GARDENING : EUCALYPTUS HIT BY AUSTRALIAN INSECT.

Byline: Joshua Siskin

In horticulture, as in life, you cannot take anything for granted, not even the health of a normally robust tree such as the eucalyptus.

So much had I come to accept the leafy bounty, if not excessive growth, of the eucalyptus trees on my street that it was almost a relief, at first, to see their leaves dropping for no apparent reason. During winter storms, leaf-laden branches from these perennially overgrown eucalyptuses fall into the street, blocking traffic and testing the limits of my back as I drag them over to the curb. With leaves falling in midsummer, I reasoned, less overall growth would occur this year. (Leaves manufacture sugar that is utilized in the growth of all parts of a tree.) At the end of the growing season, these evergreen trees would not be top-heavy with lush foliage, for once, and might even survive winter's wind and rain.

It was in the course of investigating a complaint about a neighbor's eucalyptuses, a few blocks away, that I was finally persuaded to look up into the canopy of the trees on my own street. My neighbor's eucalyptuses were not looking good at all and, lo and behold, when I stopped to take a careful second look, neither were mine. This was no midsummer swoon but a debilitating condition, fallen leaves were not replaced by healthy new growth but by stunted, yellow foliage.

At present, in the Valley, several species of eucalyptus trees are being devastated by a pest known as the red gum lerp psyllid. This Australian insect was first spotted a year ago in El Monte, having hitched a ride to California not long before then, probably on an airplane. The red gum (Eucalyptus camaldulensis) is the favorite host tree of this pest, although the flooded gum (Eucalyptus rudis), the blue gum (Eucalyptus globulus) and the red ironbark (Eucalyptus sideroxylon) are other widely grown, psyllid-infested species.

Leaves of infested trees are covered with waxy, cream-colored discs that resemble, but have no relation to, scale insects. These discs are protective coverings formed by immature lerp psyllids or nymphs. A psyllid is a miniature type of a locust, ``lerp'' is an Australian aborigine word for the protective discs that this particular psyllid secretes.

According to Robert Wallace, a Valley arborist, there is no proven means, at present, for controlling this pest. Affected trees should be deep watered to keep them growing as vigorously as possible during the infestation. A systemic insecticide known as Merit, injected into the soil beneath tree roots, has been used with some success, but it is too early to tell if such treatment will be effective over the long term. (A systemic insecticide that is taken up by tree roots will spread throughout the circulatory system of the tree and into the veins of every leaf, pests die when they suck sap from the leaves of such a tree.)

University of California entomologists are currently en route to Australia to collect specimens of a parasitic wasp that is a natural predator of the lerp psyllid. This strategy of collecting and releasing natural predators has worked successfully on at least two ornamental plant pests that plagued the Valley in recent years: the ash whitefly (parasitic wasps imported from Italy and Israel were used effectively to bring it under control) and the Eugenia psyllid - another Australian psyllid that was finally subdued with parasitic wasps collected from down under.

Parasitic wasps are tiny creatures - several could stand together on the head of a pin - that are innocuous to humans. A female parasitic wasp injects its ovipositor - a needlelike posterior appendage - into the pest victim. A wasp egg passes through the ovipositor into the pest and develops inside it. Eventually a wasp larva will hatch out of the egg and cannibalize the pest. Following pupation, an adult parasitic wasp will emerge from the skeletal remains of the pest.

Now that's a gardenia 

Can you imagine a gardenia that grows to a height of 10 feet, has heavily perfumed, tubular white flowers and never needs fertilization? Gardenias have a reputation for fussiness, but there is one species that is not fussy at all. The plant in question is Gardenia Thunbergia, a native of South Africa. It makes a superior hedge for partial sun or light-shade Valley exposures, yet it is mysteriously absent from local nurseries. If anyone knows of a source for this plant, please advise and I will pass it along to readers of this column. In the meantime, there are three wonderful specimens of Gardenia Thunbergia - which have grown together into a hedge - that are currently in bloom behind a Los Angeles Public Library building at the corner of Moorpark Street and Katherine Avenue in Sherman Oaks.