Microdon devius

[stuart]

Microdon devius (Linnaeus, 1761)

Identification ease/difficulty: 3

Status

• Vulnerable in Falk (1991)
• Vulnerable in Shirt (1987)

Sources of information

• Page 228, Pl. 9, fig 4 in Stubbs & Falk (1983)
• Larval plate Pl. 3d in Rotheray (1993)
• Map here and page 94 in Ball & Morris (2000)
• Page 339 in Stubbs & Falk (2002)

[stuart]

Species account from Provisional atlas of British hoverflies, Ball & Morris, 2000.

Microdon devius (Linnaeus, 1761)

Biology: The very distinctive larvae live in ants nests. They are hemispherical in shape and heavily armoured, and prey on the eggs and larvae of Lasius flavus in nests usually on chalk grassland. The pupae are of similar shape and are also armoured. Adults are elusive and usually encountered as single specimens when sweeping chalk grassland

Distribution: A rare species mainly found on chalk grassland in southern England, especially the Downs of Surrey and Hampshire, from which the great majority of records originate. It has also been recorded from fens at Cothill in Oxfordshire, and Redgrave and Middle Harling in Suffolk, with a number of records both old and recent from the first two localities suggesting well established populations. There are a few records from North Wales, including one recent, and a 19th century specimen in The Natural History Museum collection from Wyre Forest

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Datasheet from the Insect Red Data Book, Shirt (1989).

Microdon devius (L., 1761) VULNERABLE

Identification: Stubbs & Falk (1983), pp. 112 and 228 (genus, pl. 9:4,5).

Distribution: Mainly the North Downs of Surrey and the Chilterns, but also the South Downs, Oxfordshire and perhaps Wyre Forest, and more doubtfully north-west Wales. However, records are few and mostly old. Only known in recent years from the North Downs. The population is believed to be small.

Habitat and ecology: Chalk grassland is the normal habitat. Some localities would equate with other calcareous grassland and possibly other habitats. Scrub edge may or may not be required. The larvae live in ants` nests, feeding on buccal pellets. The literature is confused and probably unreliable as to the ant hosts, but the best candidates are in the genera LasiusandFormica, probably alsoMyrmica. The adults are normally swept from long grass or found sitting beside paths.

Status: The absence of knowledge about its ecological requirements makes the future very uncertain. Only the two North Downs sites are good prospects and one of these is extremely small. It is possible that further sites may be located on the North Downs, the Chilterns or elsewhere though the species is always very localised and elusive.Conservation: Present on two National Trust properties in Surrey, of which one is an SSSI. The management plan for the main site is probably satisfactory for this species. There is a need to survey the distribution and status more accurately, and in particular the biology.Threats: Changes in the character of chalk grassland and its ant fauna; scrub encroachment; ploughing, afforestation and other forms of land improvement. Close grazing is also probably damaging.

Author: A. E. Stubbs, using additional information from Donisthorpe )1927, pp.125-126) and Coe (1953).

[stuart]

Datasheet from the Review of Scarce and Threatened Diptera, Falk (1991).

MICRODON DEVIUS (Linnaeus) VULNERABLE

DISTRIBUTION Scattered records in southern England as far north as Worcestershire and Norfolk, also North Wales. The majority of records are for the chalk grassland on the North Downs and the Chilterns.

HABITAT Calcareous grassland and scrub.

ECOLOGY The armoured, slug-like larvae live as commensals in the nests of ants where they feed on the discarded food pellets of the adult ants. There have been reports that the yellow hill ant Lasius flavus acts as host but this needs confirmation and it is possible that various other ant species are equally good candidates. Adults recorded from May to August and may be found sitting on vegetation near their breeding sites.

STATUS Very local and declining with about a dozen known post 1960 sites, mainly from Wealdon, Hampshire, the North Downs and the Chilterns with one (possibly two) sites in North Wales (Merionethshire). It was also recorded from Middle Harling Fen, Norfolk in 1988.

THREAT Habitat loss to agriculture and intensive forestry and changes in the grazing management of calcareous grassland with a subsequent change in vegetation structure such as scrub invasion and loss of the ants.

MANAGEMENT Maintain a mosaic of vegetation types including areas of short cropped grass to encourage the ants. Use rotational grazing policies if necessary and prevent scrub invasion.