Garden Detective: Controlling aphids on locust tree

Published: Saturday, Aug. 17, 2013 - 12:00 am

We have a lot of locust trees on our property. Most are doing very well. We have lived here more than a decade and have always enjoyed one large 30-foot tree out back.

We believe it is a black locust because of the white flowers. (We also have Idaho locust.) The big tree developed a massive amount of aphidlike bugs and the tree has become coated with a substance that’s hard to describe — it’s almost as though the tree has a spraylike residue on it, all the way to the top!

We do not use pesticides. The neighbors say they haven’t sprayed anything. The tree gets very little direct water. We don’t want to lose this tree. What could be wrong?

After such a good start in the spring, it had lots of blooms and bees were thriving in it. But now, it looks as though it is dying.

– Linda Bush, Anza

According to the Sunset Western Garden Book, the black locust is a fast-growing tree to a height of 75 feet, said UC master gardener Carol Hunter. It has deeply furrowed brown bark and white fragrant flowers, which hang in clusters.

Although aphids are not commonly associated with locust trees, your description is entirely consistent with an aphid infestation.

Aphids often feed in dense groups on leaves and stems. They do not rapidly disperse when disturbed. Adults are usually 1/8 inch or less in length and are pear-shaped with long legs and antennae. They vary from green, yellow, white, brown or red to black.

The most bothersome aspect of aphids is the honeydew they produce. Honeydew is sugary water excreted by the aphids after ingesting sap. Honeydew itself is harmless to plants, except if it becomes so abundant that black sooty mold grows on it. Then the plant will suffer because light cannot reach the foliage.

Many natural enemies help to control aphids, but these predators and parasites may not always appear in sufficient numbers. However, their preservation is an essential part of a long-term integrated pest management program.

If insecticide applications are deemed necessary, choose those that are least toxic to natural enemies. Insecticidal soap and narrow-range oil kill aphids and other insects on contact. In comparison to other products, they have low residual toxicity to natural enemies that move on to plants after spraying.

Use baits or sticky barriers to control honeydew-seeking ants because ants can disrupt biological controls by protecting aphids from those natural enemies.

Another remedy is a forceful stream of water directed at the aphids early in the day, so the plant dries out before night. Knocking them off the plant will kill these soft-bodied critters; they can’t survive the fall.

Additional information about aphids is available in Pest Note No. 7404. For a copy, send a self-addressed, stamped business-size envelope to: Pest Note 7404, UC Cooperative Extension, 4145 Branch Center Road, Sacramento, CA 95827. You can also find this information online at

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