Tree Spurge | Euphorbia dendroides | Pest Rating Proposals and Final Ratings

California Pest Rating for

Tree Spurge | Euphorbia dendroides

Pest Rating: B | Proposed Seed Rating: R

PEST RATING PROFILE

Initiating Event:

Collection in Santa Barbara County.

History & Status:

Background: Tree spurge is a small shrub (to 2 meters) native the Mediterranean Region. It is a semi-succulent plant with ascending candelabra-like branches and elliptical leaves without a petiole about 17 mm long. The flowers (cyathia) are borne in spring. The floral bracts are bright greenish yellow. Like all true spurges, the branches and leaves exude an irritating white latex when damaged. Although it most likely arrived in CA as a garden plant, it is no longer available in the trade (except rarely as seed). Its distribution is limited by cold, as it is intolerant of frost.

Worldwide Distribution: California Distribution: Tree spurge is currently restricted to Southern Santa Barbara County, Ventura County, and the foothill region between Altadena and Pasadena in Los Angeles County.

United States: There are no naturalized populations of tree spurge outside of CA in the United States.

International: Tree spurge is native to the Mediteranean Region where it is found in Southern Europe and Northern Africa. It is naturalized in southwestern Australia.

This risk tree spurge will pose to California is evaluated below.

Consequences of Introduction: 3

1) Climate/Host Interaction: Evaluate if the pest would have suitable hosts and climate to establish in California.

– 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

Risk is high (3), as the plant is naturalized in more than 8 localities throughout CA.

2) Pest Host Range: 3

Evaluate the host range of the pest:

– Low (1) has a very limited host range
– Medium (2) has a moderate host range
– High (3) has a wide host range

Risk is high (3) as weeds do not require any one host, but grow wherever ecological conditions are favorable.

3) Pest Dispersal Potential: 2

Evaluate the dispersal potential of the pest:

– 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

Risk is Medium (2). The plant reproduces via rather large seeds that are thrown some distance from the mother plant. Nevertheless, its ability to disperse seems limited, as populations do not spread rapidly.

4) Economic Impact: 1

Evaluate the economic impact of the pest to California using these criteria:

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 by other states or countries)
D. The pest could negatively change normal production cultural practices

– 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

Risk is low (1) as tree spurge will be restricted to forest edges, coastal scrub, and arroyos.

5) Environmental Impact: 3

Evaluate the environmental impact of the pest on California using the following criteria:

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. Significantly impacting cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact:

– 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

Risk is High (3). In California, tree spurge has disrupted natural sagebrush scrub communities, has triggered additional treatment programs to control it, and crowds out native species that coexist with or foster rare species.

Consequences of Introduction to California for tree spurge:

Rating (Score): Add up the total score and include it here

– Low = 5-8 points
– Medium = 9-12 points
– High = 13-15 points

Total points based on above criteria: Medium (12).

6) Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included. Score: -2

-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: 10

Uncertainty:

There is no uncertainty as to whether this plant can establish in CA.

Conclusion and Rating Justification:

Conclusions of the harm(s) associated with this pest to California using all of the evidence presented above: Proposed Rating: Based on the score listed above the pest is a medium risk. Given its ability to spread much more widely and ongoing control efforts, a B rating would be justifed.

References:

Baldwin, B. G., D. H. Goldman, D. J. Keil, R. Patterson, T. J. Rosatti, and D. H. Wilken, editors. 2012. The Jepson manual: vascular plants of California, second edition. University of California Press, Berkeley.

Consortium of California Herbaria (ucjeps.berkeley.edu/consortium/). 2014.

Global Compendium of Weeds: http://www.hear.org/gcw/species/euphorbia_dendroides/

Jepson Flora Project (eds.) 2013. Jepson eFlora, http://ucjeps.berkeley.edu/IJM.html, accessed on Mar 28 2014

Responsible Party:

Dean G. Kelch, Primary Botanist; California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 654-0312; plant.health[@]cdfa.ca.gov.