Q: I have a 30-year-old myoporum tree that, I'm told by an arborist, has mites that are causing its leaves to shrivel and turn brown. It's also growing mushrooms on its trunk and has a fungus. The arborist wants to treat it with a nicotine-based spray that he says is nontoxic and harmless. I love this tree for the shade it provides and the swing that hangs from its branches. I also have fruit trees nearby, so I'm wondering: Is this spray treatment truly harmless? Am I just postponing the inevitable? Should I simply face that it may have reached the end of its lifespan? Are there any other alternatives that I could consider?
A: My research hasn't turned up a myoporum-attacking mite. The tree is sometimes infested by aphids, which make it sticky and attract the powdery black fungus called sooty mold. However, I suspect your tree is being damaged by a newly appearing pest, myoporum thrips. I saw myoporum trees with masses of thrips-curled and deformed leaves in San Diego County a couple of years ago, and Larry Costello, environmental horticulture adviser with the Cooperative Extension of San Mateo County, recently told me it has spread north at least as far as the Peninsula. When I was looking for a myoporum to photograph, I saw, to my horror, that the pest is now beginning to damage trees in San Francisco. It's a narrow, shiny, brown to black, winged insect about 1/20-inch long. Its larvae are white to orange. Like the myoporum itself, it's from Australia.
To control aphids, periodic hard sprays of water from below or a narrow-range oil spray would probably be enough. However, if, as is likely, your tree has myoporum thrips, control is more difficult. In the short run, the least toxic of the pesticides that have killed the thrips is Spinosad, a substance made by a microorganism. It's toxic to bees briefly after spraying, so should be timed to avoid active bees, but otherwise has a low toxicity profile. It would probably be most effective when the damage is only slight, before too many leaves have curled around the pests.
Read more: http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2010/01/03/HO941AK5TB.DTL#ixzz0brQqYJJY