Meet the minscule insects that picnic on leafy vegetation

  • A serpentine mine is seen on an Indian plantain leaf.

    A serpentine mine is seen on an Indian plantain leaf. Courtesy of Lara Sviatko

  • Digitate mines are seen on black locust leaves.

    Digitate mines are seen on black locust leaves. Courtesy of Stillman Nature Center

  • Digitate mines are seen on black locust leaves.

    Digitate mines are seen on black locust leaves. Courtesy of Stillman Nature Center

  • A skeletonized leaf is one where some bugs, like Japanese beetles and slug caterpillars, eat all the leaf material but not any of the veins.

    A skeletonized leaf is one where some bugs, like Japanese beetles and slug caterpillars, eat all the leaf material but not any of the veins. Courtesy of James Coyle

 

By Mark Spreyer

Updated 8/23/2022 1:09 PM

There once was a growing young miner.

Who sought safety in a leaf's inner liner.

 

Away from the birds

And hungry lizards --She digested her digitate diner.

Not surprisingly, there is a paucity of poetry available on leaf miners. With that in mind, I wrote the preceding limerick. All silliness aside, let's track down these tiny invertebrate artisans.

Free feeders and skeletons

As summer progresses, various insects nibble on foliage, and they do so in a variety of fashions. Free feeders, for example, just chew through a leaf without regard to leaf veins. Free feeders include June beetles and most large caterpillars.

Some bugs like to eat all the leaf material but not any of the veins. When they are done, the leaf is said to have been skeletonized because all that is left is a marvelously lacy, leaf-shaped doily.

Japanese beetles and slug caterpillars are a couple of insects that feed in this manner.

Mines and Oreos

This brings me to the subject of this essay, leaf miners. Think of a leaf as an Oreo cookie.

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A leaf has an upper epidermis (with cuticle) and a lower epidermis. These protective layers are the chocolate wafers. Inside is the spongy, cellular goodness and inside is where leaf miners dine.

Most leaf miners are the larvae of flies, beetles, sawflies, and moths. Each immature insect has a characteristic way of feeding. When you combine the host plant with their feeding pattern, you can often easily identify the miner in question.

In general, leaf miners are flat, legless, and sport a wedge-shaped head. The head, with eyes arranged on the side, is useful when separating a leaf's epidermal layers. Not surprisingly, these miniature herbivores have sharp jaws and teeth powered by strong muscles.

By the way, you can sometimes spot leaf miners with the use of a hand lens. Even pine needles, believe it or not, can be home to a leaf miner. If you hold an occupied needle up to the light, you can sometimes spot the little critter wiggling inside.

                                                                                                                                                                                                                       
 

As you might expect, there is more than one style of leaf mine. The categories of leaf mines come with conveniently descriptive names. We have: linear mines, serpentine mines, blotch mines, and digitate (finger shaped) mines.

Eating in the bathroom

Believe it or not, being a tiny, leaf-gut eater has its risks. For example some, like the skeletonizers mentioned earlier, find veins to be an obstacle. Other miners have to be careful not to puncture a leaf's latex cells for fear the contents pour out and drown them.

Then, where do you relieve yourself? Some miners just poop as they go. Little black clumps of frass, the term for insect larvae excrement, can be seen in a closely examined mine.

Others cut small holes in the epidermis and push their frass through, sort of a wall-less outhouse. Still others excavate little side chambers off their mines. I like to think of these feces repositories as "inhouses."

Waste disposition is not to be taken lightly. You see, some miners first feed on only liquid plant contents. Then, as they grow larger, return to the same area to eat cellular tissues. Proper frass disposal is important so as not to contaminate a future feeding zone.

Gracillariid moths

As a group, Gracillariid moths are known as leaf blotch miner moths. However, not all the mines they make are blotch mines. In fact, mine names are not exactly precise categories. For example, you can have linear-blotch mines, serpentine-blotch mines and so on. Heck, there's even a polygon miner!

Now, lets zoom in on one example of a serpentine miner. The larvae of this particular Gracillariid moth (Phyllocnistis insignis) forms serpentine mines in the leaves of Indian plantain and other members of the Aster family.

Since there are a variety of "plantains" growing in the world, let's add a few details about Indian plantain. Once common in the Chicago region, it has become much harder to find thanks to modern development. It prefers moist sites such as wet meadows, low prairies, marshes, and fens. It reaches six feet in height and has large clusters of white flowers.

While the plant may be large, its miner is miniature. Once the larva emerges from its pupa as an adult moth, it will be all of 3 mm long!

This miner deserves mention because its decorative, winding trails will be seen by many students that visit Stillman Nature Center in early autumn. These fascinating mines start as a pinpoint, where the egg hatched, and get wider as the larvae get older and larger.

The moth's scientific species name, insignis, means marked or noted. One might say that this bug's artistic, serpentine mine is its "insignature."

Locust leaf miners

I knew little of Indian plantain until I started working at Stillman. The black locust tree, by comparison, was an old friend. You see, my undergraduate degree is in forestry.

The locust miner I first saw as a student was the Locust Digitate Leaf Miner, a beetle larva.

The tiny adult beetle, 5-6 mm long, lays a group of three to five eggs on the underside of the locust leaf. The eggs overlap like shingles and are held in place with a coating of excrement.

When the little ones hatch, they bore into the leaf and feed as a group. Think of it as a kind of miner picnic. Picnicking humans might eat some eggs while enjoying the natural environment. Picnicking miners, on the other hand, come out of an egg and eat their natural environment. In time, the locust miner picnickers go their separate ways.

Up until recently, I would have gone my separate way on the subject of locust leaf miners until I learned another one of the Gracillarid moths' larvae mine their way though locust leaves. Besides locusts, this particular larva also feeds on birch, elm, hawthorn, oak, and wild cherry.

As long as we're talking about moth larvae and locusts, I thought you might like to know that black locust is a favorite food for the caterpillar of the silver-spotted skipper, our largest skipper butterfly. At two inches across, this showy butterfly seems gigantic compared to the minuscule miner moths and beetles.

Stanza mining

As suggested at the outset, poets rarely cover leaf miners. So, I'd like to close this article with a did-it-myself haiku, a Japanese poem of 17 syllables, in three lines of five, seven, and five. Traditionally, they evoke images of the natural world. Here's mine on miners:

Serpentine and blotch,

epidermis left untouched,

chlorophyll fuel.

• Mark Spreyer is the executive director of the Stillman Nature Center in Barrington. Email him at stillmangho@gmail.com.

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