Forest Preserve District of Cook County, Illinois
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Leaf Miners
Nature Bulletin No. 539-A   October 12, 1974
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

Last summer we noticed that many of the leaves on oak trees in some locations were disfigured by large whitish-brown blotches. Using a sharp penknife, we opened one of them. In the paper-thin space between the upper and lower surfaces of the leaf was a tiny caterpillar that had been feeding on the tissue of green cells between those two colorless "skins. " It was the larva of an insect called the White Blotch Leaf Miner which, when adult, is a very small white moth.

Oaks are attacked by several kinds of leaf miners: tiny beetles, flies, and sawflies, as well as moths. On the leaves they lay eggs that hatch out grubs, maggots, sawfly larvae and caterpillars which do the actual mining. Apparently all of our trees and shrubs are infested to some extent with leaf miners -- even cone-bearing trees with needles, such as pine and spruce; and broad-leaved evergreens such as boxwood and holly.

In most cases the insects are not numerous enough to worry about, or the disfigurement is not important. However, on ornamental trees and shrubs it may be so serious that control by spraying with one of the newfangled insecticides is desirable. For instance, on birches and foreign species of elms attacked by a sawfly leaf miner, most of the leaves may die and drop off.

Many other kinds of plants are infested with leaf miners -- fruit trees, grape vines, berry vines, grain crops, garden flowers and wildflowers, vegetables, and even weeds such as burdock. On vegetables and some flowers the damage done may be serious. The Spinach Leaf Miner, a fly, also lays eggs on chard, beets and sugar beets, as well as many weeds. Its maggots create blister-like blotches that render the leaves unfit for sale and use as greens. On cabbage leaves, maggots of several species of flies cause white winding trails or broad blotches. The trademark of the Columbine Leaf Miner, which also feeds on larkspur and other flowers, is a labyrinth of narrow serpentine lines.

Some of these insects will attack only one species of plant; some are limited to members of a group of related plants; others, like the spinach leaf miner, are not so particular. There is a leaf-mining beetle which infests black locust trees but feeds also on soybeans and other legumes.

There are three general types of mines: the serpentine, the trumpet- shaped, and the blotches. Each tells a tale of the life history and hungry wanderings of a larva from the time when it hatched, and perhaps as small as a pinpoint, until it became fully grown. Then, in many species, it becomes a pupa from which the adult insect emerges. In others the larva emerges and drops to the ground where that transformation takes place.

Serpentine linear mines commonly resemble the aimless crawls of a child with a pencil but increase in width until, at the end where the larva pupated, there is an enlargement like the head of a snake. Some mines flare suddenly from the starting point and are trumpet-shaped. In blotch mines the larva feeds around and around instead of straight ahead. The type of mine, and the kind of plant upon which it occurs, gives us a clue to the identity of the insect that made it. We do not know how many species of leaf miners there are. Nobody knows. All of them are very small.

"And there's never a leaf or a blade too mean To be some happy creature's palace. " (James Russell Lowell)

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