Pests Pack a Punch for Dry Plants : Drought: The chaparral is a tinderbox and the eucalyptus is full of bugs. Experts are fighting to keep the problems from flaring up further.
Make no mistake--paradise has some rough edges. In Southern California, earthquakes, brush fires and other plagues don’t make their presence known every day, but they’re lurking in the background, waiting to pounce.
Were you dazzled by the green sheen and exotic fragrance of the chaparral last winter? Well, it’s now a tinderbox waiting for a spark. A persistent fungus is continuing to take advantage of drought-weakened shrubs, creating vast dead areas that have contributed to what one forester says are the worst chaparral conditions in the last 30 years.
Is there a local stand of eucalyptus trees that you particularly enjoy? This summer, keep an ear cocked for the sound of beetles munching their way through the trees’ vital parts. Water-starved eucalyptus are prime targets for the longhorned borer, a voracious stowaway from Australia that has terrorized trees here for eight years.
“It’s still killing trees all over Southern California, and the drought has not been at all helpful,” said Tim Paine, an entomologist at UC Riverside who has led a three-year study of the beetle.
Eucalyptus has long been a favored tree in Orange County, with especially large stands in Lake Forest and the canyon areas of Laguna Beach.
For more than a century, the eucalyptus was the ideal California tree. It had no natural enemies, provided year-round shade and served as a windbreak for citrus groves. It was attractive, provided good firewood and did well under drought conditions.
Then the beetle blew into town.
The borer, Phoracantha semipunctata, arrived from Australia in a shipment of wood to a Lake Forest lumberyard in 1984. It has already killed as many as 100,000 eucalyptus trees in Southern California, said Larry Hanks, one of five UC Riverside researchers who are beginning a biological-control program.
The state’s most common type of eucalyptus, the blue gum, is also the most susceptible to the borer’s attack, say the UC Riverside researchers. Other vulnerable species include the rose gum and ribbon gum, while less susceptible varieties include the sugar gum, the ironbark and the lemon-scented gum.
But local factors, such as water stress, tree age, soil type and planting density, can combine to make any species vulnerable.
The adult borers are highly mobile, flying as far as six miles a night. Between April and November, they seek out stressed trees or recently cut or fallen branches in which to lay their eggs. The eggs, deposited under the outer bark, hatch quickly, and the larvae begin to bore into the inner bark, killing the tree as they cut off its nutrient supply.
The infestation is often so bad that the sound of the larvae munching the tree is audible, Hanks said. In as little as six weeks, adult beetles more than an inch long emerge from the tree to begin the cycle again.
Because six years of drought have weakened enough trees to keep the beetle going for decades, scientists will soon begin releasing a natural enemy of the borer.
The predator, a tiny, stingless wasp from Australia, is a natural parasite of the borer’s eggs. Because it interrupts the borer’s life cycle before the larvae hatch and begin harming trees, it is the preferred means of control, Hanks said. Wasps will be released at UC San Diego, Will Rogers State Historic Park in Los Angeles, Stanford University, Rancho Santa Fe and San Bernardino near Interstate 10.
Later this year, the scientists will release four additional varieties of wasps that specialize in attacking the borer larvae. Together, the five types of predators should knock the borer population down to 1/1,000th of its present size, said Hanks, who added that complete eradication is impossible because of the huge numbers involved.
For homeowners, the best defense against the borers is to deep-water trees regularly, experts say.
“Bark moisture content is the most important thing to keep the beetle out,” Hanks said. “If the tree is well-watered, the wood is just too turgid, like a sponge, and if the larvae try to feed in there, they drown. But once you have larvae in your tree, it’s too late.”
The adult borers are especially attracted to freshly cut logs, so pruning should be done only between late November and mid-March, when adults are inactive. Dead trees are a safety hazard and depending on their size can cost as much as $3,000 to remove.
Eucalyptus trees are not alone in their struggle to survive. Southern California’s chaparral is fighting off a fungus that foresters say will only get worse as the summer progresses. Together with foliage that has died naturally because of frost or drought, these fungus-ravaged areas have created the perfect fuel for rampaging wildfires.
The fungus, which has afflicted more than 500,000 acres throughout Southern California, is called an opportunistic pathogen, because it is striking shrubs already suffering from lack of water. It affects inland mountains more than coastal sage-scrub, foresters say, and appears to especially attack smoggy regions.
“It has a wide host range,” said Fred Brooks, a UC Riverside graduate student who has spent 18 months studying the microorganism, called Botryosphaeria dothidea.
“The fungus doesn’t stop at just native or chaparral plants. It can create problems with avocado, peach and apple trees.”
Many square miles of Orange County are at risk, especially the southern region east of Interstate 5, where chaparral and sage have so far escaped developers’ bulldozers. The broad slopes of the Santa Ana Mountains also contain many vulnerable shrubs and much dead wood.
Although heavy winter rains brought relief to suffering plants, that, too, has a downside.
“The rains caused a huge flush of new growth, up to 8 inches,” Brooks said. “What that means is that the plants need new water to support that growth.”
The rainfall also meant substantial growth of grasses, or “fine fuels.” These grasses are also unusually thick this summer, because the rains made seeds germinate, said Jon Anderson, a hazard reduction supervisor with the Orange County Fire Department.
“Then there’s the frost,” Anderson said. “We had a frost (in December, 1990) that killed a lot of broadleaf vegetation, like the laurel sumac. You can see big clumps of it up Ortega Highway and the Santiago Canyon area.”
Although many of these plants have now resprouted, officials say they still contribute significantly to the “heavy fuel” total in the mountains.
“Because of the fungus and the six years of drought, the vegetation is in its worse shape in three decades,” said Philip Riggan, a forest service ecologist based in Riverside. “It means that fires may carry into the chaparral earlier in the summer than they might otherwise. It also increases the energy of fire when it does burn.”
Chaparral covers 8% of California, and its hundreds of species are known as “live fuel” among firefighters. Since 1981, foresters in the Vegetation Management Program of the Los Angeles County Fire Department have studied the moisture content of Southland plant communities at various times of the year to determine when critically low levels exist.
“Normally, a fire has to drive the moisture out (of the plants) first, which takes heat away from the fire,” said Jim Carter, a watershed forester for the state forestry department in Riverside. “But if the brush is dead, the fire doesn’t have to. Everything becomes fuel.”
The state forestry department is how halfway through a four-year study of managing dry chaparral. For the state fiscal year that began July 1, Riggan and other foresters fear a gubernatorial veto of funds for prescribed burns, which Riggan said are crucial to the research.
