Camphor Shoot Borer; Photo by David Almquist on Bugguide.org, used with permission.

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Scientific name: Cnestus mutilatus (Blandford 1894)
Order: Coleoptera (beetles)
Family: Curculionidae (Weevils and bark and ambrosia beetles)

Summary

Camphor shoot borers are an introduced species of ambrosia beetle that attack small-diameter branches and trunks of stressed, weakened, and dying trees. They also accidentally bore holes into plastic gasoline containers, hoses, and tanks.

Description

Camphor shoot borers are small* (2.6–3.9 mm), robust, dark brown beetles. They are easily distinguished from other beetles by their cylindrical body and the shape of the abdomen, which is shorter than the head and thorax and looks as if it were cut off at an angle or squished (Figure 1). (*Camphor shoot beetles are small compared to many insects, but are the largest ambrosia beetles in North America).

Four photos of Camphor shoot borer — Figure 1. Camphor shoot borer. Figures 1a and 1b by Michael C. Thomas, Florida Department of Agriculture and Consumer Services on Bugwood.org, used under a CC BY 3.0 US license. Figures 1c and 1d by David Almquist on Bugguide.org, used with permission.

Distribution

Camphor shoot borers are native to Asia, where they are widespread. They were accidentally introduced into North America and first discovered in Mississippi in 1999. Camphor shoot borers have since spread throughout the eastern United States, including southern Pennsylvania, where they were found in 2013 (Figure 2). It is unclear if they have reached their northern limit or if they will continue to spread throughout Pennsylvania and further north.

USA and Pennsylvania maps showing locations of camphor shoot borer reports — Figure 2. States and Pennsylvania counties camphor shoot borer has been reported from, with year of first discovery indicated. Created by Michael Skvarla, Penn State. Map and dates based on information provided by Schiefer and Bright (2004), Cognato et al. (2006), Stone and Nebeker (2007), Stone et al. (2007), Gandhi et al. (2009), Carlton and Bayless (2011), Oliver et al. (2012), Leavengood (2013), Barringer (2016), and NAPIS (2009–2018). Additional unpublished county records are from the PA Department of Agriculture and specimens accessioned in the Frost Entomological Museum.

Life History and Behavior

Camphor shoot borers are polyphagous ambrosia beetles that attack many species of hardwood trees, shrubs, and vines, although sweetgum appears to be a preferred host. They have a wide host range because, like other ambrosia beetles, they inoculate trees with a fungus and feed on the fungus rather than the tree itself (Figure 3).

Figure 3. Cross-section of a camphor shoot borer gallery showing eggs and white-colored ambrosia fungus. Photograph by Joshua Basham, in Oliver et al. (2012). Used with permission.

Unlike other introduced bark and ambrosia beetles, camphor shoot borers only colonize stressed, weakened, or dying plants. They are attracted to such plants in part by ethanol, which is produced by bacteria in stressed and dying plants. Beetle attacks have been associated with plant stress caused by over- and under-watering, including natural flood and drought; poorly drained soil and tree species planted in unsuitable habitat; physical bole and branch damage; herbicide damage; recent transplantation; fire damage during a controlled burn; and, in nursery situations, inadequately managed container production, including trees grown in substrate without enough airspace. Freeze stress caused by mild winter temperatures followed by a late-spring freeze predisposes some trees to attack by other bark and ambrosia beetles and may allow successful camphor shoot borer attacks as well.

Camphor shoot borers preferentially colonize stems 2.5–3.8 cm in diameter and do not attack stems smaller than 1.2 cm or larger than 5 cm in diameter. They will colonize both branches and boles within this size range. Additionally, successful attacks begin at least 0.5 m above the ground.

In the southern United States, camphor shoot borer adults have been collected in every month from March through December, with peak emergence in April and May. In Pennsylvania, adults have been collected from May through early October; while their phenology in the state has not been examined specifically, it is reasonable to expect that peak emergence is in May and perhaps June.

Only female camphor shoot borers colonize new hosts and are consequently the sex most often seen and collected. This is because male beetles do not leave the gallery they mature in, but rather mate with their sisters after they emerge, before they disperse to new hosts.

Signs and Symptoms

Camphor shoot borers cause signs and symptoms similar to other bark and ambrosia beetles, including leaf wilting, branch die back, and tree death. Camphor shoot borer attacks may be distinguished from those of other species by the relatively large entrance and exit holes the beetles create, which are approximately 2 mm (5/64 inch) in diameter and can be measured by inserting an appropriately sized drill bit. If the infested bole or branch is split lengthwise, a distinctive t- or cross-shaped gallery with long arms that follow the grain of the stem can be observed (Figure 4); if the attack is ongoing, an adult female beetle may be found in the gallery to confirm the identification.

Figure 4. Camphor shoot borer gallery. Photograph Doug Stone, Mississippi State University, on Bugwood.org. Used under a CC BY-NC 3.0 US license.

Economic Damage

Because camphor shoot borers attack relatively small diameter stems compared to their body size, the colonized stem is often killed. If the bole is colonized, this can kill the tree above the attack and perhaps kill the entire tree, which eliminates or severely reduces the economic value of the tree. Larger trees in which individual branches are colonized can survive an attack, but may be reduced in value depending on how the killed branch(es) affect the aesthetics of the tree.

Modern gasoline contains up to 10% ethanol, which camphor shoot borers mistake for a stressed tree, such that the beetles have been known to accidentally attack plastic gasoline cans, tanks, and fuel lines (Figure 5). While many a high percentage of beetles may not bore completely through a plastic container, individual containers can be attacked by hundreds of beetles; such attacks can be so extensive that some beetles bore completely through a plastic container and render it unusable. This can be particularly problematic in certain situations, such as when beetles attack the fuel lines of a boat and a boat captain, thinking they have enough fuel, becomes stranded away from shore due to leaky fuel lines.

Figure 5. Plastic gasoline storage container with numerous holes created by female camphor shoot borers. White arrows indicate embedded beetles. Image by Chris Carlton and Victoria Bayless, in Carlton & Bayless (2011). Used with permission.

Prevention and Control

The best preventative treatment is to grow trees, shrubs, and vines that aren't stressed. This encompasses a variety of measures, including growing plant species that are adapted to the local conditions of a given site; reducing or eliminating herbicide use or at least controlling herbicide overspray around susceptible hardwoods; and minimizing transplantation shock and timing tree transplantation to avoid peak borer flight time.

Pruning out infested branches or destroying infested plants, including unsalable nursery stock, can reduce the number of beetles in the immediate area and may lessen pest pressure. However, camphor shoot borers are strong fliers and eliminating infested plants may not stop reinfestation from outside areas.

The efficacy of pesticides for control of camphor shoot borer has not been tested. Systemic neonicotinoids and pyrethroids (e.g., imidacloprid and bifenthrin) have been effective in reducing the attacks of some ambrosia beetle species but not others, so they may or may not be effective against camphor shoot borer. A surface spray of permethrin can reduce the number of attacks by other problem ambrosia beetles, so it may also be effective for camphor shoot borer. If a surface insecticide is used, it should be applied to the plant above 0.5 meters when beetle activity peaks (April and May in the southern U.S. and probably May and early June in Pennsylvania). It has also been suggested that kaolin clay applied to tree trunks may lower ambrosia beetle attacks as the stark white color is not attractive to the beetles, which fail to recognize such trees as suitable hosts.

References

Barringer, L. 2016. First records of the camphor shoot borer, Cnestus mutilatus (Blandford) (Coleoptera: Curculionidae: Scolytinae) in Pennsylvania. Insecta Mundi, 0519: 1–2.

Carlton, C., and V. Bayless. 2011. A case of Cnestus mutilatus (Blandford) (Curculionidae: Scolytinae: Xyleborini) females damaging plastic fuel storage containers in Louisiana, U.S.A. The Coleopterists Bulletin, 65(3): 290–291.

Cognato, A. I., C. E. Bográn, and R. Rabaglia. 2006. An exotic ambrosia beetle, Xylosandrus mutilatus (Blandford) (Scolytinae: Xyleborina) found in Texas. The Coleopterists Bulletin, 60(2): 162–163.

Gandhi, K. J. K., J. Audley, J. Johnson, and M. Raines. 2009. Camphor shoot borer, Xylosandrus mutilatus (Blandford) (Coleoptera: Curculionidae), an adventive ambrosia beetle in Georgia. The Coleopterists Bulletin, 63(4): 497–500.

Jones, M. E., and T. D. Paine. 2018. Potential pesticides for control of a recently introduced ambrosia beetle (Euwallacea sp.) in southern California. Journal of Pest Science, 91: 237–246.

Leavengood, J. M., Jr. 2013. First record of the camphor shoot borer, Cnestus mutilatus (Blandford 1894), (Curculionidae: Scolytinae: Xyleborini) in Kentucky. Insecta Mundi, 0308: 1–3.

NAPIS. 2009–2018. Reported status of camphor shoot borer – Cnestus mutilates. Accessed 29 January 2019.

Oliver, J., N. Youssef, J. Basham, A. Bray, K. Copley, F. Hale, W. Klingeman, M. Halcomb, and W. Haun. 2012. Camphor shoot borer: A new nursery and landscape pest in Tennessee. Cooperative Extension Faculty Research. Paper 21. 8 pp. Accessed 29 January 2019.

Ranger, C. M., P. B. Schultz, S. D. Frank, and M. E. Reding. 2018. Freeze stress of deciduous trees induces attacks by opportunistic ambrosia beetles. Agricultural and Forest Entomology. DOI: 10.1111/afe.12317

Reding, M. E., J. B. Oliver, P. B. Schultz, C. M. Ranger, and N. N. Youssef. 2013. Ethanol injection of ornamental trees facilitates insecticide efficacy against ambrosia beetles. Journal of Economic Entomology 106(1): 289–298.

Schiefer, T. L., and D. E. Bright. 2004. Xylosandrus mutilatus (Blandford), an exotic ambrosia beetle (Coleoptera: Curculionidae: Scolytinae: Xyleborini) new to North America. The Coleopterists Bulletin, 58(3): 431–438.

Stone, W. D., and T. E. Nebeker. 2007. Distribution and seasonal abundance of Xylosandrus mutilates (Coleoptera: Curculionidae). Journal of Entomological Science, 42: 409–412.

Stone, W. D., T. E. Nebeker, and P. D. Gerard. 2007. Host plants of Xylosandrus mutilates in Mississippi. Florida Entomologist, 90: 191–195.

Swirko, C. 2018. Misguided beetle causes woe for boaters, mowers. Gainesville.com Accessed 31 January 2019.

Werle, C. T., K. M. Addesso, B. J. Sampson, and J. J. Adamczyk. 2014. The knight in white armor: Can applications of kaolin deter ambrosia beetle attacks at ornamental nurseries? Entomological Society of America Annual Meeting.

Authored by Michael Skvarla, Director of the Insect Identification Laboratory & Extension Educator, February 2019.

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