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Everything you ever wanted to know about Leaf Miners!


Leaf miner is a term used to describe the larvae of many different species of insect which live in and eat the leaf tissue of plants. The vast majority of leaf-mining insects are moths (Lepidoptera), sawflies (Symphyta) and flies (Diptera), though some beetles and wasps also exhibit this behavior.

The precise pattern formed by the feeding tunnel is very often diagnostic for which kind of insect is responsible, sometimes even to genus level. The mine often contains frass, or droppings, and the pattern of frass deposition, mine shape and host plant identity are useful to determine the species of leaf miner. A few mining insects utilise other parts of a plant, such as the surface of a fruit.

Leaf miners are regarded as pests by many farmers and gardeners as they can cause damage to agricultural crops and garden plants, and can be difficult to control with insecticide sprays as they are protected inside the plant's leaves.

Common Leafminers of Trees and Shrubs

Sawfly Leafminers. Most sawflies chew on the surface of leaves, but four species found in Colorado develop as leafminers of woody plants. Adults are small, dark-colored, non-stinging wasps that insert eggs into the newly formed leaves. The developing larvae produce large blotch mines in leaves during late spring. The sawfly leafminers produced a single generation each year..

Elm leafminer (Kaliofenusa ulmi) is the most important species, being locally common in several Front Range cities where it develops on American, English and Siberian elms. Other species include: hawthorn leafminer (Profenusa canadensis) associated with Crateagus crus-galli, C. persimilis, and C. erectus ; birch leafminer (Fenusa pusilla) present in some plantings of white or gray birches; and alder leafminer (Fenusa dohrnii), a native species that develops in alder leaves.

Tentiform Leafminers. Larvae of several tiny moths (Phyllonorycter species) produce blotch mines in leaves that pucker when they dry out, somewhat resembling a pup tent. These tentiform leafminers occur on willows, poplars, and cottonwoods, hackberry and apple and leaf mines tend to be concentrated on the lower, shaded leaves. Probably two generations are normally produced. Outbreaks are rare because these insects are normally heavily attacked by parasites and other natural enemies.

Lilac leafminer. Another small moth, the lilac leafminer (Caloptilia syringella) produces a blotch mine and then folds edge of lilac and privet leaves. A related species, the boxelder leafminer, Caloptilia negundella, produces similar leaf injuries to boxelder leaves. There are two generations per year and the life cycle is likely similar to that of the lilac leafminer.

Needleminers. Several minute moths of the genus Coleotechnites have larvae that develop in the needles of conifers. Affected needles appear brown beyond the tunnels of the larvae. Most important is the ponderosa pine needleminer (Coleotechnites ponderosae) that has periodically produced outbreaks in forested areas of ponderosa pine. Related species occur in lodgepole pine, pinyon, and spruce.

Aspen leafminer. Delicate, winding mines through the upper leaf surface of aspen leaves are characteristic of another small moth, Phyllocnistis populiella. The feeding by the caterpillars that produce the mines produces insignificant damage and is notably mostly because they may attract attention.

Poplar blackmine beetle. Large black blotches in poplar leaves are produced by larvae of a leaf beetle (Zeugophora scutellaris). Adults chew small pits in the leaf. Outbreaks are extremely rare and the insect causes only minor leaf damage.

European elm flea weevil. Probably the newest leafmining insect to establish in Colorado is the European elm flea weevil (Orchestes alni). The larvae make a leaf mine in elm that superficially resembles that of the elm leafminer sawfly. The mine of the European elm flea weevil originates from a leaf vein, meanders in a serpentine form, and ultimately terminates in a leaf blotch along the edge of the leaf. Adults are tiny beetles that jump which makes pits in leaves, producing a lacy appearance of foliage when they are abundant.

Leafminers of Vegetables and Flowers

Columbine Leafminers. At least two species of leafmining flies in the genus Phytomyza produce leaf mines in columbine. One produces blotch mines while serpentine mines are characteristic of the second species. Adults are tiny dark flies and females make leaf punctures with their ovipositor so that they can drink plant fluids. The presence of small puncture marks is a indication of activity of these insects and can be useful for timing insecticides applied for control.

Vegetable Leafminers. Several Liriomyza species of leafmining flies make long serpentine mines through leaves of flower and vegetable garden plants. Normally these insects are very well controlled by natural enemies and outbreaks are almost always associated with use of insecticides.

Spinach leafminers. The only insect that regularly mines edible parts of plants is the spinach leafminer (Pegomya hyoscyami), which produces large, dark blotchmines in leaves of spinach, beets, and related weeds. Adults are small gray flies, about half the size of a house fly and they emerge in spring to lay eggs on the underside of leaves. The eggs of this insect are quite distinctive, being white and laid in small masses.

Problems are most common in gardens where spinach and beets are overwintered and continuously grown, providing host plants for the insects. Injuries most commonly occur in spring but there are two or more generations produced during the growing season.

How to get rid of them Organically?

Hopefully you have parasitic wasps that will do the work for you (another reason not to spray bee-killing insecticides in the garden!) But, if your leaves are being mined already, then you need to get to work removing and destroying the affected leaves. If you remove the leaves early, you avoid the chance that another crop will destroy additional leaves on the same or other plants. Plus, if you let the leaves remain, you may infect your soil with pupae, bringing on a fresh crop of problems next spring.

If your edibles are being mined, consider using floating row cover for your next crop to keep the adults from laying eggs. Chard, spinach, beets, and like crops are favorites of leaf miners, and a little row cover is non-invasive, non-toxic and works really well.

Can you spray? Please donít. Because leaf miners live inside the layers of the leaves, its unlikely that a spray application will even affect them. And if a product promises that it does kill leafminer larvae, keep in mind that it must have the ability to penetrate the cell layers of the leaf that you plan to eat.

Too, there are some weeds like plantain and chickweed that serve as host plants for this pest. Be vigilant in removing them from your garden.

Cultural practices such as mulching and staking of vegetables may influence both leaf miners and their natural enemies. Price and Poe (Price JF, Poe SL. 1976. Response of Liriomyza (Diptera: Agromyzidae) and its parasites to stake and mulch culture of tomatoes. Florida Entomologist 59: 85-87.) reported that leaf miner numbers were higher when tomatoes were grown with plastic mulch or tied to stakes. At least part of the reason seems to be due to lower parasitoid activity in plots where tomatoes were staked.

Oh, and when you cut off a leaf of infected chard or spinach, you can eat the part that hasnít already been eaten by the miner. Just tear the good part away and toss it in your salad. Share and share alike!

The parasitic wasp Diglyphus isaea is a commercially available beneficial insect that will kill leafminer larva in the mine. Use yellow sticky traps to catch egg laying adults. Cover soil under infested plants with plastic mulches to prevent larvae from reaching the ground and pupating

Everything you ever wanted to know about Leaf Miners!
Showing evident damage by leaf miners

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Berlin October 17, 2010
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