The Q&A Archives: leaf miners

Question: How do I rid my grapefruit tree of these pests?

Answer: Grapefruit, lemon and lime are favorite hosts of the citrus leafminer, but it feeds on all varieties of citrus and more than 20 other related plants in the citrus family. The insect primarily feeds on young developing foliage and occasionally on the rind of young fruit.

The pest occurs on host plants throughout the year and has a short life cycle. It can have six to 13 generations annually, depending on temperature and the availability of new growth flushes.

The moth is most active from dusk till dawn and rests on the undersides of leaves during the day. The insect only infests young, flushing foliage and does not attack more mature foliage. The female moth lays eggs singly on new shoots and on leaves less than a half-inch in length. The most preferred egg-laying site is along the mid vein on newly emerged leaves.

The larvae, a tiny caterpillar, hatches and begins feeding immediately in nearly invisible mines under the leaf's cuticle. As the larvae grows, its zigzagging path of parallel mines become more noticeable. Its mine has a central trail of excrement that can be used to distinguish it from mines of the native citrus peelminer.

When the larvae matures it moves to the leaf margin where it creates a silken cocoon. As the silk dries the leaf curls over the pupal cell. The larvae pupates within the mine and then emerges as an adult moth.

Beneficial insects, particularly tiny parasitic wasps, that attack the pest will increase and should provide good biological control in a year or two. Gardeners will need to be patient while this occurs and should not spray mature trees with insecticides that can harm beneficial insects.

Mature bearing citrus can withstand heavy infestation of citrus leafminer with little effect on fruit production. The loss of some new foliage is a minor problem on older trees that have a healthy canopy of mature leaves. Mines in leaves may detract from their appearance, but the green portion of leaves will still produce food for the tree and should not be removed.

Pest control products currently labeled for use on citrus in home gardens are not very effective on citrus leafminer because they do not control larvae feeding inside leaves.

Broad spectrum insecticides that leave a toxic residue on the foliage can do more harm than good. Repeated applications will kill beneficial insects that can result in a buildup of white flies, scale insects and other citrus pests.

The loss of new leaves is a more serious problem on young trees that do not have a lot of mature foliage. The growth of newly planted trees may be reduced by citrus leafminer if the infestation is heavy and prolonged. Removing the affected leaves will keep the miners from maturing and causing damage in next year's leaves.

Before you buy or use an insecticide product to control citrus leafminer, first read the label. Make sure it can be used on citrus and follow all the directions.

A new insecticide product, Green Light Spinosad, is labeled for use on citrus to control citrus leafminer, but only a maximum of six applications can be made per season. Many other insecticides labeled for citrus do not have this limitation, but they may damage tender foliage with repeated application, or if the product is mixed stronger than directed on the label.

Hope this answers all your questions!

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