Discovering white squiggly lines on the leaves of your tomatoes (Lycopersicon lycopersicum) is devastating, because those lines are the trademark of leafminer pests. The larvae of small, yellow and black flies, leafminers feed within the leaves, resulting in meandering trails. Light infestations typically cause only cosmetic damage, but severe infestations can cause leaf drop, which reduces the size and yield of your tomato crop and increases the risk of sunburned fruit. Defoliation also reduces the vigor and overall health of plants. Several control methods can help you manage and prevent leafminers in your tomato garden.
Various leafminer species exist, but the two most likely to attack your tomatoes are the vegetable leafminer (Liriomyza sativae) and the serpentine leafminer (Liriomyza trifolii). Both species overwinter in the soil as pupae and emerge as adults in the spring to feed and mate. Adult female flies puncture the leaf tissue and insert single eggs within the leaves. They prefer to lay eggs inside of mature leaves, so the pests are generally concentrated in the lower and middle sections of tomato plants.
The larvae feed on the inner leaf tissue until they mature, when they drop to the soil and dig 1 to 2 inches below the surface to pupate. The entire life cycle takes about two weeks in warm climates and up to 35 days in cooler places, so several leafminer generations can occur each year. Leafminers are difficult to control because the leaves protect them from the chemicals used in traditional pesticides.
Nonchemical Control Methods
Inspect your tomato leaves at least once a week for the squiggly white lines, crushing any mines you find between your fingertips or picking off and discarding the affected foliage in a covered trash can. Get rid of potential hiding spots by keeping your garden clean. Remove weeds throughout the growing season and pull up tomato plants right after the last harvest. Either discard the debris or plow it under. Till the soil to a depth of 6 to 10 inches before planting in the spring to help eradicate any surviving larvae. Draping a lightweight floating row cover over young seedlings prevents the adult flies from laying eggs on the plants. Covering the soil under infested tomato plants with plastic mulch prevents the larvae from burrowing into the soil to pupate.
Yellow Sticky Traps
Prevent leafminer infestations by placing yellow sticky traps around tomato plants to catch the female flies before can lay eggs. Purchase commercial traps or create your own by cutting thick cardboard or plywood into 3-inch wide by 7-inch long rectangles. Paint the material yellow, fasten them to stakes and spread a thin layer of petroleum jelly over one side. Drive the stakes into the soil about 9 inches away from tomato plants, making sure the sticky substance faces the plant but doesn't stand in direct sunlight. Clean traps once a week throughout the entire growing season, reapplying the petroleum jelly when needed.
Sprays containing spinosad, a natural, soil-dwelling bacterium, kill leafminers without harming any predatory beneficial insects. The substance will harm any bees exposed to it for about three hours following treatment, however, so spray plants around dusk when the pollinators won't be active for a while. Follow the directions on the product's label. One product recommends using 2 fluid ounces of spinosad concentrate for every gallon of water. Apply a thin, even film to the tops and undersides of leaves. Reapply every four days, as needed, but don't spray plants more than six times throughout the growing season.
Although natural, spinosad sprays can still irritate your eyes and skin on contact. Wear protective clothing, goggles and a facemask to help prevent accidental exposure. Apply spinosad on calm days when no precipitation is expected for the next 24 hours. Allow the spray to dry for at least four hours before allowing people or pets to enter the treatment area. You must wait at least 24 hours between spraying and harvesting your tomatoes, and wash the fruit thoroughly before eating it.
Attract Natural Predators
Natural enemies of leafminers include several parasitic wasps that attack the pests inside of their mines. Placing flowering plants around your garden helps lure these beneficial insects to your landscape and provides them with an additional food source and shelter. Parasitic wasps particularly like members of the carrot family (Apiaceae), including annual flowering herbs such as coriander or cilantro (Coriandrum sativum), fennel (Foeniculum vulgare), dill (Anethum graveolens) and parsley (Petroselinum crispum). Avoid using broad-spectrum insecticides on tomato plants if you want to promote parasitic wasp populations for natural pest control.