What Are Leafminers?

Images are AI-generated illustrations. The teaching diagrams are conceptual and enlarged, not field photographs or measured results. Every AI-generated image has been personally reviewed and approved by a human. Any mistakes are therefore co-signed by the human.

You turn a leaf over, looking for whatever made the pale squiggle across it. Nothing on the front. Nothing on the back. The insect has chosen a third option.

A leafminer feeds inside the leaf, eating the tissue between its upper and lower surfaces. It does this as a larva, the young feeding stage of an insect. The space it eats out is called a mine. What you see from outside is the damage left along its route.¹

You may notice the trail long before you see its maker. The surprising part is how many different insects can fit their young into that narrow space.

Several insects have had the same idea

“Leafminer” does not name one species. The young of certain flies, moths, beetles, and sawflies all feed this way. They share a habit, rather than all belonging to the same insect family.²

That distinction matters when someone recommends a treatment for “leafminers.” The insect mining a vegetable leaf may be quite different from the one mining a tree leaf. The plant's identity helps narrow the search; the tunnel alone does not always identify its maker.

Here, we will follow the leafmining flies relevant to Diglyphus, including a group called Liriomyza. Their young are tiny, pale, legless larvae. They eventually become winged flies. The larva and the fly are two stages of the same insect, not two separate pests.³

The squiggle records a meal

As a larva feeds through the leaf, it leaves damaged tissue behind. Some mines form winding trails; others spread into broad patches called blotch mines. Their appearance can change as feeding continues. Spinach leafminer damage, for example, can start as a narrow mine and expand into a patch.⁴

So the mark is more than a stain on the surface. Part of the leaf has been eaten from within. That is why wiping the leaf will not remove it—and why a mine can remain visible after the insect has gone.

Left · Larva insideRight · Empty mine

A mine tells you that feeding happened. To find out whether it is still happening, you need to look for an occupant or continued expansion. Our leafminer identification guide shows how to examine a leaf and distinguish useful evidence from an old scar.

How the insect gets inside

In the leafmining flies discussed here, the female inserts an egg into the leaf. The egg hatches, and the larva begins eating the tissue around it. It does not have to climb in through the visible end of a tunnel; it makes the tunnel as it feeds.⁴

After feeding, the larva enters the pupal stage, when it develops into an adult. In the American serpentine leafminer, Liriomyza trifolii, it normally leaves the leaf and drops to the soil or potting mix first. There, its hardened outer skin forms a small protective case called a puparium. A winged fly eventually emerges.³

Top left · EggTop right · Feeding larvaBottom left · PupariumBottom right · Adult fly

This explains how fresh mines can appear on another leaf. Adult flies can move and lay eggs elsewhere; the insects are not limited to the leaf where you first noticed damage. Other leafminer species have different details to their life cycles, so this fly's route should not be treated as a rule for every miner.

How much trouble is one tunnel?

Leafminer damage can look dramatic without seriously affecting an established plant. Many garden leafminers cause mainly cosmetic injury. Heavier feeding can affect the plant's growth. And if you are growing leaves to eat, damaged foliage matters even when the plant keeps producing more.^{1, 4}

Seedling · Less foliageEstablished plant · More foliage

Look at the whole plant alongside the marked leaf. Is most of its foliage still healthy? Are new mines appearing? Those observations help you decide how much attention the problem needs. One ugly leaf can be very persuasive; it still deserves to be outvoted by the rest of a healthy plant.

Where Diglyphus fits

Living inside a leaf does not make a larva unreachable. Diglyphus isaea is a tiny beneficial wasp whose female can pierce the leaf and attack a suitable leafminer underneath. Her young feed on the fly larva and kill it.⁵

The word “suitable” matters. Diglyphus attacks particular leafmining flies; it should not be assumed to control every moth, beetle, or sawfly that makes a mine. Identifying the insect comes before choosing its opponent.

If yours is a matching fly, follow how Diglyphus finds it. The leafminer may have concealed itself from you. The wasp has been dealing with this arrangement considerably longer.