General Plant attacker of the month September: the allium leafminer (Phytomyza gymnostoma)
The allium leafminer, Phytomyza gymnostoma (Diptera: Agromyzidae), is a pest that is spreading from Southern and Central Europe to Belgium and is an emerging problem for leek cultivation in Flanders. In our region, the allium leafminer develops two generations a year, one in spring and one in fall. The females make a large number of feeding spots with their ovipositor in the leaves while laying the eggs (Photo 1). These spots are the first visible sign that the flies are active.
Larvae of the allium leafminer dig into the shaft of the leek and cause burrows that make the leek unsaleable (Photo 2). The damage is only detected at harvest time during leek cleaning. The larvae develop into brown pupae. Pupae mainly pupate within the stems and bulbs in summer and winter, but some pupae can end up in the ground.
The host plants of the allium leafminer belong to the Allium species, such as leek, chives, onion, garlic, shallot and also wild Allium plants, but leek and chives are most susceptible to infestation. Bulbs and stems of plants with eggs, larvae or pupae are the main sources of the spread of this pest.
The leafminer is controlled by crop technical measures such as covering the Allium plants with an insect screen during the flight periods, composting plant remains to reduce the survival of the pupae, rotation with non-host plants and delayed planting of spring crops. The control of this pest is also achieved by means of current insecticides as a last option in the integrated control strategy. Recently, ILVO, together with PCG, Inagro and PSKW, completed a research project (VLAIO-PREIMINEERVLIEG) in which the integrated control of the allium leafminer in leeks was investigated.