Invasive Insects Put San Diego Ecosystems In Peril
In the serenity of the San Diego County mountains, workers armed with chainsaws are in a race against an invasive creature on a path of destruction.
The goldspotted oak borer beetle has killed 80,000 oak trees in the county over the last decade -- the Cleveland National Forest has taken the brunt of the massive die-off.
"This is probably the worst invasive insect problem these oak trees have ever experienced. And as we can see with the tens of thousands of dead oak trees, the problem is only going to get worse over time," said Mark Hoddle, extension specialist in biological control at UC Riverside.
How The Oak Borer Devours A Forest
The trees are ultimately killed after the beetles burrow their eggs in the underside of the bark and the hatched larvae eat away at the tree.
The oak borer lives for one year, but it’s capable of producing more than 100 eggs, with 70 to 80 percent hatching and producing larvae, said Hoddle.
"So when you have hundreds, or sometimes thousands of these beetle larvae feeding in these trees that are dying around us, they’re essentially cutting the tubes that move the nutrients up and down the tree," said Hoddle, as he pulled inch-long white larvae out of a tree stump in the Cleveland National Forest.
The tree ends up ultimately starving because they don't get food or water. Infested trees typically die within three to five years –- a rapid mortality for 150-year-old oaks.
When the goldspotted oak borer was first discovered in 2004 in San Diego County, researchers had no idea it would cause such catastrophic problems, said Hoddle.
The pest likely came here in a bundle of firewood from its native home in Arizona, where it's hardly visible because its predators keep it in check.
"And here in California where the climate’s great, there’s tens of thousands of oak trees for it to attack with no natural enemies -- boom, the populations have exploded and we’re seeing the consequences of that now," explained Hoddle.
Roger Covalt is supervising park ranger at William Heise County Park near Julian, the epicenter of the invasion. His crew has cut down 800 dead trees so far with hundreds more to go.
What You Can Do
Don't risk starting a new infestation of an invasive insect or disease.
- Buy firewood near where you will burn it
- Wood that looks clean and healthy can still have tiny insect eggs, or microscopic fungi spores, that will start a new and deadly infestation. Always leave it at home, even if you think the firewood looks fine.
- Aged or seasoned wood is still not safe. Just because it is dry doesn't mean that bugs can't crawl onto it!
- Tell your friends not to bring wood with them- everyone needs to know that they should not move firewood.
"The entrance drainage used to have a very full canopy of oak. Now most of the trees are down and the light shines through," said Covalt. "It’s kind of sad. We’re losing this canopy and campers miss it."
The onslaught of dead trees has created hazardous conditions for park visitors and campers. Trails and campgrounds are often closed because of unstable trees.
"Safety is a big issue," Covalt said. "They die, they fall, and we don’t want them to hurt the public. You also have an increased fire danger with the dried out tree and it’s more likely to burn if a fire comes."
Covalt is urging the public to help contain the beetle.
"The big thing is, don’t move the firewood outside of the area that’s infested," he said. "That’s a big campaign we’re doing. We have posters, we’re giving out playing cards showing the insects that are killing our trees.
Ecosystem In Peril
Thousands of acres of oak forests in San Diego County face an ecosystem collapse if the oak borer can’t be contained. The oaks provide an essential habitat for many species of plants and animals.
"Especially birds, like our acorn woodpeckers, which collect the acorns and cash them in trees. Deer feed on acorns during the winter because it’s a rich food source for them. And areas that are shaded by these oak trees often hold water and keep these areas damp, and in those areas we find unique plant and animal species that can only live in these moist areas underneath the shade of these giant oak trees."
As the oaks die out, they will be replaced by other, smaller oak species that are not susceptible to the beetle, he said.
"We’ll also see as we look around here, other species of trees which have not been able to compete successfully with the oak trees will now come to dominate these forests as well. So we’ll probably see a transition in the species composition in these trees over time."
Invasive Insects On The Rise
Invasive species are an accelerating problem in Southern California. Exotic insects become established in the region an average of every 40 days.
Hoddle blames the rise in invasive bugs on tourism and increasing trade.
"The more people move themselves around and we have more trade coming into and out of California, we increase the likelihood of these unwanted bugs coming into California," Hoddle said.
Hunting For A Predator
Hoddle is on a mission to track down the goldspotted oak borer’s enemy before it decimates oak forests all the way to Oregon and beyond. He recently tracked down the beetle's genetic finger print in the mountain ranges of southern Arizona.
"We did studies like you see on the crime shows like CSI, where you’ve got the genetic fingerprints of these different populations and then compared those genetic finger prints to the genetic fingerprint to the population that’s established here in southern California. And like CSI, we got a match," explained Hoddle.
Now, he's on the hunt for the parasite that attacks the beetle’s eggs and larvae, such as tiny little wasps that lay their eggs inside of the egg of the goldspotted oak borer. "When those parasite eggs hatch, the tiny little wasp larvae, which looks like a fly maggot, starts feeding on the inside of the goldspotted oak borer eggs killing it."
Predator surveys are underway in southern Arizona, but the process of releasing a parasite into San Diego County will take approximately five more years.
Hoddle worries that in five years there won't be any oak trees left to save.