Allium Leaf miner - Napomyza gymnostoma                                                        January 2011.
In December 2003 the first UK outbreak of the Allium Leaf miner, Napomyza gymnostoma, was discovered on leeks growing in a private garden in Wolverhampton. This led to a survey of allotments and farms growing Alliums (leeks, onions, and garlic) in the area, and a further 10 outbreaks have since been found locally. It is important to note that no outbreaks have been found on commercial Allium crops to date. The production of this note is to alert commercial leek growers on the need to be vigilant in checking their crops for this pest.
Non-commercial growers are also advised to read this note for information on ways of reducing the risk of Napomyza infesting their crops.
Geographical Distribution
Originally from central and eastern Europe, N. gymnostoma has been increasing its distribution during the past twenty years, heading eastwards to Italy and through the Balkans, and also northwards and westwards through Germany. It is now wide spread in Europe, and is recorded from Austria, Czech Republic, Denmark, Germany, Hungary, Italy, Poland, Serbia, Slovakia, Slovenia, Spain, Sweden, and Turkey.
Host Plants
Napomyza gymnostoma is a pest of Allium spp., particularly leeks, onions (bulb, and spring), garlic, and chives. It is also a potential pest of ornamental Allium spp.
Economic Importance and Damage
Feeding by larvae has been shown to cause important economic impact in Europe. The larvae mine the stalks and bulbs of Allium plants causing these plant parts to become soft and susceptible to bacterial and fungal infections, such as white rot (Sclerotium cepivorum).
The amount of damage caused by N. gymnostoma is dependent on the population level, however the pest is capable of infesting 100% of a susceptible crop. Investigations in Serbia have revealed that under such heavy infestations, with almost all plants being mined, and about 20 puparia per plant, all plants were completely destroyed. Even where population levels are low, the presence of larvae and pupae in marketed young onion and leek plants may represent an economic problem through reduction of consumer acceptance.
Commercially grown leeks and onions are amongst the five most valuable field grown vegetables in the UK, with the value of production in 2001/02 (DEFRA stats) as follows; leeks (£30.4 million), onions, dry bulb (c. £41 million), onions, green (£15 million).
Biology and Life-Cycle
In Serbia, first-generation adult flies emerge from the beginning of March to the end of April, and are attracted to host plants, where they lay eggs. Females make large numbers of feeding punctures using their ovipositors, and then use their mouthparts to feed on leaf exudates. These punctures are the first sign that the flies are active. Eggs are usually laid in the base of leaves, or in the stalks. Larvae emerge and feed from the oviposition site in the leaf, and then move downwards into the stalk and bulb, where they pupate. During their development, larvae move from one outer succulent layer to another, and never destroy the centre of the bulb. Development from feeding and oviposition, to pupation, takes approximately 30-35 days. During the summer (June-end September), the species aestivates as a pupa in infested plants. It is not known whether this pest will enter this state in cooler UK conditions. The second-generation flies emerge from the beginning of October until mid November. The larvae of this generation then develop in their host plant until the end of November, and over-winter as pupae in Allium plants.
This pest should not be confused with the leek moth Acrolepiopsis assectella, whose larvae also bore into the leaves of Allium plants. The adult stage is a grey/brown moth 8-9mm long, and therefore cannot be confused with the adult flies of the Allium leaf miner. The pupal stages of these pests are also quite different to one another. The pupal stage of the leek moth is about 10mm long, and is a cream coloured, silk cocoon; the pupae of the Allium leaf miner are about half this size, and are orange brown in colour (see figure 1). Whilst the larvae of these pests may be more easily confused with one another than the other life stages, there are significant differences (see figures 1 and 3 for comparison). The larvae of the leek moth have brown head capsules, and legs on the thorax, and pro-legs on the abdominal segments. They are yellowish green in colour, and have setae (hairs) on dark raised spots. These larvae can vary length from 1mm to 10 mm. The Allium leaf-miner larvae however have no head capsule, do not possess legs, and are white/creamish in colour.
Advisory Information
Alliums should be checked for mining damage, and split leaves. Peeling back leaf layers to reveal pupae, or slicing leeks, onions, and garlic longitudinally during November-March, and June-September, may reveal the presence of any larvae or pupae. Adults may be observed during March/April, and September/October.
To help prevent this pest from infesting Allium crops, it would be advisable to cover crops with fleece in late February. This will keep out many adult flies, which may have emerged and are searching for a new crop. Rotation with non-Allium crops would also be helpful in preventing this pest from establishing. Non-commercial growers, such as allotment holders, should avoid planting Allium crops until after the first threat of adult emergence has passed (after April). Later varieties of onions are available which will help make this possible. Prophylactic insecticide application with a systemic product, such as dimethoate may be advisable for commercial crops during March-April, and October, when adults are most likely to be flying.
Suspected outbreaks of N. gymnostoma should be reported to the Plant Health and Seeds Inspectorate. All samples will be submitted to the Central Science Laboratory for confirmation. 
The information given in this note was produced by Ellie Agallou and Dom Collins , CSL Department for Environment, Food and Rural Affairs