Redbay Ambrosia Beetle in Sassafras & Redbay

Redbay ambrosia beetles (Xyleborus glabratus) reproduce best in wood that’s dead or dying, according to a recent USDA Forest Service study.

“Redbay trees that have just died from laurel wilt are incredibly attractive to redbay ambrosia beetles,” says SRS plant pathologist Stephen Fraedrich. “A redbay tree that has recently died can attract thousands of beetles.”

When a tree dies from laurel wilt, its wood is colonized with the fungus Raffaelea lauricola. Such wood is an exceptional nursery for the redbay ambrosia beetle larvae. Beetle and fungus life cycles are perfectly synchronized into a deadly nutritional symbiosis – the fungus consumes and grows on the wood, the beetle larvae eat a fungal mycelium called ambrosia, and wherever the adult beetles go, they carry the fungus along in specialized mouth-pouches called mycangia.

“Since the fungus cannot travel on its own, focusing on the vector – the beetle – is important when trying to understand the spread of this disease,” says Fraedrich.

In some cases, sanitation cuts, combined with cutting and leaving healthy and infected asymptomatic trees in and around areas with laurel wilt may help keep populations of redbay ambrosia beetles low. The strategy has the most promise for areas where redbay trees occur near susceptible, rare and endangered species, such as pondberry; or susceptible, high-value tree crops such as avocado. Sanitation cutting and “cut and leave” management strategies could also be applicable in forests with sassafras. For landscape trees, fungicides can offer temporary protection.

“But it’s going to be very difficult to control this disease,” says Fraedrich. “One beetle is enough to start an epidemic.”

In a series of experiments, Fraedrich and his colleagues compared beetle brood production in infected and uninfected redbay bolts, or foot-long logs. Their results were published in Forest Science.

The redbay bolts were suspended about three feet in the air for up to 64 days in areas where the redbay ambrosia beetles were present, and bolts were then moved to emergence chambers in a laboratory. Any emerging beetles were collected, identified, and recorded. Three to four months after beetles had ceased to emerge, the scientists cut the bark off the bolts. Without bark, entrance holes where female ambrosia beetles had bored into the sapwood could be found and counted.

The scientists recorded the number of entrance holes and calculated the ratio of beetles emerged to entrance holes (BE/EH). They found that on average only 0.3 to 1.9 beetles emerged from each beetle hole created in uninfected redbay wood. In diseased bolts, this ratio was much higher — with 4.1 to 6.0 beetles emerging from beetle holes, on average.

To seal in moisture, the ends of some bolts were treated with paraffin wax. Beetle emergence was higher from redbay bolts treated this way.

Earlier studies had questioned whether sassafras is a good beetle host. Unfortunately, the new study reports that sassafras trees killed by laurel wilt are also good brood hosts for the redbay ambrosia beetle. The finding helps explain why the disease keeps advancing northward, where sassafras is the only known brood host. Mountain laurel is actually in the heath family and is not thought to be susceptible to the disease. And although northern spicebush is susceptible, it is rarely attacked by the redbay ambrosia beetle.

Fraedrich also contributed to a genetic study that compared two genomes of R. lauricola to genomes of seven related species. Many of the genomes had diverse repertoires of wood-decaying enzymes, but the R. lauricola genomes had a large number of species-specific, secondary metabolite, synthesis clusters.

“Genetically speaking, Raffaelea lauricola is well-equipped as a plant pathogen,” says Fraedrich. “These genetic results could help explain its success in causing disease in members of the laurel family that are indigenous to North America.” The genetic study was published in the journal Fungal Genetics and Biology.

The deadly beetle-fungus duo that causes laurel wilt disease is native to Asia. Since 2002, the disease has killed hundreds millions of trees in the southeastern U.S.

The Georgia Forestry Commission assisted in locating redbay and sassafras trees used for bolts in the study.

Read the full text of the brood production study.

Read the full text of the genetic study.

For more information, email Stephen Fraedrich at stephen.fraedrich@usda.gov.

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