An outbreak of Acanthotomicus sp. beetles has killed more than 10,000 American Sweetgum trees near Shanghai, China. (Photo credit: Lei Gao, et al)

By Meredith Swett Walker

If you’ve ever traveled overseas, you may have had the unfortunate experience of picking up a foreign microbe that made you ill. Humans, though, aren’t alone in their susceptibility to disease or injury when they stray outside their familiar milieu of microbes and hazards. Even trees suffer traveler’s illness—for instance, the American Sweetgum (Liquidambar styraciflua) went to China and picked up a beetle pest.

Meredith Swett Walker

American Sweetgum, a native of North and Central America, was imported into China at the end of the 20th century and became a popular ornamental tree in the eastern and central portions of the country. In its native range, this tree is generally disease and pest-resistant. But, in a study published online today in the Journal of Economic Entomology, Lei Gao, Ph.D., of the Shanghai Academy of Landscape Architecture Science and Planning, along with Chinese and American colleagues, document a mass attack on American Sweetgum by a previously undescribed species of bark beetle. This beetle has led to the deaths of more than 10,000 trees in the vicinity of Shanghai.

Bark beetles are members of the beetle subfamily Scolytinae that feed on, and reproduce in, the inner bark of trees. These beetles carve out a system of tunnels beneath the outer bark that are referred to as galleries. Usually bark beetles only attack aging or weakened trees, but outbreaks of certain species can be devastating to tree populations and the forestry industry. For instance, the current outbreak of the mountain pine bark beetle (Dendroctonus ponderosae) has severely affected 88 million acres of conifer forests in the Western United States and Canada. Some species of bark beetles may transmit microbes that can kill host trees such as the fungus that causes Dutch elm disease.

The American Sweetgum (Liquidambar styraciflua) has become a popular imported ornamental tree in China, but it has become the victim of a newly discovered species of bark beetle in the genus Acanthotomicus. (Photo credit: Katja Schulz, Wikimedia Commons)

The bark beetle that Gao and colleagues found attacking American Sweetgum trees in China is a previously undescribed species in the genus Acanthotomicus. Co-author Rui-Ting Ju, Ph.D., of Fudan University in Shanghai, said that the group is planning on publishing a formal description of Acanthotomicus sp. soon. Voucher specimens of the species have been sent to the Institute of Plant Protection at the Academy of Landscape Architecture Science and Planning, and Fudan University, both of which are in Shanghai, China. The School of Forest Resources and Conservation at the University of Florida and the A. J. Cook Arthropod Research Collection at Michigan State University in the United States have also received voucher specimens.

American Sweetgum appears to be highly susceptible to Acanthotomicus sp. infestations. The species has also been found infesting a handful of Chinese sweetgum trees (Liquidambar formosana). Infestation is relatively easy to diagnose. Acanthotomicus sp. typically attack the tree’s trunk but may also be found beneath bark on branches in some cases. Wounds in the bark caused by beetles will ooze resin abundantly. When the beetles have successfully reproduced, small, circular exit holes of the emerging new adults will be visible in the bark. Another sign of infestation is retention of dead leaves until winter, rather than dropping them in the fall. Acanthotomicus sp. appears to have two or three generations per year and can overwinter in its galleries as a mature larva, pupa, or adult. The researchers have found no evidence that the beetle is associated with a fungal pathogen. While Gao and colleagues plan to research methods to control the beetle, there is no currently no effective management strategy.

Small circular holes in an American Sweetgum tree’s bark indicate that Acanthotomicus sp. beetles have reproduced successfully and emerged from galleries beneath the bark. (Photo credit: Lei Gao, et al)

While American Sweetgum is planted strictly as an ornamental in China, it is an important timber species in the southeastern U.S. It is used for hardwood as well as the production of plywood. Given the poor resistance of American Sweetgum to Acanthotomicus sp., introduction of this pest to North America could be very detrimental to the timber industry. Gao and colleagues say that the discovery of Acanthotomicus sp. strengthens the case for doing “pre-invasion assessments” of potential pests. Nevertheless, Gao and Ju say the risk of Acanthotomicus sp. being accidentally introduced to North America is small. “It is nearly impossible to import [American Sweetgum] to North America from China.” Still they warn that should any sweetgum species, either Chinese or American, be imported from to China to the U.S., it should be carefully quarantined to prevent accidental introduction of Acanthotomicus sp.

Meredith Swett Walker is a former avian endocrinologist who now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. You can read her work on her blogs Pica Hudsonia and The Citizen Biologist or follow her on Twitter at @mswettwalker.