Granulate Ambrosia Beetle | Xylosandrus crassiusculus Motschulsky

California Pest Rating for
Granulate Ambrosia Beetle  |  Xylosandrus crassiusculus Motschulsky
Coleoptera: Curculionidae: Scolytinae
Pest Rating: A



Initiating Event:

Xylosandrus crassiusculus is currently Q-rated.  A permanent pest rating proposal is required to support an official pest rating.

History & Status:

Background:  Xylosandrus crassiusculus is a moderate-sized (adult females are 2-2.9 mm in length) ambrosia beetle that feeds on over 200 species of plants in 41 families, including alder, azalea, beech, cottonwood, elm, hickory, oaks, pines, maples, and carob (Sargent et al., 2008).  It attacks both stressed and apparently healthy trees, including seedlings, and is considered a pest of ornamental trees in the United States.  The beetle also utilizes freshly-cut wood (Sargent et al., 2008).  Like other ambrosia beetles, the adults and larvae feed on a symbiotic fungus rather than the wood itself.  Adult females mate with males before leaving their gallery, and they can also reproduce via arrhenotokous parthenogenesis (An unmated female lays unfertilized eggs that develop into males. The female mates with her male progeny and then deposits fertilized eggs, which develop into females) (Wood, 1982).

Worldwide Distribution:  Xylosandrus crassiusculus is thought to be native to Southeast Asia.  The species has been introduced to Europe, sub-Saharan Africa, Central America, South America, Hawaii, and the eastern United States (Flechtmann and Atkinson, 2016; United Kingdom Department for Environment Food & Rural Affairs, 2015).  Specimens were collected in 1999, 2000, and (in larger numbers) 2004 at a location in Oregon.  The source was apparently wood from the southeastern United States imported for use in railroad ties.  This led to an eradication effort that was reported to be successful in 2010.  In 2015 and 2016, however, more X. crassiusculus were found in the same area, which led to another eradication effort.  This beetle appears to have been successfully eradicated from the area by the end of 2016 (LaBonte, 2010; LaBonte, 2016; LaBonte et al., 2005).

Official Control:  Xylosandrus crassiusculus does not appear to be under official control anywhere, although it is on the EPPO Alert List, and it was the subject of eradication efforts in Oregon.

California Distribution:  Xylosandrus crassiusculus is not known to occur in California (Bright and Stark, 1973; Symbiota Collections of Arthropods Network).

California Interceptions:  Xylosandrus crassiusculus has been intercepted on plant material (including cut flowers and foliage) from Florida and Hawaii (PDR # 023370, 1396560, 1256568, 1251403, 1252446, 053195, and 150P06086553).  One interception was made from incense cedar boards that possibly originated in Louisiana (PDR # 1312482).

The risk Xylosandrus crassiusculus would pose to California is evaluated below.

Consequences of Introduction:

1) Climate/Host Interaction: Xylosandrus crassiusculus has become established in many areas of the world representing a wide variety of climates. The species may be limited to forests, as it feeds on wood and because the symbiotic fungus probably requires a certain range of humidity.  The range expansion in the eastern United States has been limited to approximately the distribution of the eastern deciduous forests, but the species apparently became established at a location in Oregon before it was eradicated there, which suggests that more humid areas on the west coast would be suitable for this species (Flechtmann and Atkinson, 2016).  The beetle feeds on many species of trees, including pines and oaks.  All of this suggests that it could become established over a wide portion of California.  Therefore, it receives a High (3) in this category.

– Low (1) Not likely to establish in California; or likely to establish in very limited areas.

– Medium (2) may be able to establish in a larger but limited part of California.

– High (3) likely to establish a widespread distribution in California.

2) Known Pest Host Range: Xylosandrus crassiusculus is known to feed on over 200 species of plants in 41 families. A wide host range is not unusual for an ambrosia beetle, probably because the fungal symbiosis releases the beetle from some of the constraints of a more typical bark beetle lifestyle (i.e., phloem feeding).  Therefore, it receives a High (3) in this category.

– Low (1) has a very limited host range.

– Medium (2) has a moderate host range.

– High (3) has a wide host range.

3) Pest Dispersal Potential: Adult female Xylosandrus crassiusculus Sibling mating and arrhenotokous parthenogenesis mean a single female can found a population.  This species is evidently capable of being moved with wood shipments.  Therefore, it receives a High (3) in this category.

– Low (1) does not have high reproductive or dispersal potential.

– Medium (2) has either high reproductive or dispersal potential.

– High (3) has both high reproduction and dispersal potential.

4) Economic Impact: Xylosandrus crassiusculus is a pest of ornamental and fruit trees in the United States.  The species causes wilting and death, especially in young trees.  In addition, as an ambrosia beetle, crassiusculus inoculates its galleries with a fungus that serves as food for adults and larvae.  This fungus may not be pathogenic, but other fungi are also carried by the beetles, including known plant pathogens.  In addition, beetle damage can allow other, opportunistic (and sometimes pathogenic) fungi to infect the tree.  This beetle also uses cut logs for development, and is known to damage them and lower their value.  Therefore, it receives a High (3) in this category.

Economic Impact: A, B, E

A. The pest could lower crop yield.

B. The pest could lower crop value (includes increasing crop production costs).

C. The pest could trigger the loss of markets (includes quarantines).

D. The pest could negatively change normal cultural practices.

E. The pest can vector, or is vectored, by another pestiferous organism.

F. The organism is injurious or poisonous to agriculturally important animals.

The organism can interfere with the delivery or supply of water for agricultural uses.

Economic Impact Score: 3

– Low (1) causes 0 or 1 of these impacts.

– Medium (2) causes 2 of these impacts.

– High (3) causes 3 or more of these impacts.

Environmental Impact: Xylosandrus crassiusculus has not been reported to have a significant impact on the environment anywhere it has been introduced. However, in tests, crassiusculus was attracted to California bay laurel (Umbellularia californica) and was apparently able to develop on this tree (Mayfield et al., 2013).  Therefore, it is possible that Xyleborus crassiusculus could pose a threat to California forests.  In addition, this beetle is known to cause damage to ornamental trees.  Therefore, it receives a High (3) in this category.

Evaluate the environmental impact of the pest on California using the criteria below.

Environmental Impact: A, E

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.

B. The pest could directly affect threatened or endangered species.

C. The pest could impact threatened or endangered species by disrupting critical habitats.

D. The pest could trigger additional official or private treatment programs.

E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Environmental Impact Score: 3

– Low (1) causes none of the above to occur.

– Medium (2) causes one of the above to occur.

– High (3) causes two or more of the above to occur.

Consequences of Introduction to California for Xylosandrus crassiusculus: High (15)

Add up the total score and include it here.

–Low = 5-8 points

–Medium = 9-12 points

–High = 13-15 points

6) Post Entry Distribution and Survey Information: Xylosandrus crassiusculus is not known to occur in California (Symbiota Collections of Arthropods Network).  It receives a Not established (0) in this category.

–Not established (0) Pest never detected in California, or known only from incursions.

–Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).

–Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

–High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Final Score:

7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: High (15)


Lack of evidence for environmental impact may only reflect environmental impact receiving less attention and research than economic impact, so it is possible that this species has an impact on the environment in areas to which it has been introduced.

Conclusion and Rating Justification:

Xylosandrus crassiusculus is a highly polyphagous pest that has demonstrated an ability to become established over a wide area and has become a pest of ornamental trees.  The species is not known to be present in California, and its potential introduction to this state poses a risk of economic and environmental damage.  For these reasons, an “A” rating is justified.


Bright Jr., D.E. and R.W. Stark.  1973.  The Bark and Ambrosia Beetles of California.  University of California Press.  169 pp.

European and Mediterranean Plant Protection Organization.  2015.  EPPO Alert List.  Accessed February 14, 2018.

Flechtmann, C.A.H. and T.H. Atkinson.  2016.  First records of Xylosandrus crassiusculus (Motschulsky) (Coleoptera: Curculionidae: Scolytinae) from South America, with notes on its distribution and spread in the New World.  The Coleopterists Bulletin.  70(1): 79-83.

LaBonte, J.R.  2010.  Eradication of an ambrosia beetle, Xylosandrus crassiusculus (Motschulsky), in Oregon.  In (K. McManus and K.W. Gottschalk, eds.) 2010 Research Forum on Invasive Species (pp. 41-43).

LaBonte, J.R.  2016.  Exotic wood boring insects program.  Oregon Department of Agriculture, Plant Protection & Conservation Programs, Annual Report.  2016: 24-25.

LaBonte, J.R., Mudge, A.D., and K.J.R. Johnson.  2005.  Nonindigenous woodboring Coleoptera (Cerambycidae, Curculionidae: Scolytinae) new to Oregon and Washington, 1999-2002: Consequences of the intracontinental movement of raw wood products and solid wood packing materials.  Proceedings of the Entomological Society of Washington.  107(3): 554-564.

Mayfield, A.E., MacKenzie, M., Cannon, P.G., Oak, S.W., Horn, S., Hwang, J., and P.E. Kendra.  2013.  Suitability of California bay laurel and other species as hosts for the non-native redbay ambrosia beetle and granulate ambrosia beetle.  Agricultural and Forest Entomology.  15: 227-235.

Sargent, C., Raupp, M., Sardanelli, S., Shrewsbury, P., Clement, D., and M.K. Malinoski.  2008.  Granulate ambrosia beetle; Xylosandrus crassiusculus Motschulsky (Coleoptera: Curculionidae: Scolytinae).  University of Maryland Entomology Bulletin.

Symbiota Collections of Arthropods Network.  Accessed February 14, 2018.

United Kingdom Department for Environment Food & Rural Affairs.  2015.  Rapid pest risk analysis (PRA) for Xylosandrus crassiusculus.

Wood, S.L.  1982.  The bark and ambrosia beetles of North and Central America (Coleoptera: Scolytidae), a taxonomic monograph.  Brigham Young University.  1359 pp.


Kyle Beucke, 1220 N Street, Room 221, Sacramento, CA, 95814, 916-403-6741,[@]

Responsible Party:

Jason Leathers, 2800 Gateway Oaks, Sacramento CA 95833, (916) 654-1211,[@]

Comment Period:* CLOSED

4/24/18 – 6/8/18


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Pest Rating: A


Posted by ls