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One can be the deadliest number when it comes to invasive species

Eastern Forest Environmental Threat Assessment Center
September 19, 2017 at 1:15pm

A picture of an Ambrosia Beetle, very small insect, among some sawdust.

Sawdust surrounds a female redbay ambrosia beetle near her entry hole in a tree. The fungus she carries spreads and chokes off the movement of water, leading to death of the tree in weeks or months.

When the redbay ambrosia beetle, native to Asia, was first detected in coastal Georgia in 2002, it didn’t set off any alarm bells. 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 because it causes laurel wilt, a swift killer that has spread to nine southeastern states since it was identified in 2004.

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

But how did the laurel wilt epidemic start, and why does that matter so much now?

University and Forest Service Southern Research Station scientists provide some hindsight in a study that was recently published in the journal 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.

A picture of a tree that has been infected by the Ambrosia Bettle, showing brown and dead leaves on the tree.

A worker fells dead redbay trees at the edge of a parking lot in Florida.

Between 2000 and 2012, the researchers collected 14 female beetles in baited traps placed 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. They then extracted and sequenced DNA from the beetles and fungus to compare the genetic building blocks among different samples. Their tests revealed that all the beetle samples were genetically the same, indicating they shared a common ancestor.

Similar to some other ambrosia beetles introduced to the U.S., redbay ambrosia beetles are considered extreme inbreeders. In fact, just one female holds great potential to start an invasion. She can produce wingless male offspring by cloning herself and female offspring sexually. The male offspring can mate with their sisters, who then fly to a new host tree to start their own families.

Researchers also found that 95 percent of the fungal samples had identical genes, further supporting their hypothesis that a single beetle was all it took to set off the laurel wilt epidemic in the United States.

Researchers and managers can often recognize potential threats from non-native 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 the fungi and may have developed disease resistance.

However, in the U.S., laurel wilt epidemic took many by surprise and it underscores the urgent need for research that can inform prediction, prevention, and management efforts.

A map that shows the Southeastern part of the U.S. that has been impacted by Laurel Wilt Mortality.

Researchers used Forest Inventory and Analysis data to estimate the impact of laurel wilt. A map shows the percentage of redbay already killed by laurel wilt in the southeastern United States.

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