Citrus Tree Damage

Water is best treatment for insects on citrus trees

Forceful spray should wash away those that feed on and damage the foliage

Gardeners who grow citrus often notice that some new leaves are damaged by insects. Close inspection may reveal aphids, citrus leafminer or Asian citrus psyllid. The place on a leaf where these insects feed is injured and may not grow at the same rate as surrounding healthy tissue. This can cause young leaves to become malformed as they grow.

Mature leaves that are damaged and malformed remain this way until they eventually fall from the tree. The abnormal leaves may look unsightly, but the green tissue continues to produce sugar, which provides energy for the tree. Pruning off damaged foliage does not help a tree unless leaves are so severely deformed that they protect pests from beneficial insects or chemical treatment.

Certain pests feed exclusively on new foliage, but the damage they cause can be minimized with vigilance and timely treatment. Check citrus weekly, especially during fall and spring, when they produce flushes of new growth. Closely inspect new shoots for insect pests and signs of their feeding.

Aphids like to feed on tender new leaves and are very common in spring. They suck sap from leaves and excrete droplets of sugary liquid called honeydew. Leaves may be coated with a sticky residue and black, sooty mold that grows on it.

Aphids multiply quickly. Their population can be reduced by washing trees with a forceful spray of water. Do this every one or two days for about a week. This will not harm beneficial insects like ladybugs and their alligator-shaped larvae, which feed on aphids. If you need to use a chemical spray, first try insecticidal soap, paraffinic oil or neem oil. These chemicals kill insects on contact and do not leave a persistent toxic residue on foliage so they are less harmful to beneficial insects.

Tiny leaves in the new flush growth on citrus can be twisted or burned back by a new pest in our area, the Asian citrus psyllid (ACP). The insect’s larvae feed on plant sap and excrete large amounts of sugary liquid. The insect can also transmit a deadly bacterial disease, Huanglongbing (HLB), which kills citrus trees in a few years. HLB has not yet been found in California, but it could soon be introduced from other areas like Mexico or Florida where it occurs. Visit

www.californiacitrusthreat.org to learn more about ACP and HLB. Call the State Exotic Pests and Disease info line, (800) 491-1899, to report possible infestations on citrus.

Larvae of a tiny moth, the citrus leafminer, can damage new citrus leaves. They feed on the inner tissue of young leaves, creating irregular silvery patches on the leaf surface. Mature larvae also fold the edges of leaves over their body when they pupate. High populations occur from summer through fall in our area, and almost every new leaf on a citrus tree may have some damage. This does not harm mature trees or reduce their fruit production. Chemical treatment may only be needed on young trees since their growth may be stunted by severe infestation. To learn more about this pest on line, search for UC Pest Note No. 74137 Citrus Leafminer.

Q: Can I combine different insecticides like Spinosad and Neem oil and use the mixture to spray my plants?

A: Different insecticides and other pesticides should not be combined unless the product label advises that this can be done. The active chemicals in different products and so-called inert ingredients may not be compatible. Combining products can produce a mixture that is less effective against pests or more toxic to plants or animals. Pesticides labeled for use on food crops also list a pre-harvest interval which is the number of days you must wait to safely harvest a crop after it is treated. This information will not be valid if you combine different products, even if the pre-harvest interval is zero days on one or both products.

Q: I planted purple leaved castor bean along one edge of my yard close to a street. Are these plants dangerous? A friend told me they were poisonous.

A: The castor bean, Ricinus communis, is a beautiful, fast-growing plant that is sometimes used as a screen. The plant produces very toxic seeds which are attractive to young children. Since eating only one seed can be fatal, it is not wise to plant castor bean in areas where young children may play or discover the toxic seed. A number of other ornamental plants would make a safer hedge or screen.

Q: My new banana plant produced a flower stock which now has a lot of small fruit on it. How can I tell when the bananas are ready to pick?

A: The individual clusters or “hands” of bananas on a flower stock ripen in sequence beginning with the oldest. A single hand of fruit can be cut from the stock when one or more of the bananas or “fingers” turn from green to yellow. The fruit will continue to ripen indoors and can be eaten when the texture and flavor is to your liking.

Gardeners who live in relatively frost-free areas of the county can grow banana varieties that are much more flavorful than the standard commercial banana sold at retail stores. More information on varieties and banana culture can be found at

www.crfg.org in the banana fact sheet prepared by the California Rare Fruit Growers.

Bananas are herbaceous plants that form large clumps. An individual stock or pseudo stem will flower after it has grown about 44 to 46 leaves. Flowering may occur any season of the year. Plants that flower during the fall or winter usually do not produce ripe fruit until the following spring or summer.

A pseudo stem that has produced fruit will not flower again. Use a large pruning saw or sharp, square-nosed shovel to cut off the old pseudo stem near ground level. Be careful not to break off the new shoots or pups growing around the base of the old stock.
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