Myoporum laetum infested with thrips

I was hoping you could solve a problem with my Myoporum laetum. I have 13 Myoporum trees as a 20-foot tall hedge. Two years ago, three of the plants started to get a strange leaf curl. The curl is very tight and affects most of the plants. My arborist said it may look awful but the plants are not dead. He thought the problem might be thrips and suggested I have the three trees sprayed; I did. It has been a year and now all 13 of the Myoporum have the awful leaf curl. The privacy this beautiful hedge used to provide is nearly gone since 80 percent of all the leaves have this tight distorted curl. Do you have any suggestions as to what is causing the leaf curl? Can the plants be saved?
— Mary Read, Simi Valley
Thrips, resembling white, yellow, brown or black threads, are probably your problem. In recent years, an Australian thrips that feasts on the Myoporum species has found its way into Southern California.
Myoporum laetum is a gorgeously domed or spherical tree with glistening, elliptical foliage, appropriate for full or partial sun exposures. If there is any hope for your trees, it would be in the arrival of natural predators, such as predator mites or minute pirate bugs, that would find your Myoporum thrips to their liking.
Unfortunately, when a pest comes to California from another part of the world, its natural predator must typically be imported as well in order to control it. The problem with spraying is that the beneficial insects that might control the pests are also killed. Often, just when a plant looks its worst is when the beneficials arrive. I would wait one more year, without spraying, to see if there are any improvements before removing the plants.
You might also want to procure beneficial insects from Ventura-based Rincon-Vitova Insectaries. You can order thrips-devouring mites and pirate bugs from www.rinconvitova.com.
Thrips are sucking insects, which puts them in the same league as aphids, mealybugs, whiteflies and scales. Their damage appears as puckered, curled or rolled-up leaves.
Sucking insects are the biggest threat to ornamental plants because of their rapid rate of reproduction. Thrips, minuscule squiggly squirmers that appear when you uncurl the leaves they inhabit, go through their entire life cycle in only 10 days. Thrips and other sucking insects experience incomplete metamorphosis, which means they hatch from eggs into nymphs, mini-versions of their adult form, and quickly grow to maturity.
Our mild climate contributes to their rapid proliferation. In colder parts of the country, insect populations are decimated by sub-freezing winter temperatures and frozen earth. Here, however, insects may over-winter as eggs or hibernate as adults only briefly before resuming full reproductive activity.
Under the best of circumstances, Myoporum laetum is not a long-lived plant in our area, going into decline at 20 to 40 years old.
The plant is called Ngaio in its native New Zealand, where the Maori people rub its crushed leaves on their skin, curiously enough, to repel insects. Myoporum appears on lists of plants that are tolerant of wind and salt spray and it is often found in beach gardens.
Inland Southern California valleys, however, are not terribly hospitable to Myoporum trees. In Newhall, a hedge of Myoporum was recently replaced with glossy leaf privet (Ligustrum lucidum), which is more cold- and heat-tolerant than Myoporum.
Tip of the week
Myoporum parvifolium is a sun-loving ground cover which, unlike the Myoporum tree, is highly suited to our area.
There are two common ground cover varieties: `Pink’ with pink flowers, growing 3 to 6 inches tall, and `Putah Creek,’ which is white-flowered and 1 foot tall. A single one-gallon plant of either type will grow 6 to 8 feet in every direction within one year, rooting wherever its stems make contact with moist ground.
Bear in mind that sun-loving ground covers of all kinds look good for around two to four years before they start dying out in spots, whether from fungus disease or thatch accumulation.

Photo credit: Mollivan Jon / Foter.com / CC BY-NC