Host range of Xylosandrus mutilatus (Blandford) in North America is reported here for the first time. Descriptive data such as number of attacks per host, size of stems at point of attacks, and height of attacks above ground are presented. Hosts observed in Mississippi were Acer rubrum L., Acer saccharum Marsh., Acer palmatum Thunb., Ostrya virginiana (Mill.) K. Koch., Cornus florida L., Fagus grandifolia Ehrh., Liquidamber styraciflua L., Carya spp., Liriodendron tulipifera L., Melia azedarach L., Pinus taeda L., Prunus serotina Ehrh., Prunus americana Marsh., Ulmus alata Michaux, and Vitus rotundifolia Michaux. Liquidamber styraciflua had significantly more successful attacks, significantly higher probability of attacks, and significantly higher number of adult beetles per host tree than did Carya spp., A. rubrum, and L. tulipifera. This information is relevant in determining the impact this exotic beetle may have in nurseries, urban areas, and other forestry systems where this beetle becomes established.
Exotic bark and ambrosia beetles are becoming established in increasing numbers in North America due to greater foreign trade and human movement (Rabaglia, unpublished data; Schiefer & Bright 2004). Approximately 4600 arthropod species (2582 species in Hawaii and 2000 in the continental US) have been introduced with approximately 95% of these introduced accidentally in association with plants, soil, or water ballast from ships (Pimentel et al. 2001). Some 360 nonindigenous insects have invaded and become established in U.S. forests with 30% of these causing $2.1 billion in damage per year (Pimentel et al. 2000).
Few bark beetles are considered aggressive species (Paine et al. 1997). Of the 500 species that have been described from North America, only 2% kill trees (Six 2003). Exotic bark and ambrosia beetles introduced into new localities may impact forest ecosystems. Ambrosia beetles fulfill an important role in the forest by initiating and promoting the breakdown of wood (Lindgren 1990).
Xyleborini is one of the largest tribes of insects, and they attack hundreds of species of woody plants worldwide (Norris 1979). Ambrosia beetles are polyphagous and have a wide host range (Beaver 1989). Five of the described species of Xylosandrus occur in North America and only X. zimmermanni (Hopkins) is native. The remaining Xylosandrus species, X. compactus (Eichhoff), X. germanus (Blandford), X. crassiusculus (Motschulsky), and X. mutilatus (Blandford), were introduced from the Old World tropics (Wood & Bright 1992; Schiefer & Bright 2004). Some ambrosia beetles can attack healthy vigorous trees and shrubs while others attack only dead or dying hosts (Roeper & French 1981; Dute et al. 2002).
Xylosandrus mutilatus was first discovered in the U.S. in Oktibbeha County, Mississippi in 1999 (Schiefer & Bright 2004). Its present distribution in the USA has been reported (Schiefer & Bright 2004; Cognato et al. 2006; Stone & Nebeker 2007). Native of Asia, X. mutilatus is one of the major pests attacking the trunk and branches of Chinese chestnut, Castanea mollissima Blume (Tang 2000). The host plants of X. mutilatus in Asia have been reported previously (Wood & Bright 1992). Several publications have recorded X. mutilatus attacking Aceraceae: Acer sieboldianum Miquel, Fagaceae: Castanea mollissima, Lauraceae: Parabenzoin trilobum (Sieb. & Zucc.) Nakai, and Proteaceae: Grevillea robusta A. Cunn. (Sreedharan et al. 1991; Kajimura & Hijii 1992, 1994; Tang 2000).
Studies on the biology and ecology of exotic bark beetles are needed to allow appropriate management recommendations to be made to the public. Schiefer & Bright (2004) stated that the range of host plants of X. mutilatus are unknown in North America. The objective of this study was to determine the host range of X. mutilatus in Mississippi.
Materials and Methods
Xylosandrus mutilatus was offered various potential host plants common to the southeastern United States. Two upland hardwood forests, Tombigbee National Forest, and a privately owned forest (Kimbrough property), were selected in Winston County, Mississippi. The initial field design used bolts (1 m in length) placed horizontally on the ground in three diameter classes (2.54 cm ± 1.27, 5.08 cm ± 1.27, and 10.16 cm ± 1.27). Having observed no X. mutilatus attacks on these horizontal hosts from Mar-Aug 2004, we modified the field design. On Aug 16, 2004, single saplings including their canopy (ground-line diameter 2.54 to 5.08 cm) in 9 species were selected and severed at ground-line: loblolly pine (Pinus taeda L.), hickory (Carya spp.), yellow poplar (Liriodendron tulipifera L.), sweetgum (Liquidambar styraciflua L.), flowering dogwood (Cornus florida L.), red maple (Acer rubrum L.), chinaberry (Melia azedarach L.), black cherry (Prunus serotina Ehrh.), and winged elm (Ulmus alata Michaux). At each location, 3 vertical, severed saplings of each species were randomly placed along a continuous transect and spaced approximately 3 m apart. They were leaning 60 to 80 degrees from the ground and supported by other living understory saplings. The severed saplings were inspected at approximately 10-d intervals for 50 consecutive d (Aug 16 to Oct 5, 2004). For each inspection period, attacks were tallied and a small colored dot was placed adjacent to each new attack. An initial attack in this study was recorded when a female created an entrance hole that penetrated the xylem. Different colored markers were used to signify the inspection period during which the attacks had occurred.
At the end of the observation period the following data (mean ± SE) were collected: (1) number of attacks per host plant, (2) diameter of stem at attack, and (3) attack height above ground-line. Proper identification concerning what beetle made the attack was confirmed by the presence of a male or female X. mutilatus and/or its unique gallery pattern as illustrated by several authors (Choo et al. 1988; Kajimura & Hijii 1994; Tang 2000).
From Apr to Oct 2005, the vertical presentation of severed saplings was replicated three times (Apr 15 to Jun 15, Jun 16 to Aug 16, Aug 17 to Oct 17), and sample size was increased from 3 to 9 saplings per species at each location. Quercus spp. replaced chinaberry due to the lack of availability of the latter. Severed saplings were inspected at approximately 10-d intervals for 60 consecutive d. At the end of each inspection period, the same data were obtained as for the fall 2004.
To determine possible host preferences, hickory, red maple, sweetgum, and yellow poplar were chosen and analyzed by the GLIMMIX procedure. Other species had few attacks and were therefore omitted. The mean number of attacks per host, the probability of a sapling for a given host being attacked at least once, and mean number of X. mutilatus adults per tree during the 2004 and 2005 fall replications were compared for significant differences among tree species. The data were analyzed with location and inspection periods as random effects by the generalized linear mixed model (PROC GLIMMIX) (α = 0.05) in SAS 9.1 version.
Xylosandrus mutilatus attacked saplings in 12 plant families in Mississippi and Alabama (Table 1). Zero attacks occurred on the horizontal bolts. The vertical severed saplings were attacked at both locations during each replication. The percentage of hosts attacked and number of attacks per host were higher during the fall than spring or summer (Table 2). For all replications, the highest numbers of attacks occurred during the first 10 d after severing (Table 2). A higher proportion of successful attacks occurred during both fall replications (Table 3). Unsuccessful attacks had a higher mean stem diameter at point of attack (2.0 cm ± 0.061) than successful attacks (1.81 cm ± 0.045). Successful attacks had a higher mean height at attack (2.171 m ± 0.063) than unsuccessful attacks (1.889 m ± 0.062).
The mean number of successful attacks per host tree, the probability of a sapling of a given host being attacked at least once, and mean number of X. mutilatus adults per host tree during the 2004 and 2005 fall replications among hickory, red maple, sweetgum, and yellow poplar were all significantly different (Table 4). Sweetgum had significantly more successful attacks, significantly higher probability of attack, and significantly higher number of adult beetles per host tree compared with hickory, red maple, and yellow poplar (Table 4). Sweetgum had a total of 114 successful attacks and 65 unsuccessful attacks compared with hickory (19 successful and 26 unsuccessful), red maple (6 successful and 38 unsuccessful) and yellow poplar (4 successful and 2 unsuccessful) during the 2004 and 2005 fall replications.
Including the findings in this paper, 20 host plant families have been recorded for X. mutilatus worldwide. This is the first report of Hamamelidaceae, Magnoliaceae, Pinaceae, Rosaceae, Ulmaceae, and Vitaceae as host families for X. mutilatus.
Several live host plants were observed being attacked. An unhealthy sugar maple (Acer saccharum Marsh) possessing crown dieback was attacked. Attacks were located within the dieback, and X. mutilatus did not attack any other portion of the tree. Attacked eastern hophornbeam (Ostrya virginiana (Mill.) K. Koch.) was observed on a recently herbicide sprayed right-of-way. Attacks occurred on a dying lower branch apparently damaged by the herbicide. A flowering dogwood, observed under attack and planted in a poorly drained area, had approximately 50% leaf drop in Jun 2004. The attacks occurred along the main stem. Potted, healthy Quercus shumardii Buckl. approximately 3 m tall located in a nursery in Alabama were attacked (J. W. Brewer, pers. comm.). Xylosandrus mutilatus attacked recently stressed and dead understory saplings associated with a prescribed burn. These host plants consisted of sweetgum, hickory spp., winged elm, and wild muscadine grape.
Xylosandrus mutilatus, like most ambrosia beetles, is polyphagous and reproduces in numerous hosts (Beaver 1989; Lindgren 1990; Maeto et al. 1999), and it appears to have low host specificity. The ambrosia beetle’s historical environment likely promoted polyphagy (Atkinson & Equihua-Martinez 1986; Beaver 1989). The floristic diversity and high decomposition rates made high host specificity less favorable in the humid tropics and similar climates where ambrosia beetles likely evolved (Atkinson & Equihua-Martinez 1986; Beaver 1989). Atkinson & Equihua-Martinez (1986) suggest that the beetles’ fungal symbiosis evolved from competition between the beetles and fungi for the same substrate.
The single stem presentation in this study was a smaller landscape disturbance as compared with man-made disturbances such as herbicide spraying and prescribed burning. From personal observations, recently dead and stressed material following a prescribed burn or herbicide application seemed to be preferred during the spring and summer periods. It has been reported that Xylosandrus crassiusculus prefers disturbed areas (Maeto et al. 1999).
No successful attacks occurred below 0.5 m, which might suggest a lower limit on attack height for vertically-oriented material. Tang (2000) stated that the attacks occurred at 40 cm or greater above the ground-line and on parts of the branch having a diameter of 1.2 to 2.5 cm. Maeto et al. (1999) reported that more than 70% of X. crassiusculus were trapped below 5 m.
In our study, only 3 out of 20 attacks were successful when stem diameter at point of attack was 3 cm or greater. This suggests an upper limit on suitable stem size. Beeson (1930) divided ambrosia beetles into 3 groups in relation to the size of the material the beetles attacked. Xylosandrus mutilatus has been classified as a shoot borer, since it may infest material up to 5.08 cm or more in diameter (Browne 1958). Browne (1958) recorded Xylosandrus (=Xyleborus) mutilatus in hosts with diameters of 1.27 to 5.08 cm; however, most attacks occurred in the 2.54 to 3.81 cm diameter classes.
Xylosandrus mutilatus showed a preference towards sweetgum when compared to the other host plants represented in this study. Even though most ambrosia beetles have a wide host range, exceptions to this general rule do occur (Browne 1958). Xyleborus uniseriatus Eggers shows a strong preference for the Fagaceae (Browne 1958). Since the introduction of X. crassiusculus into North America, this species has shown some preference towards sweetgum (Solomon 1995). Further studies to identify the factors involving recognition, suitability, and acceptance of host material by X. mutilatus in natural and man-made disturbances are needed.
The authors thank David Kimbrough for the use of his land and Kris Kovar with the Tombigbee National Forest for granting access to the National Forest. We thank Richard Baird, Gerald Baker, and Clarence Collison of the Department of Entomology and Plant Pathology, Mississippi State University, and Brian Sullivan of the Southern Research Station, Forest Insect Research, USDA Forest Service, for reviews and suggestions. We are grateful to Lamar Kimbrough, Keith Beatty, Tim Menzel, Julie Dobbs, Sarah Self, David Menough, Jeremy Norris, and Michael Estes for assistance in the field and Gensheng Shi for translating some articles. This project was funded in part by the U.S. Forest Service (Contract SRS 03-CA-11330129-222) and the Mississippi Agricultural and Forestry Experiment Station. Approved for publication as J-10948 of the Mississippi Agricultural and Forestry Experiment Station, Mississippi State University.
Host plants of Xylosandrus mutilatus in Mississippi and Alabama.
Number of attacked severed saplings and total attacks by inspection day of Xylosandrus mutilatus.
Total number and status of attacks of Xylosandrus mutilatus on severed saplings.
Host preferences of attacks of Xylosandrus mutilatus on severed saplings during fall 2004 and 2005.