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 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.
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):
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.
Since the larva is found just below the leaf surface, many topical sprays will not reach the pest. However, the 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. For me, the air seems to be the calmest around or just after dusk.
There are several scientific papers that say spinosid is safe for bees as long as it is allowed to dry for several hours before bees come in contact with the chemical. This is another good reason to spray in the late evening/dusk.
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 favored target for leaf miners (and most parasitic bugs for that matter). Therefore, if you only had one place to spray, it should be on the young leaves.
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 leftover residue in the bottle doing damage. For example, you dont want to have leftover toxic herbicide in the bottle mixing in and ending up on a plant that you would actually like to keep around. It is also nice to have a spray bottle 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 brand horticulture oil with the spinosad to the help the spinosid to stay on the leaf. I also use the horticulture oil because this oil also helps to protect the plant from a large variety of other parasites. The mix is kind of a one two punch for the bad bugs. Overall, horticultural oil is an awesome alternative to systemic pesticides.
Well, I am really disappointed to learn that Ortho Volck Oil Spray has been discontinued by the manufacturer. I know a lot of loyal customers are bummed at Ortho for discontinuing Volck. Fortunately, the main/active ingredient in this product is simple mineral oil. Several other brands have basically the same ingredients.
Therefore, I am now using “Bonide All Seasons oil” for the same indications that I used Volk oil for in the past (which is basically everything).
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 reference 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 during leafminer season.
- I was once under the impression that leafminer season was from mid-June to mid-October. However, this year, I learned the hard way that the season actually lasts into late November (at least where I live). This lesson was learned when I stopped spraying in October and a new flush on my beloved Tahitian Pummelo was ravaged by those little suckers (see recent pictures below). I am also hearing reports that the season is starting earlier as well. Therefore, my new recommendation is to spray your plants from mid-May to mid-November.
- 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 in that time between spraying then 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 in the big box stores, I go to amazon. 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 mid-May to mid-November (see above discussion).
- There are many warnings on every pesticide label. Read them.
- Even though spinosad is a natural chemical, it should still be treated with respect.
- The number one goal is to protect yourself and the people around you.
- Spray when it is not windy outside to reduce the chance of spraying yourself.
Your vulnerable bees:
- Another insecticide warning that I would like to draw your attention to (which may not be on the label of insecticides) is about bees.
- There is a mysterious disease that is killing bees called ‘honeybee colony collapse disorder’. There are a lot of different ideas about the cause of this disease. Theories include ideas about habitat loss, parasites, bacteria, viruses, fungi, stress, and pesticides.
- As a result, I have done some reading on the subject of bees and spinosad. There are some interesting conclusions from several scientific papers. Below is a summary from what I have read.
- In laboratory conditions, high concentrations of spinosad is toxic to bees.
- However, in field studies, spinosad is relatively safe to bees.
- More specifically, several papers state that spinosad is harmless to bees if bees come in contact with spinosad after it has dried.
Here are a few scientific papers to reference:
In the 2002 scientific paper, An ecological risk assessment for spinosad use on cotton from the journal Pest Management Science “Spinosad is acutely toxic to bees under laboratory conditions, but toxicity of residue studies and field studies indicate that under actual use conditions the impact on bees is minimal.”
In 2003 scientific paper The effects of spinosad, a naturally derived
insect control agent to the honeybee from Journal the Bulletin of Insectology “In field studies dry residues of spinosad were safe to foraging worker honeybees, with no adverse effects seen on mortality, foraging behaviour, brood or queen.”
Here is a similar reference from The BCPC Conference: Pests and diseases, Volume 1. Proceedings of an international conference held at the Brighton Hilton Metropole Hotel, Brighton, UK, 13-16 November 2000.
- All of this seems to basically say that it is best to spray spinosad when the bees are not around. This will allow spinosad to dry and be safe before the bees are exposed. Since bees go to their hive at night, this is another great reason to spray your plants in the evening/dusk.
- To be extra extra kind to your bees, avoid spraying plants that are currently flowering to further reduce any remaining risk.