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

****:LEAF MINERS

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)