Invasive Species Compendium

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Abstract

Biology and control of the leek mining fly, Napomyza gymnostoma.

Abstract

In 1994, an insect pest, Napomyza gymnostoma (Loew) (Diptera; Agromyzidae), established itself in the eastern parts of Austria. It has now become a serious pest of leeks (in autumn) and chives. The main injuries are feeding punctures in chives and the presence of fly puparia within leek leaves, which makes the plants unmarketable. The serpentine mines are often interrupted and are found mainly in the lower parts of the host plant. N. gymnostoma has a bivoltine life cycle with generations in the spring and autumn. In 1997, the peaks of fly emergence occurred on 28 April and on 16 September. In summer there is a long period during which no flies emerge. Hence this pest appears to aestivate. At a constant temperature of 20°C, development (egg/adult) took 73 days at photoperiods of 6-10 hours, and 112 and 120 days at photoperiods of 14 and 18 hours, respectively. Postdiapause development was induced by changes in temperature, but not photoperiod. The lower threshold temperature for development is between 3.2 and 5.1°C. Chive is highly suitable as a host plant, as it remains vegetative for most of the year and so its leaves are available to both the spring and autumn generations of flies. Experiments on chemical control showed that heptenophos (330 g a.i./ha) and phosalone (420 g a.i./ha) were the most effective insecticides for controlling larvae of N. gymnostoma. The current advice is to apply sprays as long as larvae are found feeding in the upper parts of the leaves. To control this pest without applying insecticides, farmers are advised to grow leek as far away as possible from chive. They are advised also to cover their leek crops with nets as soon as the flies of the autumn generation emerge, and to bury any plant remains containing fly pupae as deep as possible in the soil. A warning service for N. gymnostoma was established in Austria in 1997.