Leaf miners are fly, moth or beetle larvae that burrow into leaf tissue, leaving behind distinctive pale lines that are among the most recognisable of pest-damage symptoms.
There are leaf miners specific to azalea, ilex, iris, laburnum, pyracantha and syringa. In protected crops, the main threats are Liriomyza trifolii (American serpentine leaf miner) and Liriomyza huidobrensis (South American leaf miner). Both are notifiable in the UK. The latter is almost uncontrollable in some countries worldwide because of issues with pesticide resistance.
Liriomyza outbreaks in the UK have occurred on chrysanthemum, gypsophila, primula, verbena and viola. However, it is important to differentiate Liriomyza from the native chrysanthemum leaf miner, Phytomyza syngenesiae.
Horse chestnut leaf miner (Cameraria ohridella) has spread through most of Europe since it was identified on the continent 20 years ago. It is also known to attack Norway maple and sycamore. Research continues to investigate various measures, including pheromone-based monitoring and control.
Moth larvae mines can be quite extensive and heavily infested leaves can sometimes die off.
Some leaf miners that attack fruit can also hit ornamental trees. Phyllonorycta corylifoliella is often seen on pears, where it produces a mined area in the middle of the leaf, but can also affect Crataegus, Corylus, sorbus and Malus.
Identification and symptoms
Adult flies of chrysanthemum leaf miner and Liriomyza are about 2mm long but they differ in colour. The former are grey, while both Liriomyza species are black with a yellow spot between the wings on their backs.
The appearance of the mines can help in identification. Tunnels made by chrysanthemum leaf miner are white, less winding and tend to follow the leaf margin. Those of L. trifolii meander and are usually found towards the leaf tip.
L. huidobrensis mines occur at the leaf base and are often restricted by the leaf veins. Liriomyza mines are also distinguished by frass trails.
Mines produced by holly leaf miner (Phytomyza ilicis) and azalea leaf miner (Caloptilia azaleella) develop into a blotch as larvae grow. Lilac leaf miner moth (Caloptilia syringella) causes large brown blisters on lilac and privet leaves. Unusually, larvae leave their mines to pupate in leaf tips rolled up with webbing.
The adult horse chestnut leaf miner is a 5mm-long moth with white-striped brown forewings and grey-fringed hindwings. In multiple attacks, mines merge together and leaves turn brown and dry up. Damage can be confused with Guignardia leaf blotch. Phyllonorycter corylifoliella wings are a rich brown with white streaks.
Adults of Liriomyza and chrysanthemum leaf miner feed on sap seeping from perforations made by the female in the leaf. Eggs are deposited in some of the puncture holes. After about a week, the eggs hatch into larvae that tunnel into the leaf tissue, the mines getting wider as they grow.
After two-to-three weeks, Liriomyza larvae pupate on the outside surface of the leaf or drop out onto lower leaves or the ground. Chrysanthemum leaf miners pupate inside the leaf, their position given away by blister-like lesions on the outer surface. Shortening day lengths and lower temperatures in August trigger pupation.
Holly leaf miner produces one generation a year, the larvae remaining in the tunnels over winter before pupating the following spring. Lilac leaf miner pupae over winter at the base of trees and privet hedges, the small silver-white moths emerging in late spring.
Adults of horse chestnut leaf miner appear from April from overwintered pupae that can survive in fallen leaves at very low temperatures. Eggs are laid on upper leaf surfaces of the white-flowered types between May and August.
Phyllonorycta corylifoliella lay eggs on the upper leaf surface. Adults occur in April/May with larvae in June/July, both producing a second generation in September-March.
Treatment: biological control
• Diglyphus isaea, a parasitoid wasp, paralyses Phytomyza and Liriomyza larvae before laying its eggs alongside. On hatching, the wasp larvae feed on the pest larva from the outside. Usually recommended for control through the summer, when the wasp is highly active.
• Dacnusa sibirica, also a parasitoid wasp, lays its eggs in leaf miner larvae, where the wasp larvae develop, emerging as adults from the dead leaf miner pupae. Usually recommended for winter and early-season control. Mixtures of the two wasps are available.
• The nematode Steinernema feltiae swims into leaf miner mines, penetrates the larvae and releases a bacterium that kills them.
Treatment: cultural control
• Check plant consignments on receipt for signs of Liriomyza and monitor crops using yellow sticky traps.
• Control weeds (especially thistles), which can act as secondary hosts to native leaf miners.
• Where only a few leaves on specimen plants are affected, they can be picked off and destroyed.
Treatment: chemical control
Active ingredient Abamectin
IRAC code 6
Formulation Various including Dynamec (Syngenta)
Action(s) Selective insecticide that
controls larval stages only. Do not use on adiantum ferns or Shasta daisies. Spotting may occur on carnation, kalanchoe and begonia foliage. Compatible with some biological controls.
Active ingredient: Deltamethrin
IRAC code 3
Formulation Various including Decis (Bayer)
Action(s) Contact pyrethroid insecticide with residual activity. Incompatible with biological controls.
Active ingredient Lambda-cyhalothrin
Formulation Various including Hallmark WZT* (Syngenta)
IRAC code 3
Action(s) Persistent, contact and
residual insecticide. Incompatible with biological controls.
Active ingredient Oxamyl
IRAC code 1A
Formulation Vydate 10G* (DuPont)
Action(s) Systemic, carbamate nematicide. Incompatible with biological controls.
Active ingredient Spinosad
Formulation Conserve (Fargro), Tracer* (Landseer)
IRAC code 5
Action(s) Selective insecticide with contact and ingestion activity. Compatible with some biological controls.
Active ingredient Thiacloprid
IRAC code 4A
Formulation Various including Calypso*, Biscaya* (both Bayer); Exemptor (Everris)
Action(s) Systemic insecticide. Exemptor can be incorporated into the compost. Foliar applications incompatible with biological controls.
Fully updated by Dove Associates.
Use plant protection products safely. Always read the label and product information before use.
* EAMU required for use in ornamental plant production outdoors and/or under protection.
Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.