Leafminer in Vegetables: How to Spot the Tunnels and Stop Them

They feed on many crops. L. sativae likes legumes, plants of the daisy family and the potato family best, yet it has also been found on capsicum, carrot, cucumber, lettuce, melon, onion, tomato and zucchini. Heavy attacks cause leaf loss, so the fruits are exposed to sunscald and the plant tires.

How to Identify Leafminer

Hold a leaf to the light. The larva is a tiny pale maggot inside the leaf, and its path shows as a white, irregular or coiled tunnel, with a thin black line of droppings inside. Small white puncture dots on the leaf are the feeding and egg laying marks of the adult fly. The adult is a small black and yellow fly, about 1.7 mm for a female. Disease spots on leaves do not form winding tunnels, so a tunnel means leafminer.

• White, winding or coiled tunnels on the leaves, with a thin black line inside.
• Many tiny white puncture dots on the leaf surface.
• Mined leaves that dry, turn brown and fall off.
• Fruits that burn in the sun after the leaves are lost.
• Young plants that stop growing or die in a heavy attack.

How Leafminer Hurts Your Crop

The Pacific Pests fact sheet says a heavy attack causes plant stress, water loss, leaf drop, sunscald on the fruit, and even the death of young plants. Mined produce can also be rejected by buyers.

A study on onion in Nueva Ecija and Nueva Vizcaya showed why careless spraying backfires. Where farmers sprayed less, up to 41 percent of the larvae were parasitized by wasps, against up to 18.6 percent where they sprayed more. So the natural enemies already work for you, and strong sprays kill them. We found no Philippine action threshold in the sources we read.

How to Control Leafminer

1. Inspect seedlings and the first true leaves for tunnels before you plant. Do not transplant mined seedlings.
2. Control weeds and plow the soil before planting, so the pupae in the soil are exposed. Avoid planting leafminer hosts one after another in the same plot.
3. Handpick mined leaves, or crush the larva inside the tunnel, when the attack is light.
4. After harvest, collect the crop remains and burn them or bury them deep. Mulch the soil in high value crops, as Pacific Pests advises.
5. Spare the parasitic wasps. Pacific Pests names Chrysocharis, Neochrysocharis and Diglyphus as helpful ones, and an onion study found wasps parasitized up to 41 percent of the larvae. See the integrated pest management guide.
6. Use yellow sticky traps to watch the adult flies. Rotate with a crop that the fly does not like, such as corn.

When a Spray Makes Sense

Choose a spray only when the tunnels keep spreading. Avoid broad spectrum insecticides such as pyrethroids, carbamates and organophosphates, because they kill the wasps and the leafminer comes back worse. Follow the FPA registered label.

• Carbosulfan and carbaryl: Highland leafminers survive old carbamates, and the spray kills their wasps, so mines spread faster.
• Bifenthrin plus imidacloprid, acetamiprid plus bifenthrin, cypermethrin and fenvalerate: Pyrethroids kill the wasps that keep leafminer down, and it comes back worse.
• Fenthion, malathion and profenofos: Leafminers survive many organophosphates, and the spray kills their wasps, so mines spread faster.
• Fenobucarb (BPMC) plus chlorpyrifos: Two old broad chemicals that leafminers survive. They kill the wasps and mines spread faster.

Resistance. The pea leafminer (Liriomyza huidobrensis) of highland potato and vegetables survives many organophosphates, carbamates and pyrethroids. Keep one group per window of 2 to 3 weeks: abamectin (6), then cyromazine (17), then spinetoram (5) or cyantraniliprole (28).

Keep It Out Next Season

• Check seedlings and first leaves for tunnels, and never transplant mined seedlings.
• Plow the soil and clear weeds before planting, so the pupae in the soil dry out or get eaten.
• Pick and crush mined leaves while the attack is light.
• Bury or burn crop remains right after harvest, and do not leave an old crop beside a new one.
• Yellow sticky traps catch the adult flies and show when they build up.
• Rotate with a crop the fly does not like, such as corn or rice.

Natural Enemies and Biological Help

• Parasitic wasps such as Diglyphus, Chrysocharis and Neochrysocharis kill many larvae in the mines. Broad sprays kill them and leafminer comes back worse.
• Neem works on the larvae with less harm to the wasps.

Not Sure What You Are Seeing?

Take a clear photo of the insect, the damage and the whole plant, then ask Anee, the smart farm technician in anee.io, in Tagalog or English. Anee tells you what it most likely is and what to do at the age of your crop. You can also put your scouting days on the cropping calendar, so the team knows when to walk the field.

Questions Farmers Ask About Leafminer