GARDENING : PARASITIC WASPS TO THE RESCUE.

Byline: Joshua Siskin

Anyone who doubts the existence of a ubiquitous, protective, yet invisible presence in the universe should consider the phenomenon of parasitic wasps. Parasitic wasps, a group of which could stand together on the head of a pin, make it possible to grow crops. Were it not for these minute, beneficial insects, crop destroyers such as aphids, scales, whiteflies, and mites would wreak havoc on every orchard and field planted under the sun.

Parasitic wasps are not true parasites. A parasite lives off another organism without killing it. Parasitic wasps, on the other hand, cannibalize their insect hosts. To distinguish them from truly parasitic organisms, these wasps have been given the name ``parasitoids.''

Parasitoids pass their eggs through a posterior needle-like organ called an ovipositor. The ovipositor jabs into the body or into the eggs of an insect host. As the larva of the parasitoid hatches out of its egg, it consumes the contents of the insect or the egg in which it resides. This larva then pupates and eventually an adult wasps emerges from the dead host. Where large groups of aphids are found on a plant, you will see tiny jet black globs, which are the remains of parasitized aphids. Where large groups of scales are present, you will see some with exit holes made by the departing adult wasps.

There are two main groups of parasitoid mini-wasps: ichneumonids, which lay their eggs mainly in the larvae (caterpillars) and pupae (cocoons) of moths and butterflies; and chalcids, which lay their eggs not only in larvae and pupae, but in eggs, nymphs and adults of virtually every type of insect.

Whitefly knockout

Two of the most serious garden pests in recent memory were ultimately brought under control through the grace of parasitic mini-wasps. In the early 1990s, the ash whitefly had become such a nuisance in Los Angeles back yards that outdoor barbecuing was no longer possible. This prompted entomologists to go to the Mediterranean home of the ash whitefly and bring back its natural enemies - parasitic wasps that were found in Italy and Israel. In a year's time, the ash whitefly had been significantly diminished, and today it is rarely seen.

A few years later, a mini-locust known as a psyllid had decimated just about every eugenia plant in Southern California. This time, entomologists were sent to Australia, home to both the eugenia and its psyllid pest. Again parasitic wasps were found and brought back. Today, the pitted leaves caused by the eugenia psyllid are not seen nearly as much as previously.

Just when you thought it was safe to start planting again, along comes another pest that is disfiguring a variety of ornamental plants. The pest is called the giant whitefly, and it has been especially troublesome along the coast, from San Diego to Santa Barbara. It has not been seen much in the Valley; let's hope that its tropical habitat means that our drier air is not to its liking.

About two months ago, I came upon some giant whiteflies at an apartment building in Hollywood. They had turned the leaves of the hibiscus plants around the building completely white. At first, I didn't think this was an insect. It just couldn't be, I thought. The hibiscus leaves were covered with a thick white substance that looked like laundry lint. A pest control adviser inspected the plants and explained that the lint-like substance is a wax secreted by nymphs of the giant whitefly. To ameliorate this condition, the plants were cut back and thinned out. An ultra-fine oil spray was applied to the leaves, as well as diazanon. This combination of cultural and chemical procedures has, so far, eliminated most of the whitefly problem.

The new breed

Sooner or later, though, whiteflies will return to these hibiscuses unless giant whitefly predators are brought into the picture. Fortunately, as reported in this month's Pacific Coast Nurseryman, entomologist David Headrick of the University of California, Riverside, is in the process of nurturing colonies of parasitic wasps, which may help us control the giant whitefly. Headrick found two species of parasitic wasps, together with the giant whitefly, on a hibiscus plant in a botanical garden near downtown Guadalajara, Mexico. Good things come in small packages; the wasps are only 2 millimeters long.

Tip of the week: For an unusual ornamental touch in the fall color planter, consider ornamental kale and ornamental cabbage. Ornamental kale has ruffly leaves and comes in various combinations of green, white, rose and violet. Ornamental cabbage has wavy and waxy leaves that are similarly colored.