Perhaps one of the most obvious insect infestations in the garden is the handiwork of leafminer fly larva, which leave a telltale trail of destruction in their wake. Leafminers take their name from the behavior of their larval maggots, which tunnel within the vegetation of many crops, eating leaf tissues and leaving behind a winding trail of translucent and browning lines across the surface of leaves.
Adult leafminers flies are very small flies, typically around 1/10 inch long. They lay their eggs, which quickly hatch, on or inside the host plant’s leaves. The new larva will spend two or three weeks feeding on the leaves before they mature and drop out of the leaf to pupate on the ground. There are numerous species of leafminer flies in North America, and they closely resemble one another, as well as look like other native flies, so identification can be tricky.
Common leafminers that attack garden crops include the spinach/beet leafminer (Pegomya hyoscyami) and the vegetable leafminer (Liriomyza sativae). The plants that are under attack, as well as the shape or direction of the mining damage, can help you determine what leafminer species you’re dealing with. The greatest damage from leafminers is to leafy greens, such as spinach, lettuce, chard and kale, leaving the leaves unappetizing and inedible. Leafminer species will also target a wide range of fruiting crops, such as tomatoes and peppers, but minor infestations of otherwise healthy plants don’t usually cause much of a problem. Severe infections can lead to heavy leaf drop, which can eventually kill the plant.
For organic gardeners, the best strategy is to use floating row covers to prevent leafminer flies from laying eggs on vulnerable crops, though bear in mind, row covers are only effective if you plant in areas that haven’t had previous issues with leafminers. If you’ve already experienced an infestation, leafminers will emerge underneath your carefully placed covers, foiling your efforts. Row covers are especially helpful on greens crops, where the edible portions of the plants are directly affected. If leafminer damage is spotted in your garden, remove and destroy infected leaves immediately. Organic sprays with spinosad can be useful treating light leafminer populations.