Liriomyza cepae

Liriomyza cepae (Hering, 1927)

In external view the three Liriomyza species feeding on Allium (cepae, nietzkei and chinensis) are untypical members of the genus and can be confused by the unusually dark appearance and black scutellum. However, the male genitalia are distinct. A further hint might be the angulate third antennal segment.
To separate the species within this group, the investigation of the male distiphallus is necessary.

Wing length: 1.6 - 2.5 mm. Third antennal segment angulate. Frontorbital hairs rather long and bent; frontorbital bristles can vary in number. Anepisternum with a small number of bristles at posterior margin. Katepisternum with only 2 bristles at posterodorsal margin.
Male terminalia
Distiphallus broad and rounded with two short terminal tubules clearly projecting. Surstyli short and rounded, covered with several long hairs and maybe some spines. Between surstyli and inner part of epandrium a single hair or spinule on hyaline cuticle. Tip of epandrium with two spines on each side. Bacilliform sclerites well developed.
Immature stages
The larval cephalopharyngeal skeleton can be assumed to be rather similar as in the closely related species Liriomyza chinensis described by Sasakawa, 1961 (as Phytobia cepae). All the mouthparts are rather strong and thick indicating similarities of the feeding habit of cepae and that of true stem miners.

The larvae can infest not only the thick leaves but also the layers of the onion itself (Frankenhuyzen, 1977) but the damage there is low. Perhaps the wounds caused by the mining larvae can reduce the shelf life of the onions.
Frankenhuyzen reported that there is strong evidence for two generations per year. The flies and larvae can still be found in October.

Allium cepa L. (onion) and possibly other Allium species.

Records from Germany, Hungary, Netherlands, Belgium, Spain. Probably widespread throughout Europe.
It is not yet clear if the species occurs also in Asia or if any record of cepae has to be attributed to confusions with the closely related species Liriomyza chinensis.

Because of possible taxonomic confusions species records concerning leaf miner damage on onion should be handled with care, especially those from before 1973, when Spencer's book on agromyzids of economic importance appeared. The Liriomyza species causing the damage can be either Liriomyza cepae, Liriomyza chinensis (especially in the Eastern palaearctic region but also in Europe), or Liriomyza nietzkei (especially in Europe).
The larvae weaken the plants by destroying leaves. If they occur in large numbers all of these species have the potential to cause severe damage and are treated as serious pests by most authors (see literature). However, outbreaks seem to be rather intermittal and local.