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By AP Photo/California Department of Food and Agriculture
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By Photo courtesy of David Haviland
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By Photo courtesy of David Haviland
BY DAVID HAVILAND AND CRAIG KALLSEN Contributing writers
Editor's note: In light of news about Asian citrus psyllids being discovered in Kern and Tulare counties, Kern's UC Cooperative Extension farm adviser office has received numerous calls about citrus trees. John Karlik, an adviser with the UC extension, said he believes this is a result of citizens becoming more vigilant in looking for the small insect, which can carry a bacterial disease (huanglongbing) that is harmless to humans but has killed or forced the destruction of millions of citrus trees around the world.
Upon closer inspection, local citrus trees may exhibit leaf symptoms that were previously unnoticed by owners. Many sorts of leaf symptoms can be found on citrus and, in general, are not connected to the presence of the Asian citrus psyllid, Karlik said. One of the most common leaf symptoms is caused by citrus leafminer, an insect that is a relatively recent arrival in the Bakersfield area.
Citrus leafminer, an exotic pest of citrus, is spoiling the natural beauty of citrus foliage in the southern San Joaquin Valley. This worldwide pest, first found in California in Imperial County in 2000, moved up the coast and inland to sites throughout Kern County.
(This leafminer has no connection to the Asian citrus psyllid, a potentially damaging vector of citrus greening disease.)
Adult moths lay eggs that hatch into small worms that feed and cause mines in citrus leaves. In the summer and fall, when citrus trees begin to put out new growth, the new leaves become completely distorted, stunted and misshapen.
For most, citrus leafminer will likely be nothing more than a nuisance. Research has shown that mature trees can tolerate damage without affecting the number or size of fruit. However, damage from feeding by the leafminer can severely stunt young trees, and can reduce the aesthetic quality of citrus trees used for landscaping.
In most cases, control is not needed. Where the homeowner feels that it is, control can be difficult to achieve with foliar sprays because larvae are protected within the leaf from beneficial insects or pesticides. It can help to keep new leaf growth to a minimum by removing sucker growth at the base of the tree and by not over-pruning or over-fertilizing.
For bearing and non-bearing trees, homeowners can use products containing imidacloprid to help alleviate the problem. This is a systemic pesticide that can be used once during the season. Read and follow label directions closely.
Generally, because lemons and grapefruit tend to produce many new leaves early in the spring and throughout summer and fall, these varieties often show the earliest and, eventually, some of the worst symptoms. Infestations of citrus leaf miner peak in August, September and October. Citrus leaf miner is not active during the winter, and treating this pest during the winter, early spring or during bloom will not be effective and/or will be contrary to the label.
It is also important not to confuse citrus leafminer with citrus peelminer. Citrus peelminer is another new pest of citrus and is usually found on the fruit and stems of citrus and many other landscape plants. Citrus leafminer, on the other hand, feeds almost exclusively on the leaves of only citrus. Both pests produce mines that cause primarily cosmetic and superficial damage.
In other countries, citrus leafminer and citrus peelminer are attacked by several small parasitic wasps that are barely visible to the naked eye. Efforts are under way to import more of these natural enemies to California.
Once here, the hope is that these beneficial insects will become established and spread throughout urban and agricultural sites to feed on and control these pests. In the meantime, it is important to learn to tolerate less than beautiful backyard citrus trees until a long-term solution can be achieved.
More information on citrus leafminer and its control can be found at the University of California Integrated Pest Management website at ipm.ucdavis.edu/PMG/PESTNOTES/pn74137.html
-- David Haviland and Craig Kallsen are advisers for the University of California Cooperative Extension, Kern County.