Experts tackle readers’ garden questions.
Q: Years ago, we had a tree that was infested with some sort of minute insect that looked like a white powder on the leaves and secreted a sticky substance that was a real mess. My son, who was home at the time, went to our local nursery and they told him what to do and buy. I think they were stakes you put around the root system, but am not sure. Could you let me know what I should do?
Chris Ferguson, Sacramento
Sacramento County Master Gardener Rachel Tooker: Based on what you describe, some possible options include aphids, mealybugs, scale and whiteflies. At certain stages in their life cycle, all of these insects feed on plant fluids. In the process, they generate a sugary, sticky substance called honeydew. The honeydew also becomes a great host for the development of black sooty mold (a fungus). To help determine which pest is affecting your trees, bring a sample to the Master Gardener office or visit the University of California Integrated Pest Management website, ipm.ucanr.edu.
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While honeydew and black sooty mold are not usually harmful to plants, with heavy infestations, they do create a sticky and unsightly mess on trees, sidewalks, automobiles and other surfaces. In large concentrations, the insects can feed so heavily that they retard growth of the plants or cause premature fruit and leaf drop.
If honeydew is not present, but you see a white or gray dusting on the plants, another possibility is the fungal disease called powdery mildew. Powdery mildew forms most rapidly in periods with warm days and cool nights. It can be washed off with rain and subsides once temperatures are over 90 degrees. In heavy concentrations, it can cause stunting and distortion of shoots or russeting of fruit.
Once you have identified the problem, you are now ready to put together a treatment plan. In general, it is best to start with lower impact options. Pesticides – whether in the form of stakes, sprays or granules – should be a last resort. Some toxic pesticides can destroy beneficial insects and may lead to a heavier infestation of the problem pest at a later time. Fortunately, there are other cultural and biological methods that should help keep the population in check.
Start by inspecting the plants regularly for signs of pests or disease – at least twice weekly when plants are growing rapidly. Check all sides of leaves and in several different areas of the tree. Also look for evidence of natural enemies, such as lady beetles, lacewings and syrphid fly larvae. If there are high populations of these and other beneficial insects, and conditions are kept right (no spraying with insecticides) to keep the natural enemies healthy, the pest population may well be reduced within a week or two without further intervention.
If there are large numbers of ants climbing up the tree trunk, check for insects on limbs and leaves above. Ants often farm the insects for their honeydew and will defend them from natural predators. Therefore, inhibiting the ants with products such as Tanglefoot or ant bait and pruning branches that may be touching the ground are great ways to help natural predators do the pest management work for you.
If populations are localized, the best control may be to cut out and dispose of leaves or new shoots on which the insects or powdery mildew are concentrated. Because tender new leaves increase reproduction and survival, fertilizing, extensive watering and other cultural practices that stimulate plant growth should be minimized.
Another easy way to reduce aphid, mealybug and whitefly populations on sturdy plants is to knock them off with a strong spray of water in the morning. This can even work on large trees when applied with appropriate equipment. This method also works to blast off powdery mildew.
If insecticide applications are deemed necessary, choose materials that are the least toxic to beneficial insects and natural enemies. Treat only in spots where insects are most abundant to preserve natural enemies elsewhere in the yard. Insecticidal soap, narrow-range oil and neem oil applied at the correct time in the insect’s life cycle will kill them on contact and have a relatively low residual toxicity to natural enemies. Follow application instructions appropriate to your identified pest for best results.
For more information, visit www.ipm.ucanr.edu under “Home, Garden, Turf and Landscape Pests.”
Rachel Tooker is a UC Cooperative Extension Master Gardener for Sacramento County.
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 email@example.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
- Amador: 209-223-6838; 10 a.m.-noon Monday-Thursday; website: ceamador.ucdavis.edu
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- Placer: 530-889-7388; 9 a.m.-noon Tuesday-Thursday or leave a message and calls will be returned; website: pcmg.ucanr.org/got_questions
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