SCIENCE / MEDICINE : Wasps Riding to the Rescue : Scientists Hope Tiny Fighters Can Knock Out Beetles in Eucalyptus
The longhorn borer beetle, an Australian immigrant that has been chewing up Southern California’s eucalyptus trees, is about to meet an old acquaintance from Down Under, and state agricultural authorities hope that won’t be a g’day for the beetle.
To combat the borer, two researchers at UC Riverside are preparing to unleash Australian beetle-killing wasps, which state officials hope will fly, like tiny cavalrymen, to the rescue of the state’s trees.
The wasp lives only on eucalyptus longhorn borers and cannot sting or harm other life forms, according to the researchers and state and federal regulators.
The wasp does, however, “look pretty spectacular, with a needle two or three centimeters long (about 1 inch) sticking out its back end that can bore through tree bark,” said UC Riverside researcher Robert F. Luck.
Keeps Population in Check
The eucalyptus trees of Australia--where tree, beetle and wasp all came from to begin with--are not in danger from the beetle, in part because the wasp keeps the beetle population low, said Luck’s partner, Glen Scriven.
The Australian beetles first appeared around El Toro in Orange County four years ago, hitchhiking there on eucalyptus wood from Australia, agricultural authorities suspect.
With no natural enemies or effective pesticides available, the borer beetle, which can fly up to nine miles a night in its adult stage, have proliferated from San Diego to Ventura counties, according to the state Department of Food and Agriculture. Forestry officials have called the beetle a “time bomb” threatening the future of California’s eucalyptus trees, the most widely planted non-native trees in the state.
With the approval of state and federal authorities, Scriven and Luck had an Australian scientist at the University of Adelaide send them 12 immature wasps in cocoons last summer.
Breeding Less Than Expected
They planted them on a eucalyptus log kept in a sealed quarantine area on the campus. They had expected several hundred wasps by now, but the wasp-breeding process has been slower than they expected. So far they have eight--”all females, unfortunately,” Scriven said--and eight cocoons ready to hatch.
But if they can get at least two males from the cocoons, they will release half the wasps when the beetles are at a vulnerable stage in their life cycle, which could be in the next month or two, Scriven said. The others will be kept as breeding stock, in the hope of building up a larger number for the next release.
If the remaining cocoons all produce females “then we’re down the tubes” until they receive about 80 more wasps, which are expected from Australia “any day now,” he said.
The major concern in approving the wasp’s transformation into an American species was the same worry that attends the release of any foreign life form: The possibility that in its new environment the species might attack some native American insect or plant, causing unpredictable environmental consequences. Or perhaps it would show other undesireable traits, such as nesting in homes.
Some Misfirings
“The last thing we want to do is create a problem like that,” Luck said. There have been no cases of a foreign insect that had adapted to preying on a specific other insect adapting to a new food source after being turned loose in the United States, he said. But there have been several instances of insects imported to attack an undesireable plant that then went on to attack native vegetation as well, he said.
This wasp however--it is called Syngaster lepidus in Latin, but is nameless in English--belongs to a family well known for predictable behavior, Luck said.
“Although this species is not particularly well known, we know the family in general well enough to know that we don’t have to worry,” he said. “They only attack a very limited group of individuals.”
Approval of the U.S. Department of Agriculture for the wasps’ release came last month. Phil Lima, a specialist in plant and insect pests with the USDA’s biological assessment support staff in Hyattsville, Md., said that Luck and Scriven had made a sufficient case from the scientific literature that S . lepidus would not adapt to new prey.
Eucalyptus trees are popular as landscape decoration and windbreaks for their size, and also as firewood because the burning wood emits an aromatic smell. They have flourished in California in part because no native parasites had evolved to attack them.
In four years, the beetle spread from Orange to Los Angeles, San Diego, San Bernardino, Riverside and Ventura counties, and it appeared destined to continue spreading throughout the state, said state Department of Food and Agriculture spokeswoman Gera Curry.
State agricultural officials were dismayed to discover that pesticides had little effect on the beetle, and subsidized Luck’s and Scriven’s research with $40,500 in grants over the past two years, she said.
Eggs in Bark
The beetle larvae--particularly ugly insects about 1 1/2 inches long, with two horns longer than their bodies--hatch from eggs the beetles lay in eucalyptus bark. They chew tunnels as much as 1 1/2 inches wide in the cambium, the vital, delicate wood layer between the bark and the inner tree.
The beetles cannot harm most healthy trees, which produce a gum that drowns the larvae. Trees that are under stress, however, such as those that lack water or are under attack by diseases or other parasites, cannot produce enough gum, and many California eucalyptuses are suffering from drought.
When the larvae’s tunnels circle a tree, the flow of nourishment from the roots to the upper branches is blocked and the tree dies. If left unmolested, the beetles tunnel deep into the trunk, then return to the surface of the tree after about 10 days, sprout wings and fly away to mate and lay eggs in the bark of another tree.
The wasps kill the beetle larvae before they can mature.
The tunneling beetle larvae “chomp away, making a clicking sound,” which the wasps can hear at a distance, Scriven said. “The wasps land on the tree, locate the beetle under the bark, and bore through the wood with their ovipositor.”
The ovipositor evolved from the stinger that other wasps pack, and is such a specialized weapon that it threatens only the beetles, he said. With it, a wasp pierces the bark and stings the beetle larva within, paralyzing it.
“Then the wasp squeezes its eggs down the ovipositor and lays them on top of the paralyzed beetle. When the egg hatches, the wasp larva feeds on the paralyzed beetle larva, completes its development and spins a cocoon inside the larva,” Scriven said.
“It is kind of grim, if you think about it,” Luck commented.
Eventually the wasp larva hatches, chews its way out of the tree and flies off in search of other beetle larva.
The scientists have chosen a beetle-infested area on the UCR campus to make the first releases, Scriven said. They plan to gather up wasp cocoons from future generations and transplant them to other infested areas.
“Then they’ll just have to find their way to the other infestations in the state,” Scriven said, spreading naturally from one region to another as the beetles did.
Homeowners whose favorite back yard tree is being killed by beetles cannot expect the scientists to send them wasps to help out, Scriven said.
“They’re not available to the public, or we’d be swamped by requests,” he said. “We just can’t produce enough of them.”
Scriven said the wasps will probably not wipe out all the beetles, which would also result in the extinction of the wasps for lack of a food source. Instead, the wasps will probably reduce the beetle population to such a low number that the beetles will not endanger the trees and all three species will live together in balance, as they do in Australia.
