California Pest Rating for
Desert knapweed: Volutaria tubuliflora (Murb.) Sennen
Pest Rating: A | Seed Rating: R
PEST RATING PROFILE
This plant has been rated as “Q” on the CDFA Plant Pest Rating list for 2 years.
History & Status:
The genus Volutaria comprises plants in the thistle tribe (Cardueae) of the daisy family (Asteracaeae). Many of the 17 species in this genus were originally described in the genus Centaurea, the genus that includes star-thistles, knapweeds, and bachelor’s buttons; the two genera are closely related. Volutaria differs from Centaurea in lacking a terminal spine shield on the tips of the inflorescence bracts and in having flowers subtended by scales rather than bristles. Desert knapweed is a pink-flowered (sometimes white-flowered in Southeastern Morocco), annual or short-lived perennial species. It was collected from a naturalized population near Anza Borrego in San Diego County, California. At this spot, it was tentatively identified as Canary Island knapweed (Volutaria canariensis), a closely related species endemic to the Canary Islands. Desert knapweed seems to be spreading steadily in the Anza Borrego Area. Another species, Volutaria muricata, was introduced to limited localities in three counties in Southern California along the coast. We have no current information on its range and persistence. However, several species within the Centaurea group are known noxious weeds in California, including purple starthistle (Centaurea calcitrapa), diffuse knapweed (Centaurea diffusa), Iberian starthistle (Centaurea iberica), spotted knapweed (Centaurea stoebe), Malta starthistle (Centaurea melitensis), meadow knapweed (Centaurea jacea s.l.), yellow starthistle (Centaurea solstitialis), and squarrose knapweed (Centaurea squarrosa).
Desert knapweed has the largest native range of any species of Volutaria. It is widespread across northern Africa, as well as in other areas of the Region, where it inhabits drier localities and desert transition zones. Its expansion into some of these areas may be recent. It prefers nitrogen enriched soils and therefore has proven to spread rapidly along roadsides, as well as in dry farming areas and irrigated fields.
The San Diego County Agricultural Commissioner’s office initiated control measures against this plant. They are part of a coordinated effort to eradicate this plant from North America by County, State, and city staff, as well as by the non-profit organization CalIPC and private volunteers.
Worldwide Distribution: Desert knapweed occurs throughout North Africa from Morocco to Egypt, in southern Europe (Spain, Sicily, and Turkey), the Canary Islands, and in Arabia. There is a recent report of it being detected in Chile. In North America the only known populations of Desert knapweed are in southern California.
Official Control: Desert knapweed is currently listed on the California noxious weed list (under the name Volutaria canariensis; Canary Island knapweed). Desert knapweed has been recently (8/2018) as a Category A noxious weed in the state of Nevada.
California Interceptions: Desert knapweed was found after it had established along a road in the Anza Borrego Desert in 2009 (San Diego County). A new detection of a small colony along Newport Bay in Orange County was reported in 2015 and the Chula Vista plants in 2016 (San Diego County).
Consequences of Introduction:
1) Climate/Host Interaction: Evaluate if the pest would have suitable hosts and climate to establish in California.
Risk is High (3), as the plant is naturalized on roadsides in the desert, where it is spreading rapidly. Two more recent finds in Orange and San Diego counties indicate that it may invade southern coastal areas in California as well.
-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: Evaluate the host range of the pest.
Risk is high (3) as weeds do not require any one host, but grow wherever ecological conditions are favorable.
-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: Evaluate the natural and artificial dispersal potential of the pest.
Risk is Medium (2). The plant produces a moderate number of seeds that spread along roads, although large plants may produce thousands of seeds. Its appearance via some unknown pathway in such a remote area attests to its ability to spread under the right circumstances. During the 5 years that it has been detected, it has slowly increased its range in the Anza Borrego Desert. It was in Newport Bay since at least 1987, where it is currently known from seven spots. The seed lasts at least 3 years in the seed bank.
-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 Score: Evaluate the economic impact of the pest to California using the criteria below.
The presence of this plant in the Anza Borrego desert may in the future impact the spring wildflower tourist industry if the plant behaves like another noxious desert weed, Sahara mustard (Brassica tournefortii). If it infests row crops or irrigated areas, it could lower crop value or crop yield.
Score: 3 (A, B, C)
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.
G. The organism can interfere with the delivery or supply of water for agricultural uses.
-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.
5) Environmental Impact Score: Evaluate the environmental impact of the pest on California using the criteria below.
Risk is high (3) as the plant might be able to dominate desert and dry coastal areas that are home to sensitive species such as desert tortoise, blunt-nosed leopard lizard, and many rare native plants.
Score: 3 (A, C, D)
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 could significantly impact cultural practices, home/urban gardening or ornamental plantings.
Score the pest for 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 Desert knapweed: High (14)
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: 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.
Desert knapweed has been found in three counties in California. Its range at this time is limited. It receives a Low (-1) 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.
The final score is the consequences of introduction score minus the post entry distribution and survey information score: High (13)
Given that the weed history of Desert knapweed is just being deciphered, it is difficult to assess potential risk. Nevertheless, given its rapid spread in Anza Borrego it seems likely to be a major invasive. Given its long distance dispersal, its noxious relatives, and the effects of other introduced annuals such as Sahara mustard on desert ecosystems, it seems best to attempt eradication of the currently small populations.
Conclusion and Rating Justification:
Although Desert knapweed may be limited in its spread by its environmental tolerance, it may nevertheless become a severe pest within the desert and along the southern California coast in disturbed areas. This is based on its ecology in the Old World. As the species currently is highly restricted in its range in North America and eradication may be possible, we recommend that Desert knapweed be rated as A.
Calleja, J. A., Garcia-Jacas, N., Roquet, C., & Susanna de la Serna, A. 2016. Beyond the Rand Flora pattern: Phylogeny and biogeographical history of Volutaria (Compositae). Taxon 65: 315-332.
Consortium of California Herbaria. Accessed 1/31/2017: http://ucjeps.berkeley.edu/consortium/
Devesa, J. A. & Martinez, J. L. 2014. Volutaria Cass. In Devesa, J.A., Quintanar, A. & Garcia, M. A. (eds.). Flora iberica XVI: 272-278. Real Jardín Botánico, CSIC, Madrid.
Teillier, S., Macaya, J., Sisanna, A. & Calleja, J. A. 2014. Volutaria tubuliflora (Murb.) Sennen (Asteraceae), nueva especie alóctona asilvestrada para Chile. Gayana Bot. 71: 276-279.
Wagenitz, G. 1991. Volutaria canariensis Wagenitz, Candollea 46: 408.
Volutaria, a new invasive knapweed. Accessed 1/28/2017:
Dean G. Kelch, Primary Botanist; California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 403-6650; plant.health[@]cdfa.ca.gov.
Comment Period:* CLOSED
9/25/18 – 11/9/18
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Pest Rating: A | Proposed Seed Rating: R
Posted by ls