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
Insect excrement: the dark pellets left by caterpillars, the tiny black specks left by thrips, and the fine debris left by other feeders; finding it is a key sign of a hidden pest.A large palmate cannabis leaf with several long leaflets on a long petiole; fan leaves do most of the plant's photosynthesis and carry few trichomes.A coloured card coated in non-drying glue that catches flying insects, used mainly to monitor pest trends: yellow for gnats, whiteflies and aphids, blue for thrips.Tiny parasitic wasps of the genus Aphidius that lay eggs inside aphids; the larva kills the host from inside, leaving a swollen tan mummy.Slender insects about 1-2 mm long, chiefly western flower thrips (Frankliniella occidentalis), that rasp leaf surfaces, leaving silvery streaks dotted with black frass.Tetranychus urticae, the commonest cannabis mite indoors: a 0.5 mm sap-feeder that stipples leaves pale and spins webbing, breeding fastest in hot, dry rooms.A pattern of many tiny pale dots on leaves where a sucking pest such as a spider mite has emptied individual cells; the first visible sign of a mite infestation.A fungal leaf disease caused by Septoria cannabis that produces tan to grey spots with dark margins and yellow halos, starting on lower leaves.Systematic, regular inspection of plants, traps and roots to detect pests and disease early, identify them correctly and record numbers for decisions.Cleaning and disinfecting rooms, tools, containers, irrigation and people's clothing so that pests and pathogens are not carried from plant to plant or crop to crop.
