A leaf with a pale winding leafminer trail, held up to the light

Leafminers

White trails inside the leaves?

Rarely serious for the plant. Find the speck at the end, then send the wasp that reaches it.

Check that it's leafminers

Is it leafminers?

Winding trails give them away.

Pale winding trails across a houseplant leaf

1

Trails that grow.

The trail widens as the larva inside does.

A small speck at the wide end of a leafminer trail

2

A speck at the wide end.

That's the larva, still inside and feeding.

3

Pinprick dots.

Where the adult flies feed and lay. They turn white.

Not quite matching?

Marks, but no trails?

Thrips

Silver patches with black specks on the surface, not trails inside.

How to treat thrips

Spider mites

Pale dots all over, sometimes webbing.

How to treat spider mites

A geranium leaf with corky blisters on its underside

Edema

Corky blisters on the underside from too much water. Not a pest.

Leafminers it is. Meet them ↓

Meet the leafminers

A fly that starts life inside a leaf.

The adult is a small black fly, often with a yellow triangle on its back.

Females slip their eggs into the leaf. The yellow maggots eat their way through the middle. That tunnel is the trail.

What it eats

It eats the inside and leaves the walls.

The larva feeds between the two leaf surfaces. Top and bottom stay. The middle doesn't.

Where they live

Three addresses, one family.

A houseplant with leafminer trails on several leaves

On the leafAdults.

Puncturing to feed and lay.

Inside the leafLarvae.

Tunnelling between the surfaces.

In the soilPupae.

Grown larvae drop out to pupate.

How they grow up ↓

The life cycle

Born in a leaf. Finished in the dirt.

Egg

Slipped inside the leaf.

Larva

A yellow maggot, mining.

Pupa

Brown and seedlike, in the soil.

Adult

A small black fly.

Swipe for the next stage →

How fast they multiply

10

generations a year.

Warm rooms keep it going.

In warm weather the whole cycle takes about two weeks. They move plant to plant to keep it going.

A spray bottle beside a leaf with leafminer trails

Why sprays miss

Sprays hit the wrong team.

Insecticides don't do much to larvae tucked inside a leaf, but they do kill the wasps that hunt them. A fresh outbreak often follows.

How the wasp finds them ↓

A tiny metallic Diglyphus wasp on a leaf beside a leafminer trail

The one that finds them

It listens through the leaf.

Diglyphus most likely finds a larva by feeling its vibrations travel along the tunnel. Then its own young grow up right there.

One tiny wasp does this job.

What to get

Two questions, one pick.

Question 1 of 2

Are the trails still growing?

Question 2 of 2

How many plants?

Count the plants with active trails.

How to count

Count a typical six-inch plant as 1, a large 8 to 10 inch plant as 2, a dense shrub or specimen as 3 to 4.

See all picks

Two questions, one pick.
If you see	Our pick	Why
Yes, with a speck at the endLive larvae in the leaves	Diglyphus isaea	Diglyphus, for live larvae.
No, they've stoppedOld trails, no speck	Diglyphus	Nothing to treat today.

Our suggested plan

New leaves call the next round.

1
Release day
Diglyphus, at the growing trails.
Opened in the canopy.
2
Every 2–4 weeks
Another round.
While new mines keep appearing.
✓
Stop
No new mines for two weeks.
You're done.

Why do the wasps leave?

No leafminers, no reason to stay.

One fresh trail isn't an emergency. Go by the new leaves, or ask us.

How to release

Lights on. Wasps out.

Before it arrives

Confirm it's leafminers. A growing trail with a speck at the end.

Take yellow cards down. They catch the wasps too. Take them down for a week, then hang them again up among the leaves. White sticky cards catch the flies and spare the wasps, so those can stay up.

Wait out any spray.

Once dryNeem or horticultural oil

1 dayInsecticidal soap

21 daysSulfur

NeverSystemic insecticides

Something we didn't list? Send the active ingredient our way. If its label wants a longer wait, wait longer.

Release day

Cartoon: opening a tube by a bright window — **Open it in daylight.**
Near a bright window or under the lights.

Cartoon: a hand opening a tube beside a plant with trailed leaves — **At the worst leaves.**
In the canopy, where trails are widening.

Karen's noteKaren Horn, owner

No sting, and zero interest in you. Leafminer larvae are all they want.

What to expect

Watch the new leaves.

Diglyphus wasps on a leaf beside two leafminer trails

Right after release
You'll lose sight of them in seconds. They're about 2 mm.
As it takes hold
Trails stop short and stay pale. That's a stopped larva.
When it's working
Fewer fresh trails. A new wasp generation every 10 to 14 days while there's food.

New trails on new leaves? Go again.

Karen's noteKaren Horn, owner

The leaf won't repair itself. It's a leaf, not a lizard.

Why treatment fails

Old trails lie. So does a dark room.

Old pale trails on an older leaf, clean new leaves above

1

Judged by old trails.

Finished trails stay on the leaf for good.

Cartoon: a tube opened in a dark corner, away from the window

2

Released in the dark.

No light, no search.

One less common reason

A systemic spray. It moves into the mine and reaches the young wasps.

Cartoon: clipping an old trailed leaf off a plant

Prevention

Clip the evidence. Cover the seedlings.

Clip off older trailed leaves. Small seedlings can go under a light protective cloth, so the flies can't lay in them.

Field Notes

Leafminers, between the lines.

Questions

Your plant will be fine. Mostly.

Will leafminers kill my plant?

Rarely. Mining usually does little to a plant's growth. Heavy damage can slow growth and drop leaves, and it matters most on seedlings.

What about the vegetable garden?

Older plants mostly shrug it off. Spinach and chard just end up looking worse for it. Small seedlings are the ones to protect.

Can I use them with lacewings or predatory mites?

Yes. Diglyphus works inside the leaf and they work on it, so they don't get in each other's way.