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Leaf-miner: Oviposition in a leaf axil; from there a corridor descends along the leaf inner side. Part of the length of the corridor is below the surface. Often several mines per plant. Most of the feeding punctures are arranged in lines, parallel to the leaf. Pupation takes place low in the the plant, near the leaf base. The brown pupariria are situated within the mine. They can best be found by bending the outer leaves outwards, and inspecting their bases (Collins and Lole, 2005) (Bladmineerders van Europa).

Larva: The larvae of flies are leg-less maggots without a head capsule (see examples). They never have thoracic or abdominal legs. They do not have chewing mouthparts, although they do have a characteristic cephalo-pharyngeal skeleton (see examples), usually visible internally through the body wall.

Larvae unusually large (5-8 mm); anterior spiraculum with 16-18 bulbs, posterior spiraculum with 30-34 (Seljak, 1998) (Bladmineerders van Europa).

Puparium: The puparia of flies are formed within the hardened last larval skin or puparium and as a result sheaths enclosing head appendages, wings and legs are not visible externally (see examples).

Comments: A pest of leeks (Collins and Lole, 2005). It has taken until 1988 before the hostplant of the species became evident (1990). Since that year the species has manifested itself as a serious pest on various Allium species (Seljak, 1998). It looks like the problem is expanding westwards. One case concerned a leek plot in Germany, just over the Dutch border (de Goffau, 2001). Part of the damage was connected to feeding punctures that provided entrance to fungus infections, but the main problem was the presence of numerous larvae and pupariria - up to 100 in a single leek plant (Billen, 1998). The reason why this species so suddenly has turned into a serious pest is unclear. (Bladmineerders van Europa).

Anyone who thinks they may have gymnostoma on their leeks, onions or other related plants should report this to The Plant Health and Seeds Inspectorate.  There are local branches, but samples in adequately packaged containers can be sent direct to their headquarters at Central Science Laboratory, Sand Hutton, York, YO4 1LZ.

Hosts in Great Britain and Ireland:

Alliaceae
Allium	porrum	Leeks		Collins and Lole, 2005

Hosts elsewhere:

Alliaceae
Allium	cepa	Onion		Bladmineerders van Europa
Allium	porrum	Leeks		Bladmineerders van Europa

Time of year - mines: Larvae probably in the entire summer and late summer (Bladmineerders van Europa).

Time of year - adults: Currently unknown.

Distribution in Great Britain and Ireland: Leicestershire (NBN Atlas).

Distribution elsewhere: Widespread in continental Europe including Austria, Czech Republic, Danish mainland, European Turkey, Finland, French mainland, Germany, Hungary, Italian mainland, Lithuania, Poland, Sicily, Slovakia, Slovenia, Spanish mainland, Sweden, The Netherlands, Ukraine, Yugoslavia and North Africa (Fauna Europaea).

NBN Atlas links to known host species:

British and Irish Parasitoids in Britain and elsewhere: Currently unknown.