Donacia bicolora – the two-tone reed beetle
Donacia bicolora Zschach, 1788
Family: Chrysomelidae
Donacia bicolora is a metallic green or bronze beetle found on bur-reeds in summer. Ireland provides a stronghold.
In brief
- D. bicolora is known from many lakes in Fermanagh, mainly in the Upper Lough Erne system, and also from Armagh and Tyrone, with old records from Antrim
- It is found on bur-reeds (Sparganium spp.) near to water
- Adult beetles can be found from May to August, with a major peak in June
- It is listed as a UK Priority Species
- The main threats to this species are loss of marginal habitat, for example, through eutrophication and succession.
Species description
The adults are 8.0 to 11.6mm long and with a greenish-gold colour on the upperside, but with a more silvery underside. Later in the season individuals may be more blue or purple, one of the explanations for the “bicoloured” name. The larvae are cream-coloured and maggot-like but with three pairs of short thoracic legs.
Life cycle
Adults overwinter during or close to the winter months, often in tough fibrous pupal cocoons. They become active in May and die out by July. Eggs are laid at the base of hostplants in early summer and larvae develop with their rear ends attached to roots under the water. The pupae, which are also attached to the air spaces in aquatic root systems, develop in late summer and late autumn, the adults staying put inside the pupal cocoons until the following spring.
Similar species
D. bicolora is one of nineteen species of reed beetle (Coleoptera, Donaciinae) recorded from Ireland. Another species, the smaller D. simplex is more common on bur-reeds, but its infestations build up more slowly than those of D. bicolora, and last longer, with individuals found as late as November. D. marginata also occurs on bur-reeds, generally later in the summer; it is easily distinguished as most specimens have broad golden-bronze and reddish-purple stripes. The individual species of Donacia larvae cannot be identified.
How to see this species
D. bicolora is known from many lakes in Fermanagh, mainly in the Upper Lough Erne system, and also from Armagh and Tyrone. It is found on bur-reeds (Sparganium spp.) near to water, and adult beetles can be found from May to August, with a major peak in June. To see these beetles can be very difficult; one needs a long-handled sweep net or, more frequently, a good camera. Adult beetles are alert to movement and will either fall onto the water or fly off when disturbed. Relevant access permissions should always be sought prior to visiting any sites.
Current status
There are 20 sites known from 1980 onwards in Northern Ireland, mainly in the Upper Lough Erne catchment of Fermanagh, and 20 sites known from 1980 onwards in the Republic, mainly along the Shannon. This species has been lost from five English vice-counties, but continues to be found in Dorset, South Hampshire, North Hampshire, Surrey, in the latter two mainly beside the River Wey.
Why is this species a priority in Northern Ireland?
- Listed as a UK Priority Species
- Scarce, with Northern Ireland being the stronghold of both the Irish and UK populations.
Threats/Causes of decline
- Eutrophication of loughs, drainage ditches and canals, resulting in loss of marginal habitat
- Successional phenomena, such as encroachment of water margins by reedbeds and carr
- Excessive lake, canal and drainage ditch management may result in loss of habitat
- Excessive numbers of waterfowl will destroy vegetation and increase the risk of algal blooms.
Conservation of this species
Current action
There is a UK Species Action Plan which was published in 1999.
- Three of the Northern Ireland sites are ASSIs, two of which are within the Upper Lough Erne SAC
- Implementation of the Northern Ireland habitat action plans for Fens, Mesotrophic Lakes and Eutrophic Standing Waters.
Proposed objectives/actions
- Maintain all viable populations.
What you can do
Keep an eye open for metallic-finished beetles beside water causing damage to bur-reeds. Report any sightings to CEDaR at National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU. Tel: 028 9039 5256 or Email: cedar.info [at] nmni.com, with a digital photograph if possible. Note that two other species are common on bur-reeds.
Further information
Links
UK Biodiversity Action Plan
Northern Ireland Habitat Action Plans
Literature
Menzies, I.S. and Cox, M.L. (1996). Notes on the natural history, distribution and identification of British reed beetles. British Journal of Entomology and Natural History 9: 137-162, 2 plates.
Nilsson, A.N. (Ed.) (1996). Aquatic Insects of North Europe. Volume 1: Ephemeroptera, Plecoptera, Heteroptera, Megaloptera, Neuroptera, Coleoptera, Trichoptera and Lepidoptera. Apollo Books, Stenstrup.
Text written by:
Dr Garth Foster
iNaturalist: Species account : iNaturalist World Species Observations database