How Cold Is Too Cold for Redbay Ambrosia Beetles?

Warmer winters put sassafras at greater risk

sassafras
Sassafras is valuable to humans and wildlife and is a host to larvae of several butterfly species. Photo by Pennsylvania Department of Conservation and Natural Resources, courtesy of Bugwood.org.

Nonnative redbay ambrosia beetles and the fungus they carry have killed hundreds of millions of trees in the Southeast since first detected in 2002. Currently, only cold temperatures limit the beetles’ establishment and spread in the eastern United States.

A study led by Mississippi State University involving U.S. Forest Service researchers determined the coldest temperatures the beetles can tolerate and predicted where the beetles could move and survive now and in the future with warming winters.

“When the beetles bore into a tree, their companion fungus causes laurel wilt disease, which threatens the entire Lauraceae family of plants in North America,” says Frank Koch, a research ecologist with the SRS Eastern Threat Center and one of the study’s coauthors.

The laurel wilt epidemic has spread rapidly, with the greatest impacts so far seen in natural redbay and swampbay populations as well as agricultural avocado groves. “The invasion of redbay ambrosia beetles and laurel wilt was initially considered to be only a problem for coastal forests of the Southeast — not anymore,” says Koch.

Laurel wilt can kill healthy trees in just four weeks and is lethal to sassafras trees. In the past few years, sassafras across Alabama, Arkansas, Louisiana, and Mississippi have succumbed to laurel wilt. This recent development has been particularly alarming to scientists and land managers because the range of sassafras covers the eastern U.S. and extends into Canada.

redbay ambrosia beetle
Tiny redbay ambrosia beetles and their companion fungus cause laurel wilt disease and can kill healthy trees in as few as four weeks. Photo by Stephen Ausmus, USDA Agricultural Research Service.

For this study, published in Biological Invasions, researchers conducted laboratory tests of adult redbay ambrosia beetles. When subjected to freezing temperatures, some beetles experienced mortality or injury at -5 degrees C (23 degrees F), but all beetles died at -10 degrees C (14 degrees F).

The researchers then performed a field test and found that, when inside a sassafras tree that provides a buffer against winter’s cold, beetles could survive until the temperature dipped to around -11.2 degrees C (about 11.8 degrees F).

To predict where the beetles could potentially spread, researchers combined climate data from the past 30 years, climate projections for the year 2050 that assumed an average temperature increase of 1.4 degrees C (about 2.5 degrees F), and maps of the current range of sassafras. Results suggest that nearly all sassafras populations are at risk of invasion.

maps
Sassafras range and the invasion potential of redbay ambrosia beetles under current (top) and future (bottom) climate conditions. Most sassafras will be at risk as beetles spread north and survive warmer winters.

“Our models show that redbay ambrosia beetles could become established now in more than 99 percent of present sassafras populations. Just over half of present populations are located in areas where winters are typically cold enough to cause at least some beetle mortality,” says Koch.

“In a future with higher winter minimum temperatures, more than 90 percent of the land area with sassafras populations would not experience winter temperatures low enough to kill redbay ambrosia beetles.”

Insecticides and fungicides have not been able to control the spread of redbay ambrosia beetles and laurel wilt. Unless viable control options are found, cold winters remain the only factor that keeps the epidemic confined to sassafras populations in the Southeast.

Results from this study highlight the need for continued monitoring in anticipation of more widespread laurel wilt mortality in a changing climate.

Read the full text of the article.

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

Access the latest publications by SRS scientists.

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