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Wasps attacking beetles may save dying eucalyptus trees
STANFORD -- After five years of losing scores of eucalyptus trees on campus to a pest borer beetle, biologists and Stanford grounds managers have found an effective way to stem the demise of the hundred-year-old trees: they are fighting the bad predator bug with a good predator bug.
Terry Stone, grounds supervisor at Stanford, and Larry Hanks, a postdoctoral entomologist at the University of California- Riverside, are introducing into the Stanford environment several species of tiny wasps that prey on the eggs and the larvae of the beetle.
Since arriving at Stanford in 1988, the eucalyptus longhorn beetle, Phoracantha semipunctata, is breeding rampantly. Its insatiable larvae have killed an estimated thousand trees in the arboretum on either side of Palm Drive.
Rather than eradicating the eucalyptus plague, Stone expects the wasps to control the size of the borer population.
"Using the wasps, we are trying to shrink the number of beetles, so that the trees get a better chance of fending them off," Stone said. "These wasps are completely harmless to humans; they don't even have a stinger."
Currently the trees, weakened by frosts and the drought of the last seven years, are hampered in their ability to produce the toxic sap they normally exude to poison and drown insect invaders.
"We are hoping for a natural balance," said Hanks, who developed the technique with Timothy Payne and Jocelyn Millar, both associate professors at Riverside's entomology department.
"When the wasps establish themselves at Stanford, they will reduce the number of beetles," Hanks said. "The beetles, as the wasp's only food source, will in turn control the number of wasps. That way, the borers will be reduced to a small population that the trees can bear."
Pests imported with trees
The longhorn borer was first spotted in Orange County in 1984. It was probably imported as an accidental stowaway on an airplane from Australia, its natural habitat. It multiplied unchecked in eucalyptus trees, because no natural parasites or predators of the borer existed in California.
Eucalyptus trees also came from Australia, where they thrive in a complete and balanced ecosystem with the longhorn borer, wasps and other interdependent species.
Until 1984, the California eucalyptus groves had been free of major pests since first being introduced here in the 1860s.
"Because the borer had no enemies here, it spread like a wildfire. The trees being susceptible from all the climate stress worsened the problem," Stone said.
The borer pest has also ravaged eucalyptus groves in Southern California.
There, the wasp program is in its fifth year and is gradually proving successful. In all places where the Riverside entomologists had released the bug-killing wasps, they found the species had taken hold years later. Some were spread up to 10 miles around the original release site, indicating that the wasps have established colonies in that area.
These wasps are tiny black creatures, smaller than ants. They feed solely on the longhorn borer's eggs.
"They are so specialized that they don't pose any threat to the environment, or to humans," Stone said.
The beetles fall prey to the wasps in the manner that human bodies are invaded in the movie Alien: instead of nurturing their young, they simply lay eggs inside the beetle eggs. The wasp larvae emerge from their eggs within the much bigger beetle eggs and devour the soft beetle embryos as they grow. Then they pupate inside that borer egg to mature. Finally, the grown wasps break free of the now useless shells and tuck a new generation of their offspring inside beetle eggs.
Animals that reproduce in that gruesome way are known as "parasitoid," since a true parasite never goes as far as killing its host.
The Riverside entomologists envision a whole wasp armada to fight the beetle in all its developmental stages toward maturity.
"Ultimately, we want to have five parasitoid wasp species out there, one for beetle eggs and four for successively bigger stages of larvae," Hanks said.
The first wasp that Hank and Stone currently release, an as yet unnamed species of the genus Avetianille, preys only on eggs and may catch 80 percent of them. The remaining 20 percent would hatch as small beetle larvae and be attacked by the smallest larvae- specific wasp. The estimated 20 percent escaping that attack grow larger and become targets for the third wasp. The few larvae surviving that onslaught manage to grow to full size, about one and a half inches, and are promptly used as egg depositories for the fifth, and largest, wasp species to be introduced at Stanford.
"Last year, we built a closed cage in the arboretum to start releasing the egg-using wasp in a small, protected area." Stone said. "This summer, the Riverside scientists send me a little FedEx package every week. It contains beetle eggs infected with tiny wasps reared there. On August 30, we'll start putting out the first parasitoids of the larval stages, called Syngaster lepidus."
Timing is crucial
The trick in establishing wasp colonies is coordinating the timing of release, he explained.
"I receive eggs infected with wasps that are just two days short of hatching. When they emerge, they need beetle eggs nearby to stick their offspring in."
Stone achieves that by cutting fresh and healthy eucalyptus branches two days before releasing the wasp-infected eggs. These logs exude aromatic oils and lure the amorous beetles.
"The adult beetles, interested in nothing but mating, are excellent in sniffing out the eucalyptus oils," Stone said. "Whatever logs you cut, the bugs are there within 24 hours. So when the wasps hatch, there are plenty of borer eggs there."
On August 30, Hank and his colleagues also will start surveying how well last year's wasps have managed to form a stable population here.
There is no set time frame for the wasp project at Stanford, and Hanks cautioned that its future will depend on further funding. Currently the California Department of Transportation, the Department of Forestry and Fire Protection, and the Department of Food and Agriculture are the main supporters of the project.
Program may spread to other areas
The wasps may be called upon to fight the longhorn beetle elsewhere in California. Steadily creeping northward, the plague already has reached eucalyptus groves in Marin County and in the Sacramento area.
Two years ago, when the infestation peaked and Stanford grounds maintenance workers were cutting, shredding and incinerating hundreds of trees, Riverside offered to include the Stanford site into their pilot program at no charge.
"At that time, you saw dead eucalyptus everywhere. You could just peel the bark off trees six feet in diameter and see tracks of the borer larvae all over the place," Stone said. "These trees die within one season."
The trouble starts when adult longhorn borers -- sleek, dark-brown critters with long, arching antennae -- lay eggs in small fissures in the bark. One female typically deposits 300 eggs in a season.
When the larvae hatch, they dig underneath the bark and radiate out, munching their way through the so-called cambium. That is the only live layer in a tree, a thin and fragile sheath that produces bark and the annual growth rings of trees.
The cambium is crucial to a tree's survival because it shuttles sugars from the leaves to the roots and nutrient minerals from the roots to its upper reaches.
"When the larvae eat their way around the tree, like a girdle, they interrupt that nutrient flow and the tree dies," Stone said.
Gnawing through the cambium, the beetle larvae leave behind streaks of excrement consisting of finely ground sawdust.
"They move in packs and eat each other, feeding on whatever comes in their way," Stone said. "Standing next to a tree, you can even hear them crunching.
"At first the borer pest was really depressing, but now I'm optimistic. Save for other influences, like frosts, it is just a matter of time until the wasps will settle permanently."
While that biological approach to the beetle problem takes several years to work, it is the only possible solution, Stone said. It is also an ecologically responsible way of maintaining an equilibrium between various species.
The wasps will even increase biologic diversity on campus, since some of the eucalyptus trees that needed to be felled will be replaced by trees native to California, such as the less vulnerable oak and bay trees.
"This pest will ultimately help us make the arboretum as natural as possible." Stone said. "The parasitoid strategy is difficult and time-consuming in the short term, but in the longer run, it helps create a more integrated ecological system on this campus."
This story was written by Gabrielle Strobel, a science writing intern at the Stanford News Service
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