Leafminers

leafminer trail

leafminer trail

I have five citrus trees in my yard. One of the trees is a Meyer lemon about 25 years old and has been a good producer. I planted the other four trees this year: one Meyer lemon, one Valencia orange, one navel orange and one tangerine. All were purchased from a good, reputable nursery in 15-gallon pots. Now for the problem: The leaves on the older tree are getting shiny blisters that look like a snail trail. It has gradually gotten worse and is now spreading to the new Meyer that is planted about 20 feet away. The remaining three trees show signs of poor health, indentations in the leaves and yellowing of the leaves, but no other indications the disease has spread to them. I would hate to lose all five of the trees but do not know how to deal with this problem.
>Steve Woodward, Tujunga
Your lemon trees are afflicted with an infestation of citrus leafminers.
Leafminers are tiny caterpillars that develop from eggs laid by moths on leaf undersides. As the eggs hatch, larval caterpillars burrow inside of the leaves, creating squiggly trails, and puckered and blistered leaf surfaces.
Leafminers pupate inside the leaves they disfigure and ultimately emerge as 1/4-inch moths and immediately repeat their life cycle. Leafminers develop quickly from egg to adult and may produce 10 generations or more during a growing season.
Leafminers are attracted to new flushes of succulent leaf growth on shoot tips, especially during the fall. For this reason, it is a good idea to minimize nitrogen fertilization, which encourages succulent growth.
Many trees and shrubs, including citrus, show a flush of growth in October and early November as temperatures cool. During these two months, leafminers are highly active in citrus trees.
The reason your lemons have been affected while your oranges and tangerines have not is because the preferred hosts of citrus leafminers are lemon, lime and grapefruit trees.
Each year, lemons put out more new flushes of growth than other types of citrus. However, all types of citrus may be affected. A few weeks ago, I saw distinct evidence of leafminer activity on my navel orange tree.
To minimize damage, suckers that sprout from the base of a tree and water sprouts – succulent growth that shoots up vertically from interior branches – should be detached as soon as they appear.
I have learned that the best course of action for dealing with citrus leafminers, at least where mature trees are concerned, may be to do nothing at all.
Many insect pests, including citrus leafminers, are controlled by nearly invisible parasitoid wasps. Parasitoid wasps lay eggs in other insects’ eggs or larvae. As the minuscule parisitoid wasp larvae begin to grow, they feed on, and eventually kill, their insect hosts.
Most insecticides kill parasitoid wasps and other beneficial insects, preventing the natural, biological control of insect pests. Where no insecticides are sprayed, it has been shown that citrus leafminer damage decreases from one year to the next as parasitoid wasps take control of the situation.
Natural insect predators such as parasitoid wasps do not completely eliminate insect pests. The operative word to describe what they do is “control.”
If your older tree, alone, were affected, I would definitely try to wait out the leafminers for another year, at least, to see if their presence gradually declined, before spraying. Yet, where young trees are concerned, it may be necessary to spray where the majority of new foliage is affected by a pest.
Young trees may be mortally endangered by any kind of stress – from drought, to desiccating wind, to one freezing night, to pest infestation.
If you do choose to spray, you can choose from two products that have proven effective on leafminers and other pests but do no harm to beneficial insects.
These two benign insecticides are neem oil and spinosad. The staying power of these mild insecticides is brief, however, and they will have to be reapplied at short intervals to be effective. Carefully read application instructions before use.
Tip of the week
If you have trees on your property, November is a good time to examine them to see if pruning is necessary.
Santa Ana winds may blow during this month. If they do, expect to see predictable victims: top-heavy trees, hillside trees, and overwatered trees with brittle branches and shallow roots, Even without Santa Ana winds, winter storms are sure to come.
Among the trees most susceptible to limb breakage in the Valley is eucalyptus, because of its rapid growth during the spring and summer.
Enormous numbers of shoots and leaves are produced at the ends of branches, creating more weight than a single limb can support, especially when that limb is being hammered by wind or rain.
One tree that seems to make the news every winter when one of its larger specimens falls over – blocking a road or crushing a parked car – is the Italian stone pine (Pinus pinea). This majestic tree, which may reach a height of 80 feet, is recognizable by the flat top of its canopy and the attractive mottling of its bark.
Unless it receives light equally from all sides, it has a pronounced tendency to lean in the direction of greatest light exposure. An Italian stone pine that leans unmistakably to one side, and whose lean is not corrected through pruning, will be felled by a storm sooner or later.
On hillsides, trees that have a strong vertical growth habit or lean decidedly away from the slope are also prime candidates to be uprooted in storms.
The most reliable trees for Valley slopes are varieties native to California and the Southwest that do not require summer irrigation, such as California incense cedar (Calocedrus decurrens) and Arizona cypress (Cupressus arizonica). The coast live oak (Quercus agrifolia) is also excellent for planting on slopes, as the hundreds of coast live oak specimens on the hillside between the Getty Center and the San Diego (405) Freeway have proven.

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