange; pupation occurs in a large pinkish cocoon attached to a plant beneath the water surface. Habeck (1983) researched the various food plants of this species, including its possible use as a control agent of invasive aquatic plants in the USA. It is locally distributed at the margins of ponds, lakes, slow-flowing rivers, canals and ditches; commonest in the south of England and Wales, but recorded as far north as Yorkshire and Lancashire.

Acentria ephemerella

Larvae are found to 2m depth in loose spinnings among food plants which include Canadian waterweed (Elodea canadensis), pondweeds (Potamogeton spp.), stoneworts (Chara spp.) and filamentous algae. Hibernation takes place from October to May with larvae becoming fully-fed by May or June; pupation occurs in a silk cocoon at up to a metre depth beneath the water surface. Adult females are usually flightless with much-reduced wings, remaining submerged and swimming with long-fringed middle and hind legs until mating occurs on the water’s surface; fully winged females do occur and are larger than the males. It is locally abundant in ponds, lakes and slow-flowing rivers throughout Britain.

A number of other fully aquatic nymphulines have been recorded in Britain, but these are non-native species generally with sparse records associated with greenhouses specialising in aquatic plants and are detailed in Goater (1986). However, there are some other species which produce larvae with some aquatic aspect to their life history. For example, Schoenobius gigantella and Donacaula forficella (subfamily Schoenobiinae) both have larvae that feed on plants such as common reed (Phragmites australis) and reed sweet-grass (Glyceria maxima), floating to new stems on rafts made from sections of the previous stem. D. mucronellus may not raft, but feeds and pupates in the lower sections of reed, G. maxima and sedge (Carex spp.) stems, and as such may need to tolerate wet conditions, especially if water-levels are high. Similarly, within the subfamily Crambinae, the boggy heathland specialist Crambus silvella (known mainly from Surrey, Hampshire, Dorset and Norfolk) pupates in a cocoon in the soil which, in wet conditions, may again require tolerance of inundation, although this is not certain. C. uliginosellus is very local in wet bogs, but its larval biology is unknown and so its tolerance of wet conditions is less certain still.

Therefore, although it is clear that a number of species require research to determine their larval life history, Britain does have a known aquatic moth fauna, which – along with the global fauna – may expand as knowledge of larval biology increases.

References

Bouchard, R.W. (2009). Guide to Aquatic Invertebrate Families of Mongolia. Identification Manual for Students, Citizen Monitors, and Aquatic Resource Professionals. Draft Chapter 11: Lepidoptera: Aquatic Moths available here [accessed 17/11/2010].

Goater, B. (1986). British Pyralid Moths. A Guide to their Identification. Harley, Colchester.
Habeck, D.H. (1983). The potential of Parapoynx stratiotata L. as a biological control agent for Eurasian watermilfoil. Journal of Aquatic Plant Management 21: 26-29.
Manley, C. (2008). British Moths and Butterflies: A Photographic Guide. A&C Black, London.

Thanks to BioDiTRL at the University of Alberta for the image of Parapoynx.

Further reading

For more detail about one European species invasive to North America (Acentropus niveus) visit here.


For more about the diversity of aquatic moth species, see:
Mey, W. & Speidel, W. (2008). Global diversity of aquatic moths (Lepidoptera) in freshwater. Hydrobiologia 595(1): 521-528.