Order: Diptera, Lepidoptera, Coleoptera, Hymenoptera
Family: Douglasiidae, Agromyzidae, Gracillariidae, Tenthredinidae, Tischeriidae
A leafminer is the larvae of several different insects ranging from small black flies, moths, sawflies and beetles. Each is identified by the specific plant it attacks. The female lays oval eggs, which are clear before turning white, on the undersides and middle of plant leaves. After the eggs hatch, the larvae are green or black and 1/8 inches long. They immediately tunnel inside the leaves and feed between the upper and lower surfaces. Tunnels are white to brown and leave blotches on the leaves. The larvae of several species are visible through the leaf tissue. Damage causes the leaves to blister, curl, turn brown and die. Mature larvae pupate inside the leaves, on foliage or bark or may drop to the ground where they develop into adults. The larvae of small flies are the most common and do the most damage in gardens and vegetable crops.
There are two types of mines that leafminers dig in the leaves. The first is the “serpentine” mine which are elongated and bending. They are created when larvae advance in one direction during its feeding. The width and length of the mine help to identify the species. The other is the “blotch” mine, which are formed when the larvae turns around itself during feeding. They are in the shape of a circle or oval. Many mines are a combination of both, sometimes beginning with the serpentine and later forming blotch.
There are four most common types of leafminers found in North America:
The Coleoptera order includes Jewel beetles, leaf beetles, and weevils. Larvae usually are legless and females cover her eggs in a shiny black coat. They produce blotch mines and the larvae pupate within the mine in a circular or oval cocoon. Jewel Beetles are highly prized by insect collectors because of the color of their exoskeleton.
Weevils are also known as snout beetles because of the shape of their head. As adults they are damaging to crops. They damage wheat and cotton crops and are found in dry foods such as nuts, seeds, cereal and grain products.
Sawflies are part of the Hymenoptera family and create blotch mines. There are 6000 species worldwide. Females use a saw like ovipositor to cut slits through the bark of twigs where translucent eggs are wedged. She also lays eggs in leaf tissues that cannot be seen on the surface. Adults eat very little while the larvae feed on foliage and feed in groups and curl up on the leaf forming an “S” shape. Larvae pupate in the soil near the base of the plant after leaving the mines. They have a black or brown body and a threadlike antenna. Larvae have well developed head with chewing mouthparts.
The Gracillariidae family includes the Horse Chestnut Leafminer. As adults, they rest with the front part of their body raised in the air. They have normal mouthparts for feeding on plant tissue and a fully functional silk producing organ called a “spinneret”. The horse chestnut leafminer first appeared in Europe in 1985. They have shiny bright forewings with thin black and white stripes. Leaves that are infected by their burrowing stunt the trees growth and spread rapidly across the entire tree giving it an autumnal appearance. As the leaves die and drop off, new leaves grow and are again infected. They thrive in warm climates and produce six generations per year.
Citrus Leafminers is the larvae of a small moth that was first discovered in Florida in 1993. By 1995 they had moved to Texas, Central America, Western Mexico and the Caribbean Islands. They attack grapefruit trees, lemon and lime trees. Citrus Leafminers commonly only infest young foliage and pupate in the folds on the edges of the plants leaves. They rarely attack the fruit itself. A main characteristic is that the larvae leave a trail of feces in its mine. Mines are usually visible to the naked eye.
Plants that are affected by leafminers are Nasturtium and Chrysanthemum plants, Arborvitae, Azalea, Rhododendron, Holly, Roses, Lettuce, Beets, Swiss Chard, Spinach, Celery, Parsnip, Peppers, Potatoes and sometimes Parsley and Carrots. Parasitic wasps are usually used to help keep populations of leafminers in check.
Credits: Thanks to Lani Powell for making this Leafminer Information article possible!