A leafminer is any insect larva that feeds inside a leaf, between the upper and lower surfaces. On cannabis the usual culprits are small agromyzid flies, mainly Liriomyza species. The adult female punctures the leaf to lay eggs; the larva then eats a pale, winding tunnel that widens as it grows, and finally drops out to pupate in the medium.
In practice
Leafminer trails are distinctive: serpentine white or tan lines, often with a dark thread of frass down the middle. Damage on cannabis is usually cosmetic and limited to fan leaves, mostly outdoors or in greenhouses. Picking off mined leaves and destroying them breaks the cycle. Yellow sticky traps catch adults. In greenhouse crops, parasitic wasps such as Diglyphus isaea are used against thembeing reviewed. Because larvae are protected inside the leaf, contact sprays do little. Check what is registered for cannabis locally.
Don't confuse with
- Thrips: surface scraping with silvery patches, not tunnels.
- Spider mite stippling: tiny dots, no trails.
- Septoria leaf spot: round spots with dark margins, not winding lines.
