Citrus Leaf Miner | Mandarins.com.au

Citrus leaf miner (Phyllocnistis citrella) is a very common pest on Australian mandarin and citrus trees. It arrived from Southeast Asia in the early 1900s and is now found in every state. The adult is a tiny moth with a 4 to 5 mm wingspan. The moth itself is rarely seen. What you notice are the silvery, winding trails left by larvae tunnelling inside new leaves.

What it is

The adult moth lays eggs on the underside of young, soft citrus leaves. The larvae that hatch burrow into the leaf tissue and feed for five to twenty-one days, leaving the characteristic silvery trails. As larvae mature and pupate, affected leaves curl and distort, sometimes almost rolling shut along the margin. The adult moth emerges one to three weeks later, completing a generation in around four to five weeks. Up to 15 generations can occur in a single year in warm climates.

Citrus leaf miner attacks all citrus including lemons, limes, oranges, mandarins, and native finger limes.

Symptoms on new leaves

The main sign is silvery, winding tracks across young leaves. Leaves may also show:

Curling and distorting at the edges as the leaf hardens
Blistering where larvae have fed
Obvious curl and fold as pupae form along the leaf margin

The damage looks alarming but affects only new growth. Mature, hardened leaves are not attacked. Female moths lay eggs only on early flushes of new foliage.

Damage threshold

Large, established mandarin trees usually tolerate citrus leaf miner with no lasting damage. The Eco Organic Garden and Leaf Root & Fruit Gardening Services both describe leaf miner damage on mature trees as generally superficial.

Young trees and newly planted trees are more vulnerable. Heavy infestation of a small tree with few leaves can slow growth significantly. If more than about 10 per cent of new leaves on a young tree show mining, act.

Eco Organic Garden also notes that some new research suggests leaf miner tunnels can provide an entry point for bacterial diseases, including citrus canker, though this is primarily a risk in areas where citrus canker is present.

Pest oils (Eco-Oil, white oil)

Spraying new growth with a horticultural oil such as Eco-Oil or white oil deters the adult moth from laying eggs. It does not kill larvae already inside the leaves, so timing matters.

Apply oil sprays when new growth is first emerging. Spray every 5 to 14 days while new flush is present. Cover both sides of the leaves thoroughly.

Eco-Oil and Eco-Neem are available at Bunnings and most garden retailers. Eco-Neem penetrates slightly into the leaf and can kill juvenile leaf miners inside, providing broader protection. Apply both at the rate and frequency specified on the label.

Sustainable Gardening Australia recommends spraying first thing in the morning to reduce the chance of affecting beneficial insects, which are less active at that time. It also reduces the risk of foliage burn in hot weather.

Timing for spray

The female moth lays eggs only on early flushes of new growth. The key windows in most of Australia are:

Spring flush (September to November): Most active period for egg-laying. Monitor new growth closely.
Summer flush: A second flush after summer fertilising can attract a new round of egg-laying. If you fertilise in January, watch for miners on new growth in February and March.
Autumn: Significant infestations occur in late summer and autumn in warmer areas. Leaf Root & Fruit Gardening Services in Melbourne notes that February to March fertilising can produce flush that attracts both leaf miner and can increase gall wasp risk.

In cooler climates, activity is lower in winter when temperatures drop and the tree grows slowly. Minimal new growth means minimal egg-laying.

Removing infested leaves

For light infestations, remove affected leaves by hand and place them directly into a sealed bag in the general rubbish bin. Do not compost infested leaves. The larvae and pupae inside can complete development in warm compost and emerge as adults.

Sustainable Gardening Australia suggests leaving removed foliage in a sealed bag in the sun first to kill any larvae and pupae before binning.

Eco CLM pheromone traps

Pheromone traps (Eco CLM traps) use sex pheromones to lure and trap adult male moths. They are available at specialist garden retailers. They reduce the adult male population and therefore limit fertilisation of eggs. They do not harm beneficial insects and are a useful option for organic gardeners or those wanting to reduce spraying.

Managing new flush to reduce risk

Because citrus leaf miner targets only soft new growth, controlling the timing and volume of spring flush reduces pressure:

Avoid heavy nitrogen fertilising in late summer and autumn. This produces new growth at the peak leaf miner activity period.
Use light, steady fertiliser applications rather than a single heavy feed that triggers a burst of soft growth.
After harvest, prune only what needs removing. Over-pruning stimulates regrowth that is immediately attacked.

The Seed Collection and Leaf Root & Fruit both note that over-pruning can worsen leaf miner by stimulating exactly the kind of soft flush the moth prefers.

Quarantine of new growth

When bringing new citrus plants home from a nursery, inspect them carefully for any signs of leaf miner activity before placing them near established trees. Larvae inside leaves are invisible from outside; look for the silvery trails. Plants with active infestation can spread the pest to your established trees.