Garden Detective

Garden Detective: Messy aphids have her up a tree

Garden Detective: Asian wooly hackberry aphids can make a mess. This heavily infested hackberry leaf shows the honeydew produced by the aphids dripping from the tip.
Garden Detective: Asian wooly hackberry aphids can make a mess. This heavily infested hackberry leaf shows the honeydew produced by the aphids dripping from the tip. Courtesy of UC Statewide IPM Program

Q: I have a huge, beautiful, tall, good shade tree in our front yard that is infested by aphids. I don’t want to remove the tree, but I want the sticky sap gone. How do I treat this issue?

Delilah Mergupe, Sacramento

A: The variety of your tree will help determine specifically what insect is producing the sticky substance, according to UC master gardener Annie Kempees.

Many species of aphids are active in Sacramento on a variety of plants. Asian wooly hackberry aphids are a pest in Chinese hackberry. The UC IPM website includes photos of common aphids by habitat. See them at http://ipm.ucdavis.edu.

Aphids are small, soft-bodied insects with long, slender mouth parts. With these mouth parts, they pierce stems, leaves and other tender plant parts to suck out plant fluids.

They also secrete large quantities of a sticky substance known as honeydew. This honeydew often turns surrounding plant material black with the growth of sooty mold fungus that inhibits the plant’s ability to photosynthesize and provide food for itself through its leaves.

Wherever the honeydew lands becomes a sticky mess: the ground, other leaves, your deck, your car.

Aphids have many natural enemies such as lady beetle, lacewings and syrphid fly larva, to name a few. Substantial numbers of any of these natural control factors can mean that the aphid population may be reduced rapidly and should be watched closely.

If you have noticed that your tree looks water-stressed, leaves have yellowed and are dropping, death of twigs and limbs, and bark cracking and gumming, you may have an infestation of scale, not aphids. Scales feed by sucking plant juices and some may inject toxic saliva into the plants. When numerous, some species weaken a tree or plant, causing it to grow more slowly.

The importance of infestations depends on the scale species, the plant species and cultivar, environmental factors, and natural enemies.

Ants are often associated with aphid and scale populations; they will protect these pests from their natural enemies in order to feed on the honeydew.

If you see ants crawling up aphid- or scale-infested trees, visit your local garden center and find a product called “Tree Wrap.” Purchase a tube of a sticky substance such as Tanglefoot, and some thin, wood sticks for application. Avoid using your fingers for this task; the substance is very sticky. These two products used together will minimize the amount of ants gaining access to the pests and the honeydew. If the tree wrap becomes covered in dead ants and live ones are crawling over the dead ones, remove the used wrap and reapply the tree wrap and sticky substance.

With the ants becoming less able to gain access to the tree, the natural enemies of aphids and scale will be able to more quickly reduce the pests’ populations.

Additional information on control of aphids and scales is available online at http://ipm.ucdavis.edu.

Garden questions?

Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties. Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to h&g@sacbee.com. Please put “Garden Detective” in the subject field and include your postal address. To contact UC Extension directly, call:

  • Sacramento: (916) 875-6913; 9 a.m.-noon and 1-4 p.m. Monday-Thursday
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  • Yolo: (530) 666-8737; 9-11 a.m. Tuesdays and Fridays, or leave a message and calls will be returned
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