Saratoga News
Gardening
Raywood ash grows rapidly, but only gets about 30 feet tall and 20 feet wide.
Leafhoppers infest local trees, and can be difficult to control
By Tony Tomeo
The topic for this week will be arboriculture, or the horticulture of trees. A particular tree problem is the recent proliferation of leafhoppers. They have infested many local trees, particularly ash and birch trees in the northern Santa Clara Valley. I expect they will eventually get to areas not yet afflicted before they go away. Symptoms of infestation are very similar to those caused by the scale infestations that had so severely afflicted tulip trees and deciduous magnolias.
In entomological terms, a leafhopper has "piercing, sucking" mouthparts, as scale insects and aphid do. These types of mouthparts effectively impale leaves and suck out the cytoplasm within. Leafhoppers leave no obvious chewing damage on the foliage and are usually relatively harmless. Foliar discoloration is typically minor and is only seen by close inspection. Leafhoppers are only causing noticeable discoloration now because of the severity of the infestations. The main problem of infestation is that, like aphid and scale, leafhoppers excrete sticky honeydew, which rains down from infested trees onto pavement, parked cars or whatever happens to be in the area. Honeydew is not only messy, but it eventually becomes infested with unsightly black, sooty mold.
Unlike scale insects, which do not move after inserting their mouthparts, leafhoppers are quite mobile. As their name implies, they hop, but can also jump, fly, scurry and probably ride a bicycle--whatever they need to do to get from tree to tree. This makes control more difficult. Infestation can affect large areas very rapidly, including trees that were sprayed with insecticidal oil in winter. The oil kills overwintering leafhopper eggs, but does nothing to adults that come in from other trees. Leafhoppers are sometimes difficult to see because they try to leave the scene of the crime when foliage is handled. A typical leafhopper is only about half an inch long and is usually not riding a bicycle.
Fortunately, leafhopper infestation is only rarely severe enough to warrant pesticide application. Natural predators normally control them quite effectively. Pesticides are not completely effective and can kill the predatory insects that are frantically trying to control the problem themselves. Afterward, the leafhoppers tend to proliferate faster than their predators because there are so few predators to control them. Application of real insecticides (toxic chemicals) by a certified pesticide applicator should be used only for infestations that are so severe that they might damage the affected trees, or are producing an intolerable amount of honeydew.
Incidentally, I need to mention a problem associated with the use of cigarette butts as an aphid repellent as I discussed in a past column. The Master Gardeners of Santa Clara County have informed me that although tobacco can be somewhat effective when applied to the soil around small potted plants, it can unfortunately introduce the tobacco mosaic virus to solanaceous plants (those of the family Solanaceae) like tomato, pepper, potato, petunia, angel's trumpet and nicotiana (flowering tobacco).
Tree of the Week: Raywood ash
Most of the time, Raywood ash, Fraxinus oxycarpa" is not dripping honeydew, and is quite a nice, clean shade tree. The finely textured compound leaves turn burgundy red in autumn, and later fall within only a limited time. (Foliage does not linger in the canopy for a long period.) If watered too frequently, particularly in dense soil, verticillium wilt can become a problem, causing dieback of substantial limbs.
Horticulturist Tony Tomeo can be contacted at 408.358.2574 or at LGHORTICULTURE@aol.com.
