Phytomyza (Napomyza) gymnostoma is a new pest of Allium in Europe: addition to the EPPO Alert List
In Europe, observations made during the last 20 years suggest that Phytomyza (Napomyza) gymnostoma (Diptera: Agromyzidae –leaf miner) should be considered as a new important pest on onion and leek, and that increasing significance may be expected. The EPPO Secretariat decided that it could be useful to draw EPPO member countries’ attention to this new problem of Allium crops.
Phytomyza (Napomyza) gymnostoma (Diptera: Agromyzidae – Allium leaf miner)
Why: In the last 20 years, damage to Allium crops caused by Phytomyza (Napomyza) gymnostoma has been reported by an increasing number of European countries. P. gymnostoma was first described in 1858 in the region of Poznan (Poland). In 1976, it was transferred to the genus Napomyza by Spencer and back to Phytomyza in 1994 by Zlobin.
Where: In 1976, this species was reported in Denmark, Sweden, Poland, and in the Mediterranean Basin but no damage was observed. Since the 1980s, P. gymnostoma has become a pest of Allium plants in several countries for reasons which remain unknown, first in eastern Europe and more recently in western Europe. Today, the pest is widespread in Europe.
EPPO region: Austria (damage reported in 1994), Croatia (1990s, now reported as the most important and most frequent pest of onions), Czechia, Denmark, France (2003 in Alsace), Germany (1994), Hungary (1986), Italy (Friuli-Venetia Giulia in 1999 and Veneto in 2001), Poland (1997), Serbia and Montenegro (1992), Slovakia (1990), Slovenia (1994), Spain, Sweden, Switzerland (near Basel 2003), Turkey, United Kingdom (2003 in a private garden in Wolverhampton, near Birmingham).
On which plants: Allium species, more particularly leek (A. porrum) but also chives (A. schoenoprasum) and to a lesser extent onion (A. cepa), garlic (A. sativum), shallot (A. ascallonicum).
Damage: Larvae of N. gymnostoma mine the stalk and bulbs of Allium plants which become soft and susceptible to fungal or bacterial infections. Frequently, severe plant deformations (split leaves and stalks, distortions) are observed. Females make large numbers of feeding punctures using their ovipositors, and then use their mouth parts to feed on leaf exudates. These punctures are the first sign that the flies are active. P. gymnostoma overwinter as pupae attached to plant tissues. At the beginning of spring, adults emerge. Adults are small greyish flies of 3 mm long, with a largely yellow head. Wing length varied from 2.9 in males to 4.0 mm in females. Legs are dark with yellowish knees. Eggs are laid within plant tissues, usually at the leaf base. Larvae mine the leaves (moving downward into the stalk, and eventually to the bulb), and pupate at the end of their galleries. During summer, the pest aestivates as pupae within the plants. Another generation of adults emerge at the end of summer – beginning of autumn. In spring, damage is observed after the first adult flight. On leeks for example, which are usually small plants at this time of the year, few larvae can kill a plant, so an uinfested field can rapidly show a large number of missing plants. In autumn, plants are larger and tolerate higher levels of populations. Although, no figures are given, damage caused by P. gymnostoma is considered as economic. It is reported in Serbia that the presence of about 20 puparia per plant can lead to complete plant destruction. In addition, the presence of larvae in young onion and leek plants may render them unmarketable.
Dissemination: Adults can fly, but more studies are needed on flight periods and distances. There is no data on the possible role of infested bulbs in spreading the insects.
Pathway: Allium plants for planting or bulbs infested by P. gymnostoma.
Possible risks: Allium crops are widely grown in European countries. In many European countries, P. gymnostoma is mentioned as an emerging and economically damaging pest. More studies are needed on control measures (rotations, destruction of plant debris, chemical control, possible use of parasitoids). It can be concluded that more attention should be paid to this potentially damaging pest of leek and other Allium crops.
EPPO RS 2005/064
Panel review date - Entry date 2005-04
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