Citrus Leaf Miners- Hi Ho, Hi Ho, Destroying They Will Go

I am a serious citrus aficionado. In my opinion, citrus are the most rewarding of all backyard fruits for a number of reasons. They are attractive, they have sweetly perfumed flowers, they are green year round, they are easy to grow with a minimal investment of time and care, the fruit in most cases has a very long hang time on the tree, and the thick citrus skin protects them from so many other fauna that would like to share your fruit. You certainly can't say that about those delicate, flaccid figs hanging on your trees. In fact with three varieties of orange trees, one can be assured of the ability to eat an orange a day all year long in San Diego. That was one of the major enticements to move to San Diego during the years of early development here.

Now that I have extolled the benefits of raising those marvelous citrus, and have you ready to rush to the nursery to obtain more of those rewarding trees, many of you are rather perturbed by the present condition of your citrus. I get more questions about Citrus leafminer in September than on any other subject combined. The reason? The citrus look like they have been to war. In a way they have. They really do look like they have been to Hell and back. Blame it all on those nasty little leaf miners. I always thought of miners as those fun little friends of Snow White. These miners are anything but friendly.

Tracks in citrus leaves caused by Citrus leafminer

In addition to the tracks in the leaves, curling is also a symptom of leafminer

When I was a youngster, citrus were the gods in the pantheon of garden divinity. They always looked good, were easy to care for, and gave delicious fruit. These days, it seems every time I turn around there is some new nasty little beast making my plants look awful, or outright killing them. Citrus leafminer is a fairly new arrival. Citrus leafminer originated in southeast Asia. It first burst on the scene in San Diego county from Mexico in the year 2,000, and has marched northward ever since.

The offending insect, Phyllocnistis citrella, is rarely if ever seen by gardeners, but oh do we see the damage caused by them. We in San Diego are not suffering damage to our citrus trees alone. It affects all citrus growing regions of the world these days.

Life stages of the Citrus Leafminer

The adult stage of the pest is a very tiny moth, and is very elusive, usually hiding on the undersides of leaves. The larval stage is when damage is caused. A single leaf can have up to 6-8 tunneling larvae! No wonder they can make our citrus look so dreadful. The larvae penetrate citrus leaves, and begin to tunnel through thin leaf tissues, will pupate between the top and bottom half of the leaves and then emerge as adult moths.

Older, well established trees can usually sustain the damage, but newly planted and drought stressed unhealthy trees can be severely affected. The life cycle of the leafminer coincides with our warmer weather, and with the flushes of growth we experience on our citrus trees here.

The larvae are only active and ready to penetrate your leaves at a particular stage of growth. I find they don't attack leaves that are very new, fresh, and supple. Nor do they attack leaves that have matured and hardened off. The stage of leaf growth when the leafminer are interested in your trees is a very narrow window of time. They can be controlled with Spinosad...IF, and this is a really big IF, if you spray at just the time the leaves are attractive, and when the pests are active. One of the best controls is to allow natural predators a whack at the pests, so be careful spraying Spinosad as it can kill the very predators that may be of help to you.

Pheromone traps can be of great help in control of Citrus leafminer, but they are expensive, and have a narrow window of time that they are effective, about three weeks, because the sex attractant loses potency fairly quickly. So, get to know your trees, their cycles, and the stages at which leaves can be attacked. Pheromone traps and sticky traps can also be helpful in determining presence of adult miner moths.

I am going to recommend in severe cases to use Imidacloprid, available as Bayer Fruit and vegetable control. Imidacloprid is a dreaded neonicotinoid. Yes, I am aware that many people dislike its use, boycott big box stores and nurseries that carry plants that have been treated with this insecticide. In certain applications it is very helpful, when applied at the right time. DO NOT APPLY THIS PRODUCT PRIOR TO THE FLOWERING OF YOUR TREES, OR WHILE THEY ARE IN FLOWER. That is when it will be toxic to bees and pollinators. Once the fruit has formed the bees will be safe and the Imidacloprid will only be damaging to the pests living off the juices or tissues of your precious trees. Look, I'd rather not use insecticide, and I am 99% organic, but don't neglect a valuable tool in your arsenal because of preconceived notions. Imidacloprid also controls a very destructive pest causing severe and irreversible damage, the Glassy winged sharpshooter.  Poison is poison, whether natural or chemical. Use any insecticide sparingly. Only when absolutely necessary. Only as a last resort. Only following the label instructions to the letter.

Things to remember:

Citrus leafminer attacks all citrus.

Once you see the damage, nothing can be done to fix the problem. Leaf damage which has already occurred will remain through the life of the leaf. Prevent the problem next year.

Encourage natural predators to be present in your garden.

Keep your trees healthy, well fed, and well watered. I know we live with drought, but don't skimp on citrus. If underwatered, they will look awful and be much more prone to attack by pests and disease. They will also be less able to make a recovery, should pests attack.

Keep the leaves clean and free of dust. Washing the leaves on a semi-weekly basis helps with almost all pest and disease problems. 

Many people strip the leaves off the trees, thinking they are doing something beneficial, DON'T! Let the leaves alone, they still create food for the tree, and removing them does nothing to help in controlling the leafminer population.

Don't cut them back severely after an infestation! Much like the admonition above, cutting them back severely does nothing to help. You really destroy any structure the tree had, and it is likely you aren't making cuts with building a good scaffold for the future.

Avoid high nitrogen fertilizers and Miracle-Gro type fertilizers which make tree's growth too "hot and soft". Use a high quality organic food with balanced N-P-K, so the tree gets the nutrients it needs to harden off leaf tissues.

Mulch, mulch, mulch ( You knew I was going to say that, didn't you? It is my mantra).