A River Never Sleeps

Kick at the golden and yellow-green leaves of fall covering the trail while you walk down to your favorite fishing run. Almost certainly you will see snake-like markings imprinted on some of them. These are signs of a leaf-mining insect.

Leaf miners are insects that spent part of their life feeding upon the fleshy, inner substance (parenchyma) of the leaves. There they hide, safely out of view, between the upper and lower surfaces of the leaf.

As they tunnel along through the leaves, the insects leave behind a recognizable pattern or signature on the leaf surface. Apparently, most species of leaf miner has its own characteristic pattern.

The most common leaf patterns are linear mines (long, narrow, more or less winding); serpentine mines (narrow at their beginning and gradually enlarging with the larger end terminating in a head-like blotch), trumpet mines (narrow at the beginning and enlarge rapidly in a curve shape), and blotch or disk-shaped mines - the insect feeds in circles instead of straight ahead.

Deciduous leaves, grass blades, and pine needles are all attacked by leaf mining insects. Many different kinds of insects are leaf miners. There are leaf mining flies (eg Anthomyidae spp), leaf mining beetles (eg buprestid spp), and there are numerous leaf mining moths (eg Eriocraniidae, Nepticulidae, Opostegidae, Heliozelidae, Tischeriidae, Lyonetiidae, Phyllocnistidae, Gracillariidae, Gelechiidae, Incurvariidae).

For all these leaf mining insects, it is the larval stage which does the mining. When fully grown, larvae will either leave the leaf, or the larvae will pupate within the leaf and it is the adults that tunnel out and fly away. As might be expected, most of the leaf mining insects are very tiny.

Some of the nepticulid moths as full grown adults have wing spans of no more than 3 mm. It is believed that leaf mining arose as a way of escaping insect-eating songbirds, and parasitic flies and parasitic wasps. Leaf miners are quite common, and it is said that the leaves of most kinds of shrubs or trees are invaded by these little creatures.

In general each species of leaf miner targets one particular plant species, or a group of closely related plant species.

It’s interesting to look at the plants along the trail to the river, and realize that a whole secret world of insects is going on there.

The importance of leaf miners to fly fishermen - that is to say, to trout - appears to be unknown at this time. But with these insects proliferating in forests along streams, it seems common sense to assume that at some point in their life cycle leaf miners become available to trout.

How many of us have ever paused along a stream in the fall, to contemplate the leaves tumbling in the current. To us, these are signs of winter coming on. But you have to wonder if, to the fish, they are a reminder of previous feasts.