Many gardeners are presently experiencing a surge in damage to their citrus trees because of the citrus leafminer (CLM). This relatively new pest, originating in Southeast Asia, has been around since 2000 when it was first seen on Oahu, but now infests all the Hawaiian Islands. Though similar to the leafminer that attacks tomatoes and other vegetables, the CLM is a species which prefers the common citrus varieties such as Valencia and navel oranges, lemons and limes, mandarins, calamondin, grapefruit and pummel.
The CLM is a moth whose larval stage lives inside young terminal leaves.
Feeding as they meander throughout the leaf, the larvae cause the tunneling or mining patterns found in leaves. After pupating, the adult emerges as a tiny, 1/2 inch, silvery moth with a black spot at the tip of each wing.
Damaged leaves slow the growth of young trees. Frequently leaves appear curled or distorted. Older leaves, which have become hardened, are not susceptible to the tunneling unless pest populations are extremely high. Generally, fruit loss does not occur in mature citrus trees.
Note: aphids often feed on new citrus shoots. Do not confuse the curled leaves of the CLM with that caused by aphids. Leaves distorted from aphid damage are tightly curled and will have no tunneling or larval mining patterns.
Control: At present, there are no practical chemical control recommendations for backyard citrus; treatments are difficult and rarely effective. Since the larvae are within the leaf, most pesticides would be of little use. In addition, foliar applied pesticide treatments will greatly decrease the population of natural biological control agents, parasites and predators in the environment. Pesticide applications typically cause a resurgence of other pest populations, typically mites.
Fortunately, at the same time this pest came to Hawaii, a tiny parasitic wasp arrived with it. And because of its presence, citrus leafminer damage, for the most part, has been minimized. Worldwide, there are numerous species of natural enemies of the CLM and biological control has provided the most effective management for this pest.
Here are some cultural measures that will help minimize CLM populations:
— Moths are attracted to new flushes of growth. Avoid pruning, especially young trees; pruning stimulates trees to produce new growth. Minimize the pruning of older trees to no more than once a year.
— Do not pick off damaged leaves because undamaged portions of the leaf will continue to produce food for the tree.
— Although nitrogen fertilizer is needed, be aware that high nitrogen fertilizer applications will stimulate new flushes of growth which are attractive to the CLM. Concentrate fertilization in the winter months rather than in the summer when populations are higher.
For more detailed brochures on the citrus leafminer, visit the University of California website at
http://ucanr.org/freepubs/docs/8321.pdf and the State of Hawaii Department of Agriculture website at http://hawaii.gov/hdoa.
Two readers share ideas for corks:
Make leis! She wrote, “I was given two very original and lovely leis recently that were made with wine corks separated by colorful beads. The bows were of raffia. Two mainland sisters visited me a couple of days later and preferred my cork leis to the shell ones they had been given on their arrival.”
Another reader wrote, “I am a small commercial cymbidium grower and would like to use them (corks) with cinder when I repot. Contact: watt@hawaiianisp.com; www.alwaysanthuriums.com.”
Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years. Email your questions to Sakovich at askthegardenguy@earthlink.net.
