Garden Detective: Spider mites a mighty vexing pest

Q: Each year, the leaves on my potted Brugmansia (angel's trumpet) get mottled, turn yellow, die and fall off. Spider mites? I did see some webbing and minuscule dots, much too small to see with the naked eye. I have put on approximately four applications of Bayer Advanced Rose and Flower Care, which says it kills spider mites. I also spray the underside of the leaves every day when I water it. The leaf wash seems to be helping some. Is there any other systemic product I can use for this problem? Because the plant is under stress, it's not flowering like it should.

A: Spider mites are common pests of landscapes and gardens, and can be found feeding on many fruit trees, vines, berries, vegetables and ornamental plants, says Sacramento County UC master gardener Liz Haines. Although related to insects, mites are members of the arachnid class, along with spiders and ticks.

To the naked eye, spider mites look like tiny moving dots; however, you can see them easily with a 10X hand lens. Mites cause damage by sucking cell contents from leaves. In high populations, visible damage to the leaves can be seen as stippling and yellowing of the leaves; sometimes the leaves have a bronze color. As feeding continues, the leaves turn yellow and drop off.

Often leaves, twigs and fruit are covered with large amounts of webbing. Dust on leaves encourages the mites; as they prefer dry conditions, apply water to pathways and other dusty areas at regular intervals.

Spider mites have many natural enemies that often limit populations. Using broad-spectrum insecticides and systemic insecticides can actually lead to mite outbreaks by killing beneficial insects. Forceful sprays of water, insecticidal oils or soaps can be used for management. Always monitor before treatment.

Spraying the underside of the leaves is a beneficial way to manage spider mites; however, check your soil before watering. Daily watering may be excessive, resulting in plant stress.

Brugmansia is a woody shrub that grows 5 to 15 feet tall, bearing long, fragrant trumpet-shaped flowers that attract hummingbirds. They require full sun to flower. They are heavy feeders and respond to frequent fertilization. All parts of the plant are poisonous, so take care when small children are in the garden.

For more information on spider mites, send a self-addressed, stamped, 42-cent business-size envelope to: PN7405 UC Cooperative Extension, 4145 Branch Center Road, Sacramento, CA 95827.

Q: Carpenter bees are attacking my flowering cherry tree, which occupies a prominent space in the front-yard landscape. It has a beautiful display of pink blossoms in the spring and red leaves. They're boring deep holes in larger limbs.

The holes are numerous, and in a recent discovery I found a network of holes and tunnels in the limb. Today, when I cut off another limb I discovered a network containing two or three large black bees and a couple of equally large brownish-red bees. So far the damage is confined to one side (north) of the tree.

How can I get rid of the carpenter bees and save what is left of the tree?

A: The presence of carpenter bees around buildings and wooden structures can be annoying or even frightening; however, males cannot sting and females rarely attack, says Sacramento County UC master gardener Mary Griggs. Carpenter bees frequently attack dead wood or softer trees such as redwood, cedar, mulberry, ash and pecan. They avoid most harder woods.

Prevention is the main approach to managing carpenter bees. Fill the holes with steel wool and caulk, then put some diluted latex paint over the holes.

If the infestation is not severe, it might be best to just leave them alone. Carpenter bees are generally considered beneficial insects because they help pollinate various crop and noncrop plants.

If infestation is high or risk of damage is great, insecticides may be used. Treat active nests with liquid or dust formulations of insecticides or desiccant dusts. There are liquid formulations and dusts labeled for use against carpenter bees. Be sure to read and follow instructions carefully, as these dusts can cause serious lung irritation.

For additional information on carpenter bees, please send a self-addressed, stamped, 42-cent, business-size envelope to PN 7417, UC Cooperative Extension, 4145 Branch Center Road, Sacramento, CA 95827.

Evaluation of the overall health of your tree as well as your cultural practices is recommended. The master gardeners are available to discuss this from 9 a.m. to noon and 1 to 4 p.m. Monday through Friday at (916) 875-6913.