Onion Leaf Miner
· Onion · Garlic · Leek
The infestation can occur at any growth stage and is first visible on the leaves. Because there are two generations every year, the first symptoms occur in spring and autumn. Initial symptoms are related to the punctures made by the females using a egg-laying structure and appear as tiny marks on the leaf sides. These marks are directed towards the leaf endings and form a characteristic linear pattern. Also the plants may acquired a waved, twisted and deformed appearance due to the feeding damage. The symptoms, related to the feeding of the maggots, appear as tunnels on the leaves and bulbs. In Allium crops with larger leaves the detection of maggots is easier by pulling back the leaves. The damage on the leaves and bulbs can function as a source of fungal or bacterial infections.
Symptoms are caused by the feeding activity of the adult and immature insects of Phytomyza gymnostoma. The adults are small (about 3 mm), longish, gray or black colored flies with a typical yellow or orange spot on the top and front. In spring the females puncture the plant tissues and lay small, slightly oval, white eggs on the stems or the base of leaves. The hatching maggots are creamy-white or yellowish. They start to feed on the leaves, stems and bulbs by boring tunnels and once they have reached a size of about 8 mm, they move into the stems and bulbs, or on rotten plants debris on the ground, where they pupate. The pupae are dark brown and 3,5 mm long. This second generation emerges in autumn and the whole process repeats. Often the second generation is more harmful than the first and can lead to heavy yield losses.
Organic products containing spinosad may be used as foliar spray applications to control the infestation by Phytomyza gymnostoma. Adults can be captured using yellow sticky traps or yellow plastic bowls containing soapy water.
Always consider an integrated approach with preventive measures together with biological treatments. If insecticides are needed, systemic and contact products should be applied when the adults are active. Several products are available as sprays, among others formulations containing azadirachtin, cyromazine, dinotefuran, lambda-cyhalothrin, zeta-cypermethrin or spinetoram. Dimethoate and a sequence of dimethoate and imidacloprid were shown quite effective against the pest. Other products inlcude fenitrothion and cyromazine, alone or with an oily mix.