By: Richard Flowers, ACCNP Green Thumb Nursery- Ventura
Countless times everyday people come up to me with a sample of their citrus leaves. The leaves have squiggly lines all throughout the surface, the leaves are curled and distorted. They want to know what is wrong and how do I cure it? The problem they are experiencing is called Citrus Leafminer. Citrus Leafminer is a tiny worm that chews between the layers of the leaves causing unsightly and noticeable trails inside the leaf. If you look closely enough you can see the creature working or eating its way between the leaf. Fresh damage of the Citrus Leafminer gives the leaf a silvery translucent color. Once the mining has stopped, it dries up and turns a tannish color, this is old damage. You may wonder how does this little worm get between the leaf. The adult is a moth which lays its eggs on the leaves, these eggs eventually become the larvae (worm) that penetrates inside the leaf then begins doing its damage. Citrus Leafminer larvae feed by creating shallow tunnels or mines in young leaves only. As the name suggests, it is most commonly found on Citrus.
Citrus Leafminer, Phyllocnistis citrella, was found in California in the year 2000 and is native to Asia. The adult Citrus Leafminer is a very small, light-colored moth, less than 1/4 inch long. It has silvery and white iridescent forewings with brown and white markings and a distinct black spot on each wing tip. The hind wings and body are white, with long fringe scales extending from the hindwing margins. The damaging larval stage (what you can sometimes see) is found only inside mines of new Citrus leaves. Most people see the unsightly trails that form a maze on the leaf surface. As it feeds and develops, the larva leaves a frass (feces) trail, observed as a thin dark line, inside the meandering mine just under the surface of the leaf. In its last stage the larva emerges from the mine and moves to the edge of the leaf. It rolls the leaf around itself and pupates in preparation for adulthood. So when you see the curled leaves it will soon become ready to change into a moth (the adult) then the life cycle starts all over again.
Adult Citrus Leafminer moth do not damage plants and live only 1 or 2 weeks. Adult moths are mostly active in the morning and the evening and spend the day resting on the undersides of leaves, but are rarely seen. Eggs hatch about 1 week after being laid. The newly emerged larvae immediately begin feeding in the leaf and initially produce tiny, nearly invisible, mines. As the larva grows, its path of mines becomes more noticeable. Citrus Leafminer develops best at temperatures between 70º to 85ºF and greater than 60% relative humidity, but will readily adapt to most California conditions. In the moderated Coastal communities of Southern California the leaf miner can be active almost year round.
Citrus Leafminer prefers to do its damage only in the tender, young, shiny leaf flushes of growth and they tend to shy away from older leaves that have hardened off, meaning these leaves are not susceptible unless extremely high populations are present. I have noticed that mature trees (more than 4 years old) that have a dense canopy of older foliage can sustain them and can tolerate damage on new leaves during part of the growing season with negligible effect on tree growth and fruit yield. On the other hand, newly planted and young trees that do not have much mature foliage and produce more flush year-round, thereby can support larger Citrus Leafminer populations. Young trees may experience a reduction in growth, however even young trees with heavy Leafminer populations are unlikely to die.
In inland areas, summer heat seems to suppress Leafminer populations, but in cooler coastal areas, the insect population may remain high from summer through fall or at anytime when the weather is mild. Despite the new flush of growth of Citrus trees attacked by Leafminer it will look unsightly, it is highly recommended to leave it alone and let the natural enemies of the Citrus Leafminer feed on and parasitize the larvae in the mines, rather than trying to control this pest with insecticides. Most often the natural enemies will be tiny parasitic wasps. It is important to encourage these beneficial insects by planting nectar plants from which they feed on like Daisies, Salvias and Lavenders because these parasitic wasps lay eggs in the Leafminer and after the egg hatches the larvae consumes the Citrus Leafminer, which then develops into an adult parasitic wasp. On backyard citrus trees, Citrus Leafminer rarely causes serious damage and management is normally limited to practices that limit succulent growth and protect natural enemies. To limit such new succulent growth, I suggest not to fertilize excessively with high nitrogen fertilizers. Citrus trees in a regular fertilized lawn are more apt to have Citrus Leafminer damage. Do not apply nitrogen fertilizer at times of the year when Leafminer populations are high as flush growth will be severely damaged, such as in the summer and fall, instead when fertilizing use a material like an organic Citrus and Avocado Food and/or Liquid Kelp, which strengthens the leaf tissue.
It is best to remove vigorous shoots known as water sprouts and suckers that often develop on branches and above the graft union on the trunk of mature trees because these are potential areas where adult female moths will lay eggs. Another method to discourage large amounts of succulent growth favored by Citrus Leafminer is to not over-water your tree (more water equals more succulent growth favored by Leafminers). Also pruning during summer encourages more growth which should be avoided because this is when the Leafminers are more active. Avoid pruning live branches more than once a year, so that the cycles of flushing are uniform and short.
Instead time your pruning to not coincide with new flushes of growth and flying adult populations. The idea is to have the leaves harden when the warm weather arrives. Mature (hardened) leaves equals no or less leaf miner activity. Pheromone or sticky traps are an excellent tool used to monitor whether or not you have active populations of adult moths in the area. These traps are used to catch the adult before they lay eggs thus preventing the larvae and subsequent damage that follows.
Insecticidal treatment for Leafminer is difficult to achieve because larvae are shielded within mines of the leaf. Many times the best method includes proper cultural care as a preventive as described above. Only spray if it is absolutely necessary. You can use the sticky traps to determine if you need to spray. I have included some chosen insecticides to use in the event it is necessary.
An organic insecticide includes using a product called Spinosad (easily remembered as Captian Jacks). The highly desirable Captain Jacks controls the larvae while at the same time is safe for natural enemies. The Leafminer consumes the bacteria in the Spinosad and gets ill then dies. Captain Jacks residues do not last very long, and this insecticide might need to be re-applied every 7 to 14 days and is limited to apply 6 times per season.
Another method is using All season Horticultural and Dormant Spray Oil which can kill (suffocate) any eggs that are there at the time of application. The Leafminer still needs to breath in the tunnel they created. The oil can seal the end(s) of the tunnel, not allowing fresh air in. If the miner is young, weak, etc. then it can die in the tunnel, however some Leafminers can survive . It is important to apply when they are young for greater effectiveness. You may need to apply it on a weekly basis. Do not apply when it is hot and sunny as plant injury may occur. Be sure to thoroughly cover all leaf surfaces. As with any insect or disease it is important to continuously monitor for ongoing conditions which is key along with any other good cultural control.
If one chooses to use Imidacloprid-Systemic (Bayer Advanced Fruit, Citrus, and Vegetable Insect Control) applied to the ground at the base of Citrus trees which provides the longest period of control, 1 to 3 months and should only be applied once a year. To protect bees, avoid applying during the period 1 month prior to or during bloom.
A note on using any insecticide. It is best to read, follow and understand all printed documentation the insecticide label has before applying the chemical.
To be able to achieve an accurate diagnosis of any plant related problem, it is advised to bring in fresh samples of the concern in a sealed sandwich bag. You can also accompany the samples with good quality photos that depict the issue.
If you have any bug or plant related questions feel free to stop by your favorite Green Thumb Nursery we will be happy to help you out.
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