Why is it a problem? The citrus leaf-miner is mainly a problem in young (seedling) trees, including in nurseries, where at high densities it retards growth and can result in secondary infections. Mature trees are better able to withstand the attack.
Where and when is it a problem? It is present in all mandarin orchards, but only rarely requires control.
Adults are small (about 3 mm long, wingspan in flight of 4.5 mm), delicate moths with narrow paired forewings and hind wings fringed with long hairs. The upper surface of each forewing has a black dot at the tip. The hind wings are narrower than the forewings and are covered with silvery scales. Because flight generally occurs at night, the adults are rarely seen in daylight except when they are disturbed, generally by human activities. When this occurs, flight is short and rapid. Eggs are flat, slightly oval, and about 0.33 mm long and deposited singly, on the underside of leaves near midrib, usually at the base of leaf. Pupae are yellowish-brown, about 2.5 mm long and found in mines and rolled onto the edges of the leaves.
Larvae feed on the undersides of the leaves under the epidermis causing serpentine mines initiating near the vein. Silvery serpentine leaf-mines have a characteristic black line in the middle caused by faecal matter. The mining on leaves cause leaf-curling and distortion, and sometimes secondary infection by fungi and bacteria. Growth of plants can be retarded if the damage to flushes of nursery trees and young leaves persists. At infestation levels with high humidity larvae may also mine green twigs. Mining on the fruit is very uncommon.
Confusion with other pests: This is the only leaf-miner commonly found on citrus. The serpentine mines are characteristic.
Lifecycle: Eggs are usually laid singly on the underside of the leaves and hatch in 2-10 days. Newly emerged leaflets (10-20mm) are the preferred egg laying site. Old leaves are avoided for egg-laying. On hatching, larvae burrow into the leaf, making their mines. Larvae never leave their mines to form other mines or move between lower and upper side of leaves. When larvae complete their feeding they mine near the edge of the leaf causing the leaf margin to fold over. Pupation takes place at the edge of the leaf within a folded portion. The pupa remains in the mine until it emerges as an adult. Depending on the climate there can be several generations per year, with the lifecycle being completed in under three weeks under optimal conditions. Aestivation (over-wintering) takes place as larvae or pupae. Mating occurs at dusk and in the early evening.
Dispersal: Adults fly.
When can damage be expected? The citrus leaf-miner favours new growth. Outbreaks are most common in nurseries.
Hosts: The citrus leaf miner is restricted to the citrus family (Rutaceae). All types of citrus leaves (e.g., grapefruit, pummelo, lime, lemon, and orange) are fed upon. In addition, the citrus leaf miner may feed upon kumquat and calamondin leaves.
Leaf miners are only occasionally a problem. In most cases good orchard management and regular inspection should prevent it becoming a problem.
Fortnightly inspect young seedlings for leaf-mine attack during the warmer months. Remove leaf-mined leaves as they are found.
Effect of variety
Citrus varieties do differ in susceptibility, but this has not been quantified in Bhutan.
- Some control can be achieved by collection and destruction of fallen leaves in the winter. This may be particularly useful in nurseries.
- In nurseries if infections are low, hand picking and destruction of infected leaves can be carried out.
- Adopt best-practice irrigation and fertilization to limit excess flush growth in late summer and autumn.
- Prune only once a year to reduce sprouting and induce a uniform and short cycle.
- Remove vigorous shoots below and above the graft union on the trunk of mature trees as they produce new leaves for a long period of time.
- Horticulture Mineral Oil (0.5%) has been successfully used elsewhere. Preliminary work in Bhutan suggests it will be very effective here as well. Care in application would be needed to avoid phytotoxic effects. It is currently not available in Bhutan.
- Larvae are normally attacked by a large variety of wasp parasitoids. It is therefore important to minimise the use of insecticides in citrus orchards.
- Chemical control on mature trees is generally not needed nor recommended due to the cost of multiple applications. Difficulty in killing larvae in the mines, build of resistance and destruction of natural enemies all combine to make this method non-viable. Spraying can in fact benefit the leaf-miner by killing natural enemies.
- In case of serious infection in nurseries, the systemic insecticide Dimethoate should be sprayed at the rate of 2 ml per 1 litre of water.
Image acknowledgements: NPPC.