A leaf miner is a tiny larva that lives inside a leaf. It eats the soft green tissue between the top skin and the bottom skin of the leaf. From the outside you see a pale, winding tunnel or a pale blotch, often with a thin dark line of droppings inside. On cannabis, an important group is the larvae of small flies in the genus Liriomyza.
“Leaf miner” is not the name of one species. It covers the larvae of many small flies, moths and beetles that feed this way. What they share is where they feed. Insects such as mites and caterpillars feed on the leaf surface, but a miner larva stays sealed inside the leaf the whole time it eats. That is why a spray aimed at the leaf surface so often misses it.
This article explains how to recognise leaf miners, how fast they breed in warm weather, how to check how many are active, and which controls work. The information comes from extension fact sheets of several US universities and an official pest report from Australia; they are introduced where they are first used and listed at the end.
Which species attack cannabis
Most leaf miners on vegetable and herb crops are fly larvae in the family Agromyzidae. Three species do most of the damage worldwide. All three are polyphagous, meaning they feed on many unrelated plant species:
- the American serpentine leafminer (Liriomyza trifolii)
- the vegetable leafminer (L. sativae)
- the pea leafminer (L. huidobrensis)
The entomology fact sheets of the University of Florida (UF) call these three the most damaging polyphagous leaf miners of horticultural crops. All three came from the Americas and have spread widely from there. UF also notes that L. sativae, once found only in the Americas, now occurs in many areas of Asia and the Middle East.
Cannabis is a confirmed host. The International Plant Protection Convention (IPPC) is the international plant-health treaty, and member countries post official pest reports through it. One such report from Australia records L. trifolii on Cannabis sativa (hemp). The same report lists cotton, safflower and several common weeds as hosts, and gives a host range of more than 400 plant species.
To name the exact species you need a microscope and an entomologist (an insect specialist). For day-to-day management, the species rarely changes what you do. So you do not need a species name before you act.
Translator’s note: this article does not confirm which of the three Liriomyza species are recorded on crops in Thailand. For a grower this matters little, because the controls below work the same way for all of them. If you need a confirmed identification, for example for a GACP audit or an export buyer, keep a few mined leaves with live larvae in a sealed bag in the fridge and ask a university entomology department or the Department of Agriculture (กรมวิชาการเกษตร) whether they can identify them.
Signs to look for
- White dots come first. The adult female pierces the leaf with her ovipositor (the egg-laying tube at the tip of her body) to feed and to lay eggs. The University of Georgia notes that these punctures give the leaf a stippled look, meaning many tiny pale dots, and that only 10 to 25% of them receive an egg. The rest are places where the female fed.
- Winding tunnels (serpentine mines). A thin, winding pale line that grows wider as the larva grows. UF describes the tunnel widening from about 0.25 mm to about 1.5 mm.
- Droppings inside. A dark thread or dotted line of droppings (called frass) runs along the tunnel.
- Blotches. Some species, and heavy infestations, make broad pale patches (blotch mines) instead of neat lines.
- Exit slits. When the larva is fully grown, it cuts a small half-moon slit, usually in the upper surface, and leaves the leaf to pupate (turn into an adult).
- Leaf loss. Heavy mining reduces the green area that makes the plant’s food from light. Badly mined leaves can dry out and drop.
What to do: when you see white dots on the upper surface of young leaves, check those leaves again a few days later for the first thin tunnels.
Telling leaf miner damage from other problems
| What you see | Likely cause | How to confirm |
|---|---|---|
| Winding pale tunnels that widen along their length, with a line of droppings inside | Leaf miner (serpentine mine) | Hold the leaf up to the light. A live larva is often visible at the wide end |
| Irregular pale blotch, papery, with droppings between the two leaf surfaces | Leaf miner (blotch mine) | Peel back the leaf surface. The larva is inside the leaf, not on top |
| Many tiny white dots with no tunnels | Adult leaf miner punctures, or feeding by thrips or mites | Check the leaf undersides with a hand lens. Use yellow sticky cards to catch adult flies |
| Round brown spots with yellow rings around them | Fungal leaf spot | The spots have no tunnel and no droppings, and may show dark specks |
| Holes or ragged leaf edges | Caterpillars or beetles chewing the leaf | The damage goes right through the leaf |
The rule is simple. If the damage lies between the two leaf surfaces and you can see a track, it is a leaf miner.
What to do: before you treat anything, hold one damaged leaf up to the light and look for a track and a larva.
Life cycle, and why heat matters
UF gives the details for the vegetable leafminer:
- The female inserts each egg just under the leaf surface. The egg hatches in about three days.
- The larva feeds inside the leaf through three active growth stages.
- The fully grown larva usually cuts its way out, drops to the ground and pupates a few centimetres down in the soil.
- At 30 °C the whole cycle, from egg to adult, takes about 15 days.
The adults are tiny yellow-and-black flies, less than 2 mm long.
This short cycle is why warm weather matters. UF notes that in warm climates these flies can breed all year round, with many generations overlapping. So outdoor cannabis in a region that stays warm all year can show mines on the lower leaves for most of the season, not in a single wave.
Translator’s note: daytime temperatures in much of lowland Thailand and Southeast Asia reach 30–35 °C for much of the year, and many indoor rooms run close to 30 °C. Use the 15-day figure as a planning number: a new generation of flies can appear about every two weeks, with no cold season to break the cycle. UC IPM likewise notes that the life cycle takes only about two weeks in warm weather. Because the larvae drop into the soil to pupate, the pots and ground under a mined plant are part of the problem. What to do: plan your checks at least weekly, so you catch each new generation while it is still in its first tunnels.
Monitoring
- Hang yellow sticky cards at plant height. They catch adult flies and warn you when flies arrive from the plants and weeds around your grow.
- Count mines that still have a live larva, not all mines. The University of Georgia notes that counting all mines can overestimate activity, because many mines are empty. Mines with live larvae tell you how much damage is still coming.
- Check the lower leaves. The University of California’s Statewide Integrated Pest Management Program (UC IPM) notes that most mines are on the older leaves near the bottom of the plant. Some mines are easier to see from the leaf underside.
- Look for dead larvae. A darkened, shrivelled larva inside a mine is often a sign that a parasitic wasp has attacked it. That is good news: natural control is working.
What to do: once a week, turn over ten lower leaves per bed, count live and dead larvae, and write the numbers down with the sticky-card count. After a few weeks you will see whether pressure is rising or falling.
Control
Remove mined leaves early
The larva lives inside the leaf, so picking off a mined leaf takes the larva out with it. On a small number of plants, picking leaves plus sticky cards is often all you need. Put the picked leaves in a bag and take them out of the grow area. Do not drop them on the ground, because the larvae pupate in the soil below.
Translator’s note: a clone or young plant bought from another farm can arrive with larvae already inside its leaves, together with other pests, diseases such as hop latent viroid (HLVd), or the wrong variety. Keep new plants apart for at least one to two weeks and check the leaves against the light before they join your main room. What to do: inspect every incoming plant leaf by leaf, and remove any leaf with a tunnel before it goes in.
Protect the parasitic wasps
Tiny parasitic wasps are the main natural control of leaf miners. These wasps lay their eggs in or on the larva inside the mine, and their own young kill it. UF lists wasps in the families Braconidae, Eulophidae and Pteromalidae. It names Diglyphus species among the most important in North America. UC IPM names Diglyphus as the main genus in pepper crops.
Both UF and UC IPM give the same warning. Broad-spectrum insecticides, meaning sprays that kill most insects rather than one target pest, kill these wasps as well. Using them against other pests is a common cause of leaf miner outbreaks. UC IPM’s guidance for peppers says that if at least 50% of the larvae are parasitised, the crop will most likely tolerate the damage. Pepper guidance is useful here because the same Liriomyza flies attack peppers, and few such guides exist for cannabis. Growers buy and release Diglyphus wasps commercially, mainly in greenhouses.
Translator’s note: we could not confirm a commercial supplier of Diglyphus in Thailand. Bringing living insects into the country needs permission; see sourcing beneficial insects legally before you order anything from abroad. The practical route for most Thai growers is to protect the wild wasps that arrive on their own. What to do: count dead, darkened larvae in your weekly check; if half or more of the larvae are dead, hold off on any spray and keep picking leaves.
Clear weeds and debris, and screen out the flies
Broadleaf weeds such as nightshade, lambsquarters and pigweed carry the same flies. The University of Wisconsin–Madison Extension recommends destroying weeds that host the flies and removing infested plant material. It also suggests covering highly susceptible crops with floating row cover, to keep the adult flies from laying eggs.
Translator’s note: in the Thai rainy season, weeds around an outdoor plot grow back within weeks, and vegetable or herb plots next door may also host the same flies. Greenhouses can use insect netting on vents and doors; see greenhouse growing in the tropics for pest screening and ventilation together, since fine netting also reduces airflow. What to do: keep a weeded strip around the plot, clear it again after heavy rains, and remove fallen leaves from under the plants.
Sprays: what they can and cannot do
The University of Georgia describes chemical control of leaf miners as difficult for two reasons. The larvae are protected inside the leaf, and resistance to insecticides is severe. Wisconsin Extension adds that a spray must go on early in the insect’s life cycle to have any effect.
UC IPM lists two options acceptable in organic production of peppers:
- azadirachtin, the active compound extracted from neem seeds
- a spinosad product; spinosad is an insecticide made from compounds produced by a soil bacterium
Whether any product is allowed on cannabis, and at which growth stage, depends on local registration. Many labels exclude flowering plants entirely.
Translator’s note: in Thailand, use only pesticide products registered with the Department of Agriculture (กรมวิชาการเกษตร), and use them exactly as the label says. Neem (สะเดา) is familiar to Thai farmers, but a home-made or unregistered neem mix is not a registered product. No official list of products approved for cannabis exists, so check the registration, the label and your GACP auditor first; see what you may legally spray on cannabis under Thai GACP. Avoid spraying flowers at all, because residues on flower can fail a lab test. What to do: if you decide to spray, time it for the first tunnels, before larvae are fully grown, and record the product, its registration number, the date and the dose.
How serious is it on cannabis?
In most crops leaf miners are secondary pests, meaning they rarely become the main problem. Colorado State notes that leaf-mining insects often cause damage that is easy to see but usually does not affect plant health much. The University of Georgia also calls them secondary pests that natural enemies usually keep down.
Two situations need closer watching:
- Young plants and clones. Each leaf is a larger share of a small plant, so the same number of mines costs it more.
- Grows where heavy spraying has killed the parasitic wasps. Without them, nothing holds the flies down.
What to do: give seedlings and clones the closest weekly check, and remove any mined leaf on them at once.
Common mistakes
- Spraying the leaf surface and expecting it to reach the larvae. Contact products, which work only on what they touch, do not reach a larva already inside the leaf.
- Using broad-spectrum insecticides “just in case”. This is the classic way to turn a small leaf miner problem into an outbreak, because it kills the wasps that were keeping it small.
- Counting old, empty mines. They show past activity, not current pressure. Count only mines with live larvae.
- Ignoring the weeds around an outdoor plot. The flies may be breeding right next to your plants.
Check the current local rules on which products may be used on cannabis before you apply anything.
Share what you find
If you have leaf miners in your grow, post below. Include a clear photo of a mined leaf held against the light, your weekly counts of live and dead larvae, your temperature, and whether the plants are indoor, greenhouse or outdoor. Other growers can then help you judge whether to pick, wait for the wasps, or treat.
References
- University of Florida, entomology fact sheets on the American serpentine leafminer (Liriomyza trifolii) and the vegetable leafminer (Liriomyza sativae).
- International Plant Protection Convention (IPPC), official pest report from Australia: Liriomyza trifolii detected on Cannabis sativa (hemp) and other hosts.
- Colorado State University Extension, fact sheet on leaf-mining insects.
- University of Georgia, extension guidance on leaf miners.
- University of California Statewide Integrated Pest Management Program (UC IPM), pest management guidelines for peppers: leafminers.
- University of Wisconsin–Madison Extension, guidance on leaf miners.