One is the Deadliest Number

A single pest introduction led to widespread redbay mortality

dead redbay trees
Redbay trees in a Florida forest have succumbed to laurel wilt. Photo by Bud Mayfield, U.S. Forest Service, Bugwood.org.

When the redbay ambrosia beetle, native to Asia, was first detected in coastal Georgia in 2002, it didn’t set off any alarm bells. “Ambrosia beetles rarely damage healthy trees, so this find was initially considered unimportant. But the subsequent epidemic of laurel wilt in the Southeast has caused a re-examination of the significance of these insects in forest health,” says Frank Koch, a research ecologist with the U.S. Forest Service Eastern Forest Environmental Threat Assessment Center.

All ambrosia beetles carry fungi that serve as a food source for adults and larvae living in tunnels under the bark of host trees and shrubs. Usually, these fungi aren’t harmful, but the Raffaelea lauricola fungus carried by the redbay ambrosia beetle is one major exception. It causes laurel wilt, a swift killer that has spread to nine states since it was identified in 2004.

As many as 300 million redbay trees—a third of the population—have already died, according to Forest Inventory and Analysis data. Redbay is just one of 14 susceptible host species in the laurel family, which also includes avocado and sassafras, among others.

But how did the epidemic start, and why does that matter so much now? Koch is among university and Forest Service researchers who provide some hindsight in a study that was recently published in Biological Invasions. They hypothesized that the tiny beetle and its companion fungus began their rampage after a single introduction, likely near the Port of Savannah, and their results support this.

Between 2000 and 2012, researchers collected 14 female beetles in baited traps places in infested redbay stands in Florida, Georgia, Louisiana, and South Carolina. They also collected 57 fungal samples in these same states between 2004 and 2010. Next, they extracted and sequenced DNA from the beetles and fungus to compare the genetic building blocks among different samples.

fungal staining
Redbay stem with vascular staining from R. lauricola. The fungus chokes off the movement of water. Affected trees wilt and die in weeks or months. Photo by Bud Mayfield, U.S. Forest Service, Bugwood.org.

All the beetle samples were genetically the same, indicating they shared a common ancestor. Similar to other ambrosia beetles introduced to the U.S., one female holds great potential to start an invasion.

“Some ambrosia beetles, including the redbay ambrosia beetle, are extreme inbreeders,” explains Koch. A female can produce male offspring by cloning herself, known as parthenogenesis. She can also produce female offspring sexually. “To establish viable colonies, females can produce a few dwarfed and wingless males that rarely leave their brood chamber. They can mate with their sisters, who then fly to a new host to start their own families.”

Researchers also found that 95% of the fungal samples had identical genes, further supporting their hypothesis that a single introduction of a redbay ambrosia beetle carrying the R. lauricola fungus was all it took to set off the laurel wilt epidemic in the United States.

Another recently published study, co-authored by Southern Research Station research plant pathologist Stephen Fraedrich, reports similar findings for 125 R. lauricola samples from the United States. These researchers also tested samples from the fungus’ native range in Japan and Taiwan and found these to have high genetic diversity, suggesting a form of sexual reproduction not found in the U.S. samples. The authors stress the importance of preventing a second introduction of the fungus in the United States, which could allow the two fungal strains to mix, potentially creating a more aggressive disease.

“The ecological impacts of laurel wilt are arguably among the most spectacular ever recorded for a clonal pest and an associated pathogen. This is the only example of which we’re aware where an insect-fungus symbiosis has resulted in such major and extensive impacts in a relatively short time period,” says Koch. And the epidemic comes on the heels of a number of devastating forest pests and pathogens creating ripple effects throughout ecosystems and robbing present and future generations of ecological and economic benefits.

redbay ambrosia beetle
Sawdust surrounds a female redbay ambrosia beetle near its entry hole in a tree. Photo by Stephen Ausmus, USDA, courtesy of Flickr.

Researchers and managers can often recognize potential threats from invasive species by observing impacts in their native ranges. Interestingly, laurel wilt has not been identified in Asia, perhaps because host trees and shrubs have co-evolved with the redbay ambrosia beetle and R. lauricola and may have developed disease resistance. Though the U.S. laurel wilt epidemic took many by surprise, it underscores the urgent need for research that can inform prediction, prevention, and management efforts.

“For vectors of pathogens, especially insects that transmit fungal pathogens of trees, little is known about how their reproductive strategies and genetic diversity affect their ability to invade new ecosystems,” says Koch. “Genetic information studies are becoming increasingly useful for examining the establishment and spread of invasive diseases and pests.”

Read the full text of the article.

For more information, email Frank Koch at fhkoch@fs.fed.us.

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