Citrus leafminer (Phyllocnistis citrella) is a pest on both native and cultivated citrus trees. It attacks all citrus, including lemons, limes, oranges, mandarins and native finger limes. This pest is a small silvery-white nocturnal moth which is only about 2mm long with a 4.5mm wingspan.


The adult female moths start laying eggs around 24 hours after mating, can lay up to 20 eggs per night, and more than 50 eggs during their entire life. They lay eggs on flushes of new growth during summer and autumn. They prefer to lay their eggs on small, newly emerged leaves that are around 10-20mm long, and they lay their eggs under the leaves. The leafminer larvae hatch from the eggs after 2-10 days and start tunnelling into the immature leaves and feeding, leaving silvery trails and causing the leaves to twist and curl.

The citrus leafminer larvae, which is flat, yellow in colour and around 3mm long, stays in the same leaf its entire life, feeding inside the leaf for 5-6 days. It then tunnels to the edge of the leaf, causing the leaf edge to fold over for protection. It pupates in the fold on the leaf edge, and its pupal stage lasts for 6 days, after which it hatches and emerges as an adult.

The entire life cycle of the citrus leafminer moth takes approximately two to three weeks to complete under optimum summer and autumn conditions. At other times, such as in late autumn, winter and spring, the pest’s life cycle can take two or three times longer.

 

Damage caused by citrus leafminer

The characteristic damage caused by citrus leafminer is the twisting and curling of young new leaves with the tell-tale silvery trails running through the leaf. Essentially, itnew foliage growth.

On a mature citrus tree, which are over four years old, damage to new growth does not have a significant impact as these trees have a dense canopy with a lot of tough older leaves to sustain them, so there is negligible impact tree growth and fruit yields. The only concern is that the tunnelling of the leaves can act as an entry point for pathogens and diseases.

Where citrus leafminer is of great concern is to young citrus trees and potted citrus, which do not have extensive mature foliage, and produce more new growth each year as they mature and grow to size. Since they produce more new growth, young citrus trees can support greater pest populations, which can affect new foliage and therefore reduce overall growth. It is unlikely that even heavy leafminer infestations will kill a young citrus tree, but they will certainly reduce its vigour and yields.

 

Controlling citrus leafminer

If we take an Integrated Pest Management (IPM) approach to controlling citrus leafminer, we are able to implement multiple pest control strategies simultaneously to deal with the problem more effectively.

Cultural controls – disrupting the pest’s environment

Citrus leafminer moths only attack new growth flushes of citrus trees in summer and autumn, once the leaves harden off the pest can’t mine into the leaves, so there are a few ways we can change conditions so that they’re not as favourable to the pest.

  • Fertilise in late winter to promote strong spring growth, as spring is a time when citrus leafminer is absent or in extremely low numbers.
  • Reduce the new growth flushes in late summer and autumn, the pest’s peak periods, by not overfertilizing, or overwatering in summer and autumn, use just enough fertilizer and water to support normal growth, but no more than that. Avoid using high nitrogen fertilizers during this period.
  • this keeps the new growth flush cycles short and consistent, reducing the window of opportunity for pests. The best times to prune citrus in cooler climates where frosts are encountered is mid to late spring, once the risk of frost has passed. In warmer frost-free areas, pruning is best carried out in late winter to early spring after fruiting. Note, this is a damage minimisation strategy as these pruning times will lead to new growth which will coincide with the period when pests begin to
  • Prune out vigorous shoots, such as water sprouts growing from branches, and suckers growing from the trunk below the graft union, as these produce new leaves for a prolonged period of time, providing a pest breeding site.

Mechanical and physical controls – destroying the pest directly

  • Remove and destroy (bag and bin) leaves which have been attacked by leaf miner, preventing the pupating pest from hatching.
  • Use citrus leaf miner pheromone traps to catch the male moths and prevent them mating, reducing the pest population.

Biological controls –  management of a pest through the use of their natural enemies

  • Generalist predators such as green and brown lacewings feed on citrus leafminer.
  • Citrus leafminers are killed by tiny nonstinging, naturally occurring parasitic wasps such as Cirrospilus and Pnigalio These wasps lay their eggs inside or on top of the leafminer larva, or in its tunnel in the leaf. When the wasp egg hatches the wasp larva consumes the leafminer larva.
  • Support beneficial predator and parasite insect populations by NOT spraying citrus with broad-spectrum insecticides and avoid other practices that disrupt natural enemies whenever possible.

Chemical controls – using chemicals to control pests

Using chemical sprays to control citrus leaf miner is difficult because the larvae are protected in their mines inside the leaf and the pupae are also protected by their shell and the rolled leaf edge.

Spraying with Insecticides such as organophosphates (e.g. maldison, malathion), carbamates (e.g. carbaryl) and pyrethroids (e.g. tau-fluvalinate, cyfluthrin, bifenthrin, permethrin) will only kill off natural predators and disrupt their activity, leading to outbreaks of other pests such as scale, whitefly and mites and have a number of other undesirable environmental impacts.

The NSW Department of Primary Industries is quite specific in their advice about the need to spray:

“Sprays are usually only required for control on young or vigorous trees in summer and autumn. Spraying immature flush on mature trees is generally only warranted for aesthetic reasons or to protect prolific growth that may occur if trees are heavily pruned in summer.”

What they’re saying is that you should only spray mature citrus trees eafminer for cosmetic purposes, or after they’ve been cut back hard in summer. Only young trees and trees putting on a lot of new growth really justify spraying.

If you need to spray, the NSW Department of Primary Industries recommends using horticultural oil, also known as ‘white oil’ or ‘petroleum oil’. According to NSW DPI:

“Horticultural mineral oils (HMOs) and agricultural mineral oils (AMOs) are generally as effective as insect growth regulators and broad-spectrum organophosphates and carbamates. They are the only products recommended for general use in nurseries, home gardens, and orchards.

Sprays applied to both susceptible and mature leaves will control a range of others pests (e.g. armoured scales, mites and some thrips) simultaneously and improve control of citrus leafminer. The oils are effective because adult female moths avoid sprayed surfaces and this leads to reduced egg laying.”

Spraying horticultural oil on new flush growth citrus leaves deters citrus leafminer moths from laying their eggs, as they avoid surfaces sprayed with oil. It won’t kill larvae mining inside the leaf. Natural plant-based horticultural oils such as Eco-oil are a more sustainable alternative o synthetic petroleum-based horticultural oils as they have a lower environmental impact.

When to spray horticultural oil:

Begin spraying new summer growth as soon as it emerges, before the leaves reach 10 mm in length.

NOTE: Do not spray horticultural oil when temperatures are near or above 35°C because heat increases risk of leaf injury through drought-stress. Only use on healthy, unstressed plants, do not apply to plants suffering from heat or moisture stress.

Thoroughly spray both the upper and lower surfaces of new leaves. Spray every 5-7 days in warmer weather, and every 10-14 days during cooler weather.

The number of times you will need to spray will depend on the citrus variety and how long the tree produces flushes of new growth. Lemon trees, for example, will usually require more sprays than other citrus tree such as mandarins, oranges or grapefruit.

Stop spraying when most of the new leaves produced within a flush cycle start to harden or are longer than 40 mm.

 

Insecticides containing neem oil (of which the active ingredient is azadirachtin) such as eco-neem and spinosad/spinetoram such as Nature’s Way Fruit Fly Control and Success Ultra will penetrate slightly into the leaf and kill the larvae that are tunnelling within the leaves, while being relatively safe for beneficial insects. These products are particularly useful for citrus trees that are ornamental as well as productive.

In terms of toxicity to beneficial insects, neem oil is safe, whereas spinosad is slightly toxic and spinetoram even more so, so it’s important to take the necessary precautions when spraying the latter two products, so as not to harm beneficial insect populations and create explosions in pest numbers.

Remember, in integrated pest management (IPM), toxic chemicals are always the last resort, and using all pest control strategies in tandem will increase the chances of effectively managing the citrus leafminer pest.

 

References