A rapidly spreading infestation of Australian beetles is threatening to kill millions of eucalyptus trees in Southern California unless natural enemies of the pest are imported from Australia, according to a UC Riverside entomologist.
The beetle, known as the eucalyptus longhorn borer, was first detected in an Orange County eucalyptus grove near El Toro in October, 1984. In the last 21 months it has spread to every Southern California county, entomologist Robert F. Luck said.
Scientists and state officials, hampered by a lack of funds to fight the infestation and an almost total absence of research on the subject, have only recently began to grapple with the problem. Entomologists from UCR began studying the infestation in March, 1985, and their preliminary findings indicate that "hundreds" of trees have already died in Orange County, according to Glenn Scriven, Luck's research associate.
Eucalyptus trees in Southern California, long associated with the landscape of the region, have been free of major pests since they were planted in the 1860s as windbreaks and wood lots for railroad ties.
The widespread infestation and absence of local natural enemies to the destructive beetles has left state officials scrambling to find a solution to the problem.
"The beetle has spread pretty rapidly because a lot of these trees are cut and transported throughout Southern California for firewood," Luck said. "The larvae are also transported in the process."
The pest's larvae bore feeding tunnels beneath the eucalyptus bark and into branches, interrupting the flow of nutrients and water. Female beetles usually lay the eggs on older, dry or diseased trees, and on freshly cut logs, Luck said.
San Diego County entomologist George Opel said the beetle has been detected at San Onofre, Oceanside and in Balboa Park. Opel said the insect "has done no appreciable damage in the county yet," but officials are keeping a close eye on the large eucalyptus forest in Balboa Park.
Luck and Scriven are leading efforts to find ways to eradicate the pest and recently returned from a 3 1/2-week trip to Australia, where they studied the beetle's predators in the wild. Infestation is not a problem in Australia, where eucalyptus trees are a native plant and where the pest has a host of natural enemies ranging from wasps and bees to other beetles, Luck said.
Don Henry, head of the state Department of Food and Agriculture's pest detection division, said the state has no eradication plans. Henry said the state places its priority on "pests of agricultural significance" and added that spraying the tall trees would be financially "impractical" because it would have to be done twice a month during the spring and summer, when the beetles are most active.
Meanwhile, Luck said that the larger, older trees planted at the turn of the century are particularly vulnerable to the beetle's larvae.
"Potentially, the larger, older trees run a good risk of being killed by the beetle," he said. "They become more vulnerable during the summer months and if they are not watered regularly."
Scriven said trees that are watered regularly, and younger ones, hold up well against the insect and actually repel the larvae. Healthy trees respond to the larval boring by secreting large quantities of gum, which smothers the larvae.
But the damage on a stricken tree can be extensive. Beetles repeatedly attack the same tree until "galleries" of larvae riddle the inner bark and cambium--the living tissue under the bark. In some cases a single larva can "girdle" the trunk and kill the tree, Luck said. Attacking larvae can also leave huge lesions on the sides of the trunk that eventually kill the tree.
In the first report published on the infestation, Luck, Scriven and Eldon Reeves, Riverside County Department of Agriculture entomologist, wrote in the July-August issue of California Agriculture magazine that a single adult female longhorn borer beetle can lay 300 eggs. As many as three generations per year can be produced in Southern California's climate, the entomologists said.
Luck said he is still attempting to cut through the of state, federal and Australian bureaucracies to obtain permission to import the beetle's natural enemies. He said that it will probably be next spring before the first shipments of Australian wasps, bees and beetles needed to combat the infestation arrive in Southern California.
But even then, Luck estimates that the predators' success rate at wiping out the beetle will probably be no more than 50%.
"As long as there are many water-stressed trees around, the problem will continue," he said. "They'll pick off these trees and there's not a lot we can do about it. . . . Frequent watering seems to be the key . . . but natural enemies is the next best approach. And even then, the success rate can only be guaranteed at 40% to 50%."