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C. horticola makes small punctures in the leaf epidermis and whitish, irregularly linear mines. Pupae or exuviae may be present at the end of mines; frass occurs at short intervals along the mines in irregular, well-separated grains.
Rot and decay of seedlings is a secondary effect. Mining causes stem wilt in seedlings.
Research on organophosphorus insecticides has been a major industry and results suggest that heavy use of sprays and granular dispersions are of limited effect. Chemical treatment of seedlings, preventing the initial laying of eggs is of great value, as this enables the plants to develop healthily.
When the plants are established, subsequent attack by the flies will be less serious, and natural predators will keep populations low. In greenhouses, the introduction of Hymenoptera parasites can be extremely effective, especially during the first period of leaf mine development (see Natural Enemies). Irradiated (and hence sterile) males are also a very effective method of reducing egg-laying. Limited success has also been obtained by exposing yellow sticky traps close above the growing plants, as the males tend to fly low between plants and in general are more mobile than the females. For more detailed information see the following references: Robb and Parella (1984), Süss and Agosti (1986), del Bene (1989), Heinz and Parella (1990) and Zoebisch and Schuster (1990).
C. horticola is a very serious pest almost everywhere. In northern Europe it often causes serious damage, especially on glasshouse crops of tomatoes and lettuce (Harris, 1976; Pakal'nishkis, 1984); in Mediterranean areas it causes damage both in glasshouses and in the field, especially to Cicer, Brassica and Pisum species (Griffiths, 1967), and similarly in Japan to Allium, Cucurbita, Mentha and Vicia species (Sasakawa and Yoshida, 1975). Infestations on seedlings are especially serious as the plants quickly lose the ability to develop, and wilt or die. Infestations on older plants can be less serious in effect, but yields are often reduced.