The redbay ambrosia beetle Xyleborus glabratus is one of the most destructive invasive insect species in North America. The beetle is associated with a symbiotic fungus Raffaelea lauricola T.C. Harr., Fraedrich & Aghayeva (Ophiostomatales: Ophiostomataceae) which is extremely pathogenic to North American trees in the family Laura-ceae (Fraedrich et al. 2008) and causes a disease known as laurel wilt. Since the introduction of this symbiotic couple in the early 2000's, laurel wilt killed millions of trees, mostly in the genus Persea, throughout most of southeastern USA (J. A. Smith, pers. comm.).
One of the important unknown aspects of the biology of this invasive species is its conspicuous specificity to Lauraceae. Most ambrosia species are broad host generalists (Hulcr et al. 2007). Current data on the original host specificity of X. glabratus axe ambiguous. Published records of tree species from which the beetle was collected includes mostly Lauraceae, but also occasionally unrelated plant families (Wood & Bright 1992; Rabaglia et al. 2006), and it is not possible to confirm the accuracy of many of these records. The current hypotheses can be summarized as follows:
H0: The beetle is an unusual ambrosia species even in its native region, specialized on Lauraceae.
H1: As most ambrosia beetle species, the redbay ambrosia beetle is a host generalist in its native region. The specificity to Lauraceae in North America is a rapid evolutionary-ecological change.
Determining the correct hypothesis is important for many reasons. For example, knowing the original host specificity of the beetle may explain its unusual semiochemical ecology (Kendra et al. 2011). Importantly, it may also help us predict whether the symbiotic couple requires Lauraceae for its survival in invaded ecosystems, or if it is likely to persist in other tree families when Lauraceae are eliminated.
Here we present previously unpublished host records of Xyleborus glabratus deposited in the National Zoological Museum of China, Institute of Zoology, Chinese Academy of Sciences, Beijing. These are the first published records of this beetle occurring on mainland China. Identity of all specimens identified as X. glabratus was checked for correctness. Chinese host tree names were associated with scientific names according to Chang (1998), Cheng & Fu (1978) and Li (1982).
The collection at the Institute of Zoology in Beijing contains 45 specimens of Xyleborus glabratus, mostly collected by Dr. Fu-sheng Huang and identified by Dr. Hui-fen Yin. This is more than the number of records from the rest of Asia by Wood & Bright (1992). Specimens originated in the Sichuan, Hunan and Fujian provinces. This collection displays greater morphological variation than the American non-native population, particularly in terms of beetle size, but the larger or smaller beetles are not confined to specific host species or locations. The majority of specimens were collected from trees in the family Lauraceae (Table 1).
The host list indicates that Xyleborus glabratus is not a strict specialist to trees from the family Lauraceae, but it strongly prefers them. While this is an unusual behavior for an ambrosia beetle in the tribe Xyleborini, there are several other xyleborines for which preference for a particular host group has been reported (Kirkendall 2006; Hulcr et al. 2007). This also confirms that the apparent bias to Lauraceae in the host tree summary in Wood & Bright (1992) is correct, and it explains the observation of Dr. J. Peña (UF Tropical Research and Education Center, pers. comm.) who found X. glabratus in Taiwan only in Cinnamomum osmophloeum Kanehira. The 2 collections from non-lauraceous trees may represent uncommon behavior of the species.
The CAS collection labels do not contain any information on whether the trees were killed by the beetle. However, no records of this behavior in Southeast Asia have ever been reported. Healthy populations of the host tree species reported here occur in the areas where the X. glabratus specimens were collected, thus the authors doubt that X. glabratus displays tree-killing behavior in its native range.
Hosts trees of Xyleboeus glabeatus specimens in the National Zoological Museum of China.
The records explain the strong attraction of X. glabratus to phoebe oil, extracted from trees of the genus Phoebe (Kendra et al. 2011) and to many North American Lauraceae (Hanula et al. 2008). It also appears that occasionally the beetle is capable of colonizing non-lauraceous hosts, and it remains unknown whether or not it may eventually disappear from American ecosystems when all lauraceous of suitable size hosts have been eliminated. The origin of several X. glabratus from a pine tree (Table 1) is unclear, it may be an unusual or erratic behavior of those particular specimens, or an erroneous record.
We thank Drs. Jiang-hua Sun, Min Lu and Ji-an Yao for facilitating access to the collection. JH was funded by the University of Florida Emerging Pathogens Institute, the USDA Forest Service, and the National Science Foundation.
The redbay ambrosia beetle Xyleborus glabratus is unusual among ambrosia beetles because of its host specificity to Lauraceae, but it isn't clear whether this is only a feature of the invasive American population. Our examination of the extensive collection in the Chinese Academy of Sciences suggests that Xyleborus glabratus strongly prefers Lauraceae also in its native Asia. These are also the first published records of the species from continental China, which highlights the value of entomological collections.
Key Words: host specificity, laurel wilt, Persea, symbiotic fungus, Theaceae, tree-killing behavior
El escarabajo ambrosia Xyleborus glabratus es una especie insólita entre los escarabajos ambrosia, debido a su especificidad de hospede-ros de la familia Lauraceae, pero no está claro si esto es sólo una característica de esta pobla-ción invasiva en los EE. UU. Nuestro examen de la extensa colección de la Academia de Cien-cias de China sugiere que Xyleborus glabratus prefiere fuertemente hospederos de la familia Lauraceae también en su nativa Asia. Estos son también los primeros registres publicados de las especies en China continental, lo que des-taca el valor de las colecciones entomológicas.
Palabras Clave: especificidad de hospedero, mar-chitez del laurel, Persea, hongo simbiótico, Theaceae, comportamiento de matar árboles