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Citrus Leafminer: Diagnosis and Treatment

Citrus Leafminer

(Phyllocnistis citrella)

 

Citrus Leafminer Overview:

Citrus Leafminer is a common pest for nearly all citrus growers.  The disease deforms leaves and reduces the plants ability to photosynthesize.  Citrus Leafminer can stunt the growth and fruit production of young trees.  In severe cases, Citrus Leafminer infection can even cause total tree defoliation and eventual tree death to a young tree.  The disease has been spreading around the world from its native Asia.  In the year 2000 it was discovered in California for the first time and now it is widespread.

Citrus Leafminer larva

Citrus Leafminer infection: Pictures from UC Davis http://anrcatalog.ucdavis.edu/pdf/8321.pdf

 

Citrus Leafminer Diagnosis:

  • Infected leaves are deformed, rippled and curled but generally remain green.
  • On closer inspection, you will see a squiggly whitish line (sometimes a dotted dark line) on the leaf surface which is a cardinal sign of Citrus Leafminer.  This thin line represents the parasites feeding trail and feces of the Citrus Leafminer.  This fecal streak (yuck) is most commonly found on the under surface of young leaves (see images above and below).
  • However, in cases of severe infections even older old hardened off leaves can be infected.
Citrus Leafminer (Phyllocnistis citrella)

Citrus Leafminer Diagnosis: Pictures from UC Davis http://anrcatalog.ucdavis.edu/pdf/8321.pdf

 

What causes Citrus Leafminer?

  • The disease is caused by the larva of a tiny moth (Phyllocnistis citrella).
  • The larva feeds just under the surface of the leaf.  The thin line that you see is the path where the larva has been… and the poo it has left behind.
  • Since this path of the parasite is located just below the surface of the leaf, this is the larval mine (and thus leafminer has become the name of the bug).  The actual culprit larva can often be found at one end of this mine (see arrow in the picture above).
  • There are often several leafminer larva per leaf; commonly 2 to 3 larva per leaf.
  • At the end of the larva stage, the leafminer will emerge from the mine and will curl the leaf around itself, apparently as a form of protection as it enters its pupa stage.

 

Biological control of Citrus Leafminer:

  • There are many predatory insects that will feed on the citrus leafminer larva. In California many of these parasitoids are generalists.
  • However, there are more-specific leafminer enemies that are found in other parts of the world.  These leafminer specific eating insects are currently being evaluated for importation into the United States.
  • The enemy of my enemy is my friend. – Ancient Proverb

 

Cultural Control of Citrus Leafminer:

  • Since the citrus leafminer larva thrive on young leaves, eliminating new growth during the time that the larva are around may help to protect your tree.
  • Therefore, another important option to control the citrus leafminer is to reduce a summertime leaf flush.
  • The proper timing of pruning and fertilizing will help to prevent the tree from having a flush of new growth during the vulnerable time of the year (summer and fall).
  • Although this option may help, I do not believe it will be enough to fight the onslaught by itself.

 

Pheromone traps for Citrus Leafminer:

  • There are Citrus Leafminer traps which emit a pheromone/chemical that attracts the male moth and then traps the moth.
  • These traps are most often used to monitor the amount of leafminer moths in the environment so to optimize treatment times.
  • However, some people have advocated using them as a primary means of defense.  The rational is that if you pull the males out of the population, there will be no mating and therefore no larva to hurt the plant.  This sounds good, on paper; in fact it was my own personal first step in the battle.  However, for me using the pheromone traps didn’t seem to do anything to protect my plants.   There are just too many moths in the environment for a few traps to do anything significant to the mating population.

 

General considerations regarding insecticides for Citrus Leafminer:

Some people have suggested that you can just leave the leafminer alone and the plant will survive.  However, this option is definitely not my experience.  I have several young citrus trees that took a major hit a few years back and this ‘hands off’ approach is not in any way recommended by me.  Therefore, in my strong opinion, chemical control may be your only option.  None the less, many insecticides are nonspecific and can/will reduce the populations of beneficial insects.  This is always a difficult balance to be mindful of.

 

Citrus leafminer insecticides (options):

Systemic chemicals:

There are several systemic chemicals that you can put directly in the ground to be absorbed into the trees through the roots.  However, I am not thrilled with this option because I am not confident about how long it will be in the plants system or to what degree it will be inside the fruit that I eat.   I would also expect this type of insecticide to dramatically change the soil microbial environment for the worse.

Topical Sprays:

Since the larva is found just below the leaf surface, many topical sprays will not reach the pest.  However, topical/spray spinosid (which is a natural insecticide) is able to penetrate this leaf barrier and kill the parasite.  In addition, spinosad is supposedly safe for the leafminers natural enemies.  Unfortunately, spinosad doesn’t last that long in the environment and needs frequent reapplication.

Azadirachtin is another natural insecticide that is said to be effective against the leafminer larva.  However, I have not tried that one.

General spraying considerations:

Whatever spray method you use, don’t spray when it is windy outside.  This will waste chemicals and you will likely end up spraying yourself in the process.  Also make sure to spray the bottom of the leaves as this is a favorite spot for the leafminers to live. Although I tend to spray the entire plant, new tender leaf growth is the main target in spraying.

I recommend that you get (and label) a separate garden sprayer for each of your different garden needs.  That way you don’t have to worry about spraying residue of something like an herbicide on a plant that you would actually like to keep around.  It is also nice to have one with an angled spray nozzle so you can easily get the bottoms of the leaves.

 

My method of controlling Citrus leafminer:

  • For me, I am most comfortable with the balance struck with the spinosid option.
  • I use to use volck oil with the spinosad to the help the spinosid to stay on the leaf.  I liked volck oil because horticultural oil also helps to protect the plant from other parasites.

UPDATE 1/20/14:
Well, I am really disappointed to see that Ortho Volck Oil Spray has been discontinued by the manufacturer.  Fortunately, the main active ingredient in the product is simple mineral oil.  Several other brands have basically the same product.  

Therefore, I am now using “Bonide All Seasons oil” for the same indications that I used Volk oil for (which is basically everything).  Overall, horticultural oil is an awesome alternative to systemic pesticides and I know a lot of loyal customers are bummed at Ortho for discontinuing Volck.  

Anyhow, if you are really interested in learning more about the fine details of oil insecticides, the “Using Oils as Pesticides” article is a rather complete outline from “agrilife extension” of Texas A&M University. 

  • Specifically, I now mix spinosad and Bonide All Seasons oil  in one spray bottle (by recommended amounts of each as seen on the label).
  • I mark my calendar and spray my citrus leaves every 3 weeks from mid-June to mid-October.
  • That being said, I sometimes spray more frequently when I see a flush of new growth between scheduled spraying times. The new leaves are the main target for the leafminer and if they emerge after you spray they will be totally unprotected. Also, if it happens to rain between spraying, you might what to spray again, because the rain may wash off the spinosad.

Since I had some trouble finding spinosad, I just got it on Amazon and this Monterey brand spinosid has worked extremely well for me.  I have also found horticultural oil  to be totally awesome way to address just about every sap sucking bug.

 

When to treat for Citrus Leafminer:

  • In California, the disease is most active in the summer and fall.  If you are using a ‘natural insecticide,’ such as spinosad, you will have to respray at regular intervals.
  • For example, spray every 3 weeks (as directed on the label) from June to October.

 

Insecticide warning:

  • There are many warnings on every pesticide label.  Read them.
  • The number one goal is to protect yourself and the people around you.

 

Your vulnerable bees:

  • However, another insecticide warning that I would like to draw your attention to (which may not be on the label)  is about bees.
  • Any pesticide that reaches the flowers may be picked up by bees.  Any chemical that can kill an insect may also kill bees.
  • If the bees bring an insecticide back to the hive, it may lead to ‘honeybee colony collapse disorder’.  No one wants that.
  • Therefore, to protect your buzzing-fruit fertilizing friends, avoid treating plants that are flowering.  This is a particularly difficult issue when using systemic chemicals that last in the plants system for a long time-perhaps into the next flowering cycle.  Another more painful option is to remove flowers before treating, but this is adamantly unpractical for many.

 

Further Citrus Leafminer reading:

http://anrcatalog.ucdavis.edu/pdf/8321.pdf

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74137.html

http://ag.arizona.edu/crop/citrus/insects/leafminer.pdf

 

 

About Thomas Osborne, MD

Dr. Osborne is a Harvard trained Radiologist and Neuroradiologist who loves to share his insight about medicine and gardening.

16 comments

  1. I liked this article so much that I bought Sinosad right away from Amazon and have sprayed several of my citrus trees which have been damaged by leaf miners. I will repeat again monthly. I am very impressed by your knowledge and willingness to share. I will reciprocate over the coming months. Thanks.

    • Thomas Osborne, MD

      Hey Sateesh
      Thanks for the great feedback.
      I am happy to hear that you have found it helpful.
      Looking forward to reading your other comments.
      Thanks!

  2. Hi Dr. Osborne.

    Love your article about leaf miners. I planted a few citrus last year and a few had been damaged by leaf miners. I also ordered your treatments from Amazon. I’ll treat the citrus next week.

    Thank you for your research and articles.

    Betty

  3. Hello !! I live in La Crescenta and have read what you say about the citrus leaf miners with great interest. I am wondering when you would say is the proper time to fertilize and prune the citrus trees to try to avoid the leaf miner infestation. I have had my trees for several years and have been fertilizing per package instructions once monthly from January to September. As for pruning, I snip off a few leaves or a little branch or two or three about once or twice a month, mostly removing the disfigured leaves. Since it’s always the new growth that I have been removing, I’m thinking that maybe I am interrupting the trees’ cycles, and that it’s not good for the trees to be forced to depend only on their old leaves. How harmful is it, do you suppose?

    • Thomas Osborne, MD

      Hey Kathy

      Thanks for the great questions.

      What is the proper time to fertilize your citrus?
      The idea of managing your fertilizing schedule in relationship to the leaf miner season has to do with the timing of new leaf growth.
      Young leaves are the most susceptible to leaf miner infection… and fertilization will encourage new leaf growth (flush).
      Producing young leaves is costly for a plant and if those leaves are not working to their full potential because of a leaf miner infection), then the plant gets a poor ROI (Return On Investment).
      So if you can time the fertilization in such a way to avoid the leaf miner season (June to October), you can avoid a lot of negative issues (in theory).

      However, this is a tricky issue because depending on the type of fertilizer you use, there may be a variable amount of lag time for its growth stimulating effects on your tree.
      Different citrus trees also respond differently to the growth stimulating factors of fertilizer.
      So to use this technique, you really need to know how your particular plants responds to the specific fertilizer you use… so you can schedule and account for the lag time of the trees growth response.

      Hot weather:
      However to further complicate things,
      Warm/hot weather will stimulate a leaf flush regardless of your fertilizer timing.
      Furthermore, when it is really hot your trees will need/demand additional watering, which may also stimulate new growth.
      Finally, since the summer is the best time to get your trees to grow, you may not want to put the brakes on that growth opportunity.

      Important side note:
      Young leaves are also the most susceptible to the damaging effects of cold weather.
      Therefore, I try to avoid fertilizing in a way that would promote a cold weather (winter) flush.

      So what do I do?
      I try to do the major fertilizing in early spring and right about now (early fall).
      I do give smaller amounts of fertilizer through out the growing season.
      I also will fertilize or augment at just about anytime if I notice signs of a particular nutrient deficiency.

      I also spray with the spinosad-oil mix that I have outlined in the article.
      I spray every 3 weeks during the leaf miner season.
      The most important thing to do is spray the young leaves.
      However, I spray the whole tree because that mix will also take care of other pests such as scale, aphids, etc.
      This spray is by far the best thing I do to fight leaf miner.

      Pruning citrus leaf miner infected leaves:
      Pruning plants (of any kind) can stimulate new growth.
      This is not the best option during leaf miner season (see above discussion).

      However, this brings up the issue of pruning leafminer diseased leaves.
      In general, I would leave the leaf miner infected leaves alone.

      Very early on in my growing experience, I red somewhere on line that you should remove citrus leaf miner infected leaves (I dont do this now).

      Therefore, at that early time, I tried to remove all of the leaf miner infected leaves on a tree (this is before I discovered my spinosad-oil spray mix).
      By the time I was half way through the process, I realized that this was a loosing battle and I stopped.
      However, as a result I my pruning, a new flush of leaves came out soon after that and all of those leaves were systematically ravaged by those freekin leaf miners.
      You just can win this way.

      Pruning citrus at all:
      Overall, pruning a citrus tree is a rather controversial topic.
      There are people who stand strongly on both sides of the issue.
      Some people say you don’t ever have to and/or shouldn’t trim a citrus (except to remove dead branches).
      There are also others that have outlined extensive methods of creating the ‘perfect’ pruned structure for a citrus tree.

      So where do I stand?
      I dont think you can lump all citrus trees into the same bucket when it comes to this issue. I think there is a lot of variation in the way each different citrus tree grows and responds to environmental factors.

      However, if I was to generalize…
      I will always remove dead branches and suckers below the graft union.
      I will trim/prune a citrus tree if it is getting in my way (making it difficult to walk on a path for example).
      I will trim off broken branches (broken branches can often be prevented if you remove some fruit from a heavily-weighted down over-burdened branch).
      I will not trim a citrus to ‘open it up’ to the air in the way you would for a stone fruit (peaches, plums, apricots, etc).

      I don’t intentionally trim to expose the trunk of my citrus trees to direct sunlight. Many have advocated this method, but I think it is a terrible idea. Citrus bark is thin and can easily become sun burnt. If you must expose your citrus trees trunk to direct sunlight, I would paint the trunk white to protect it from the sun. I would use the same method that I use on fig trees (I talk about this in my Mission Fig Tree Cultivation article.

      Hope this helps.

      Best,
      Tom

  4. Sorry !! Forgot to give you my name. Kathy

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