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Feeding punctures, small discoloured spots on the upper leaf surface, caused by adult females using the ovipositor to puncture the leaf surface in the early spring or autumn are the first symptoms of damage. Larval mines can subsequently be seen in the leaves and bulbs. Infested host parts are soft to the touch and susceptible to secondary plant pathogens and infections that could exhibit their own symptoms. Infested plants are smaller and have dried leaves. Photographs of damage symptoms on leek foliage are provided in Darvas et al. (1988).
Good crop husbandry can reduce the damage caused by P. gymnostoma. Covering Allium species with fleece in late February will help to exclude newly emerging adult flies that are searching for a host. Crops should be rotated with a non-Allium species and later planting can help to control the pest. Working in Poland, Sionek (1999) showed that planting Allium after mid-May reduced the damage caused by P. gymnostoma. Chemical treatment with a systemic insecticide during spring and autumn, when the adults are active, is an option. In Italy imidacloprid, dimethoate, spinosad, cyromazine and an imidacloprid treatment preceded and followed by dimethoate were tested for efficacy against P. gymnostoma in the field. Most treatments significantly reduced the numbers of larvae or pupae per plant. Dimethoate and the dimethoate/imidacloprid sequence treatments were particularly promising (Talotti et al., 2003). Also effective are fenitrothion and cyromazine, alone or with an oily wetter (Talotti et al., 2004).
In Austria, organic farmers are advised to grow leeks as far away as possible from chives. They are also advised 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 (Kahrer, 1999).
P. gymnostoma was first reported in Poland in 1858 (Loew, 1858) but was not regarded as an economic pest and was not included in the monograph Agromyzidae of economic importance (Spencer, 1973). It was first reported as a pest in Hungary in 1988 (Darvas et al., 1988). Following this report, more was learned about the basic biology of the pest. P. gymnostoma is capable of infesting almost 100% of a crop. Studies in Serbia revealed that nearly all plants in a field of Allium were mined with approximately 20 pupae per plant. All plants were completely destroyed. Even at lower populations, the presence of mines on young plants may reduce the quality and marketability of produce (Spasic and Mihajlovic, 1997). Economic losses can therefore be serious and result from feeding damage lowering the marketability of produce.
Economic damage was first observed in northern Italy in autumn 1999 in Friuli-Venezia Giulia and during 2001 in Veneto (Zandigiacomo and Dalla Monta, 2002). Damage can be seen in Allium grown in private gardens and commercial crops with severe damage observed in organic farms (Zandigiacomo and Dalla Monta, 2002).
In Croatia Delia antiqua was considered the most important dipteran pest on onions but P. gymnostoma has now become the most important onion pest (Mesic and Baric, 2004).