If you notice, upon repeated observation here and there, that a particular plant species is being ravaged by an insect pest, wait a few years. If you happen to have a representative of this plant species in your own garden, doing absolutely nothing about it may be the best form of pest control. In due course, natural predators – ladybugs, lacewings, pirate bugs or parasitic wasps – are likely to find the offending pest and begin to feast upon it. Or, if the plant’s natural predators are only found in its land of origin, entomologists (insect experts) may decide to import those predators and, after instituting a breeding program, release huge numbers of them as a biological control tactic.
Typically, the imported insect predator is a parasitic wasp. Parasitic wasps are so tiny as to be virtually invisible. As many as 30 of them could stand together on the head of a pin. Parasitic wasps lay their eggs in eggs, larvae (caterpillars), or adult insect pests. After the parasitic wasp larvae hatch, they cannibalize their egg, larval, or adult insect pest hosts.
In 1988, the ash whitefly, a type of scale insect, was a major pest in ash trees and other ornamental plants throughout Los Angeles. The whitefly infestation was so severe that it posed a health hazard since you could hardly breathe in some areas without inhaling whiteflies. A parasitic wasp from the Mediterranean habitat of the ash whitefly was imported and, after two years, had the ash whitefly under control.
Also in 1988 came the proliferation of the eugenia psyllid, an aphid relative that causes pitting and puckering of eugenia leaves. Eugenia (Syzygium paniculatum) is the preferred plant for use as a medium to tall hedge or screen. Its foliage has a unique luster and new growth is a shiny bronze. In 1992, a parasitic wasp from eugenia’s native Australia was imported and first released at Disneyland, whose parking lot is hedged with thousands of linear feet of eugenia plants. The wasp ultimately helped control the psyllid and planting of eugenia hedges, which had been avoided for years, is happening once again.
To keep the psyllids at bay, however, it is recommended that, following pruning, eugenia clippings be used as a mulch under the plants. Just as the psyllid pests are concentrated in new growth, so, too, are the parasitic wasps that consume them. Thus, new top growth that is removed during hedge trimming should be left underneath in order that subsequent new growth will be inhabited not only by psyllids, but by the wasps, flying up from the mulch below, that control them.
In 1992, the giant whitefly appeared in San Diego and arrived in Los Angeles soon afterward.
The favorite plant host of this pest is hibiscus, but it is found on many other ornamental plants as well.
The San Diego Zoo was concerned about the giant whitefly since hibiscus grown on site is fed to many of its animals, especially reptiles such as iguanas and giant tortoises. Parasitic wasps from Texas and Central America were subsequently introduced and have helped to control the giant whitefly at the zoo. Incidentally, if you decide to feed hibiscus to your reptile, make sure the plants used are pesticide free.
Sometimes, biological control has unavoidably adverse consequences. In California, Scotch broom (Cytisus scoparius) was introduced as an ornamental plant but, due to its ability to produce lots of viable seed, became a weed. Scotch broom started popping up everywhere and still does, easily recognized by its outstanding neon yellow flowers in the spring and a fountainesque growth habit. Scotch broom soon took over vast acreages of pasture land throughout the state and control measures were sought. The genista caterpillar was brought in to munch the Scotch broom, a highly successful venture except that the caterpillar also munched the much more attractive, fragrant and less invasive sweet broom (Cytisus or Genista spachiana). So if you want to grow sweet broom, be prepared to spray the caterpillars that accompany it.
Use an organic pesticide containing either BT (Bacillus thuringiensis) or Spinosad.
If you seek a less troublesome yellow bloomer than a broom, I would recommend a yellow bush snapdragon (Keckiella antirrhinoides). This special shrub is on glorious display at the Theodore Payne Foundation Nursery in Sun Valley. It does well in both partial sun and shady locations.
One of the rewards of horticultural knowledge and experience is the ability to use familiar plants in unconventional ways. For example, let’s say you have a small to medium sized yard that is mostly lawn. You are thinking of taking out the lawn because of the upkeep involved. Michael Kappel, a master gardener in West Los Angeles, recently replaced several hundred square feet of partially shaded lawn with mondo grass (Ophiopogon japonicus). While this low mounding ground cover, which is actually a member of the lily family, does not accept heavy foot traffic, it does provide a steady sea green vista of predictable serenity and uses less than half the water of a conventional lawn.
Older clumps of mondo grass tend to turn brown around the edges or just get shabby, but this can be remedied by cutting them back with a hedge shears in early spring just before fresh, new growth resumes. In the front of Kappel’s house, a slope leading down to the street that was formerly planted first to a lawn and then to a variety of different species, has now become the exclusive domain of Bougainvillea ‘Orange King.’ Kappel prunes all vertical shoots of the bougainvillea to encourage exclusively horizontal growth. Horizontal shoots are pinned to the ground with U-nails so that, ultimately, his front slope will be nothing but a supine expanse of orange bracts.
Tip of the week
Shrimp plant (Justicia brandegeana) is a satisfying ornamental, both for its prawn-shaped bracts and its drought tolerance. It grows to around three feet tall and may be used as a stand-alone plant in a container or in the garden, or as a low hedge. Its colorful bracts are nearly always on display, a trait it shares with several of its counsins, namely pink Justicia carnea, orange Justicia spicigera, silver Justicia betonia, golden Justicia aurea and yellow and pink Justicia ‘Fruit Salad.’