Leafminers

    Leafminers are small, yellow maggots that form trails beneath the leaf epidermis as they feed. The adults are small flies, about 1/16 inch (16 mm) long, mostly black with some yellow markings. The adult flies cause small, whitish stippling on the leaves as they feed and lay eggs. The egg hatches in about 3 days and the larva begins to mine the leaf as it feeds. After 4-7 days, the larva drops to the soil to pupate.

    Leafminers are normally a secondary pest that is kept in check by natural enemies. However, both the vegetable leafminer (Liriomiza sativae) and the celery leafminer (L. trifolii) may become primary pests in cucumber fields where intensive pesticide use has destroyed their natural enemies. This trend began in the 1950s where widespread DDT and closely related compound applications eliminated the natural enemy populations of leafminers. Tiny, wasp like parasites are the main natural enemies of leafminers. Because of the establishment of effective natural enemies and the resistance of leafminers to insecticides, the major emphasis with leafminer management is the conservation of its natural enemies. This means that registered pesticides should be applied only when necessary and should be selected for maximum control of the target pest with minimum adverse effects on beneficial insects.

   A monitoring program consists of placing white styrofoam or plastic pans at the soil level below the plants throughout the field to collect larvae as they drop and pupate. Insecticides are recommended when average counts rise above 20 pupae/pan/day. Yellow sticky traps can also be used to monitor leafminers in the greenhouse. Thresholds need to be determined by the grower based on the particular environmental and crop conditions. Because contact insecticides won't reach the larvae inside the leaf, systemic insecticides are recommended. Because of their high reproduction rates leafminers are prone to develop resistance to pesticides. The celery leafminer is more difficult to control because it develops resistance to pesticides faster than does the vegetable leafminer. For best results from pesticide treatments use the recommended rates, rotate pesticide families when possible, and obtain thorough foliage coverage. Cultural controls include field or greenhouse sanitation, growing less susceptible varieties, and weed control around the field to eliminate alternate hosts.