Tropical Whiteweed | Ageratum conyzoides L.

California Pest Rating  for
Tropical whiteweed | Ageratum conyzoides L.
Family:  Asteraceae
Pest Rating: C |  Proposed Seed Rating: None

Initiating Event:

Tropical whiteweed was intercepted in Yolo county in October 2017 (PDR 570P066111862). It has not yet been rated. A pest rating proposal is required to determine a permanent rating for this pest.

History & Status:

Background: Ageratum conyzoides L. (tropical whiteweed, billy goat weed) is an erect, 30 to 80 cm tall, annual herb with shallow, fibrous roots. The stem is cylindrical and is covered with short, white hairs; it becomes strong and woody with age. The leaves are pubescent with long petioles and they are arranged oppositely. The fruit is a ribbed or angled, black achene that had rough bristles with upward turning spines. Tropical whiteweed has great morphological variation, and appears highly adaptable to different ecological conditions. It is a common pantropical weed that can extend into subtropical and warm temperate zones, where it grows during the summer2.

Worldwide Distribution: Tropical whiteweed is native to South and Central America and probably also the West Indies. It is introduced in Asia, Africa, North America, Europe, and Oceania2.

United States: Tropical whiteweed is known from Connecticut, Georgia, Kentucky, Maryland, Mississippi, North Carolina, Alabama, California, Florida, Hawaii, Louisiana, and Missouri5.

California Distribution: Tropical whiteweed has not yet been detected officially in California. Only one voucher from San Diego County is submitted3.

Official Control : Tropical whiteweed is listed as a harmful organism in New Zealand 7.

California Interceptions: Tropical whiteweed was recently intercepted in Yolo County in October 2017 (PRD 570P066111862)6. It has previously been intercepted in California during nursery inspections (300P06 039955, 1317560) and a dog team intercept (340P06128213)6.

The risk Ageratum conyzoides (tropical whiteweed) would pose to California is evaluated below.

Consequences of Introduction:

1) Climate/Host Interaction: Tropical whiteweed is adapted to Central and South America and it would presumably thrive in similar climates. It may be able to establish in a very limited part of California. It receives a Low (1) in this category.

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.

2) Known Pest Host Range: Tropical whiteweed does not require any one host, but grows wherever ecological conditions are favorable. It receives a High (3) in this category.

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

3) Pest Dispersal Potential: Tropical whiteweed reproduces by seed. It has no photoperiodic requirement for germination and in some areas one-half of the seeds will germinate shortly after they are shed. Each plant can produce up to 40,000 seeds that can be dispersed by wind and water. They may be dispersed longer distances by vehicles and in contaminated agricultural produce2, 4. It receives a High (3) in this category

Evaluate the natural and artificial 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.

4) Economic Impact: Tropical whiteweed is considered to be an important weed of crops in the tropics and subtropics in open situations. Year-round flowering and the production of large quantities of seed allow it to compete with crops, which could lower the crop yield and value. It occurs in cultivated land, roadsides, and in forest edges. Tropical whiteweed is also an important alternate host for pathogens and nematode pests of various economically important crops. For example, it is a symptomless carrier of Burkholderia solanacearum, which is a bacterial pest of potato in India. It is a host of the banana nematodes Radopholus similis and Helicotylenchus multicinctus in Brazil and of the root knot nematode, Meloidogyne javanica, in many parts of the world. It is also the host of the Tomato Yellow Leaf Curl Tanzania Virus (TYLCTZV) and the Ageratum Yellow Vein Virus2. It receives a High (3) in this category.

Evaluate the economic impact of the pest to California using the criteria below.

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.

G. 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.

5) Environmental Impact: Tropical whiteweed thrives best in rich, moist, mineral soils with high humidity and it tolerates shading. It is not tolerant to soils with poor fertility1.It is likely to trigger new chemical treatments by ranchers and land managers. It receives a Medium (2) in this category.

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

Environmental Impact:  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 significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Environmental Impact: Score: 2

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 Ageratum conyzoides (tropical whiteweed): Medium (12)

Low = 5-8 points

Medium = 9-12 points

High = 13-15 points

6) Post Entry Distribution and Survey Information: Tropical whiteweed is not considered to be naturalized in California, as only one voucher from San Diego County has been submitted. It considered as localized distribution in California and Received a Low (-1) in this category.

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:

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:

The final score is the consequences of introduction score minus the post entry   distribution and survey information score: High (13)


Tropical whiteweed has only been documented from San Diego County and has been intercepted a couple of times. This weed has been growing in California for years and has not escaped; therefore, the uncertainty about this species is low.

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 potential weed with a distribution in limited areas. A “C” rating is recommended, as the plant is invasive, but it is of limited adaptability in the dry climate of California.

  1. Global Invasive Species database. Accessed October 20, 2017.
  2. Invasive Species Compendium: Distribution maps for plant pests, Accessed October 20, 2017.
  3. 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. Accessed October 20, 2017.
  4. Pacific Island Ecosystem. Accessed October 20, 2017.
  5. Plant in USA.  Accessed October 20, 2017.
  6. Pest and Damage Record Database, California Department of Food and Agriculture, Plant Health and Pest Prevention Services. Accessed October 20, 2017.
  7. USDA Phytosanitary Certificate Issuance & Tracking System (PCIT) Phytosanitary Export Database (PExD). Accessed October 20, 2017.


Javaid Iqbal,  California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 403-6695;[@]

Responsible Party:

Dean G. Kelch, Primary Botanist; California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 403-6650;[@]


You must be registered and logged in to post a comment.  If you have registered and have not received the registration confirmation, please contact us at[@]

Comment Format:

♦  Comments should refer to the appropriate California Pest Rating Proposal Form subsection(s) being commented on, as shown below.

Example Comment:
Consequences of Introduction:  1. Climate/Host Interaction: [Your comment that relates to “Climate/Host Interaction” here.]

♦  Posted comments will not be able to be viewed immediately.

♦  Comments may not be posted if they:

Contain inappropriate language which is not germane to the pest rating proposal;

Contains defamatory, false, inaccurate, abusive, obscene, pornographic, sexually oriented, threatening, racially offensive, discriminatory or illegal material;

Violates agency regulations prohibiting sexual harassment or other forms of discrimination;

Violates agency regulations prohibiting workplace violence, including threats.

♦  Comments may be edited prior to posting to ensure they are entirely germane.

♦  Posted comments shall be those which have been approved in content and posted to the website to be viewed, not just submitted.

Pest Rating: C |  Proposed Seed Rating: None

Posted by ls