Canada thistle has three approved biological control agents in California: the stem gall fly, Urophora cardui (Diptera: Tephritidae), the stem weevil Hadroplontus litura (formerly called Ceutorhynchus litura; Coleoptera: Curculionidae), and the rust pathogen Puccinia punctiformis (Pucciniales: Pucciniaceae). The stem weevil holds the most promise of the permitted insects.

The stem gall fly was first released in 1977 and now occurs in several counties in northern California (Fig. 1). However, it rarely attacks more than a handful of plants where found. Females lay eggs in young stems which stimulate the formation of a gall (Fig. 2). Multiple larvae develop inside each gall and hibernate inside the gall. Adults emerge in late spring. Adults have clear wings with a thick black "W" marking. Semi-shaded sites are slightly preferred to those in full sun.

The stem weevil was first released in 1971 but it failed to establish (Fig. 3). New release efforts that began in 2012 have resulted in a population of the stem weevil that persists at one location near Etna in Siskiyou County. Adults emerging in late winter feed on the young leaves of plants (Fig. 4) and females deposit eggs in the young leaves in the late winter and early spring. The larvae mine down the leaf midrib into the root crown and lower stem (Fig. 5). Mature larvae exit the plant and pupate in the soil. The exit holes allow access for small insects and pathogens which can further damage the plant. The underground parts of the plant usually do not survive the winter, and it has been suggested that H. lituramay reduce overwintering survival of Canada thistle plants. Adults are black with a white 'T'- or thunderbird-shaped marking on their back (Fig. 3).

The Canada thistle rust is found occasionally on plants in Siskiyou and Lassen counties (Fig. 6). It infects the leaves and stems but appears to have little impact on plant survivorship or seed production. However, there is a study currently underway to artificially inoculate plants with the rust in the fall when the plants are translocating nutrients down to the root system. If the rust infects the root, the disease will be translocated throughout the clonal root system and the thistle patch will die out. Results of this control method are still pending and will not be available until after 2021.

Currently, the most common insects on Canada thistle in California are two accidentally-introduced species, the seed head weevil, Larinus carlinae (formely called L. planus; Coleoptera: Curculionidae) (Fig. 7), and the seed head fly, Terellia ruficauda (Diptera: Tephritidae) (Fig. 8). Larvae of both species feed on the developing seeds and reduce seed production. Larinus carlinae, the more common insect, completely destroys all seed when its larva is present in a head. It can become locally abundant, and, at high population levels, it can destroy over 90% of seed produced in a patch of plants. The seed head fly is less common (attack rates usually less than 20%) and does not provide population-level impacts.

The seed head weevil (Rhinocyllus conicus), which was released in California and other states primarily to control musk and Italian thistles, attacks Canada thistle at low levels. Generally 2-12% of the early seed heads can be found with R. conicus eggs. However, few R. conicus larvae mature to adults because feeding by adult L. carlinae kill young buds and result in high mortality of young R. conicus larvae that are present in the heads.

Biological Control Agents


Common name




Hadroplontus litura [= Ceutorhynchus litura]

stem weevil



First released in CA in 1971, re-released in 2011

Larinus carlinae[=L. planus]

seed head weevil

widespread in northern CA


Accidental introduction, found in CA in 2014. Not a permitted agent.

Puccinia punctiformis

Canada thistle rust



Present but not intentionally released – experimental use permit for CA in 2019

Terellia ruficauda

seed head fly

Widespread in Siskiyou, Lassen, Modoc cos.


Accidental introduction, found in CA in 1942. Not a permitted agent.

Urophora cardui

gall fly

recovered in Siskiyou and Lassen cos.


First released in CA in 1977

Rhinocyllus conicus

seed head weevil



Released as an agent on musk thistle. Not a permitted agent.

How the Technique Is Employed

The stem gall fly has too little impact in California to be recommended for use as a control organism. Collect galls in the fall, winter or early spring and place them at new sites. It would be best to rear out adults indoors in a cage to prevent the unintentional movement of parastoids.

Of the permitted insects, the stem weevil holds the most promise. Adults should be collected in early spring to release when stems are short (<2 in. tall). Use fingers, forceps or aspirator.

The fall inoculation of the Canada thistle rust has shown good results in Colorado where summer rains are common. Whether it will work in California is under study.

The two seed head insects likely occur at most infestations of Canada thistle in Siskiyou and Lassen counties. Look for signs of the seed head weevil by examining old flower heads for the presence of a small white larva when opened. These insects invaded California on their own and are not permitted for release. The weevil is known to attack at least 4 species of native thistles (Cirsium spp.) in other western states, but in California it has been found only on Canada thistle to date.

Special Tips

Dead plants with seed heads laying on the ground at the end of the season may contain live insect larvae, so it is best not to remove them until adults have finished emergence in the spring.


The stem weevil, stem gall fly, rust, and the adventive seed head fly attack only Canada thistle and have not been found on native thistles.

The seed head weevil has been reported to attack four nontarget native thistles (Cirsiumspp.) in the western USA. However, in a recent survey in California, it was found only on Canada thistle and not on any native thistle species.

Where Can I Get These?

The use of the rust as a control agent is currently under study and not available for general distribution.

Commercial vendor of the Canada thistle stem weevil:
Integrated Weed Control (1-888-319-1632) website: www.integratedweedcontrol.com


Coombs, E.M., J.K. Clark, G.L. Piper, and A.F. Cofrancesco, Jr. 2004. Biological Control of Invasive Plants in the United States. Western Society of Weed Science, Oregon State Univ. Press, Corvallis.

Pitcairn, M.J. 2018. Weed biological control in California, USA: review of the past and prospects for the future. BioControl 63: 349-359.doi.org/10.1007/s10526-018-9884-6

Villegas B., C. Gibbs, J. Aceves, M.J. Pitcairn. 2015. Release of the root weevil, Ceutorhynchus litura, on Canada thistle in northern California. In: Pickett, C.H. (ed.) Biological Control Program Annual Summary, 2011-12. California Department of Food and Agriculture, Plant Health and Pest Prevention Services, Sacramento, CA, pp. 47-48.

Winston, R., R. Hansen, M. Schwarzlander, E. Coombs, C. B. Randall, and R. Lym. 2008. Biology and Biological Control of Exotic True Thistles. USFS Forest Health Technology Enterprise Team, FHTET-2007-05. 130pp.

Contributing Authors

Dr. Michael J. Pitcairn, Program Manager, California Department of Food and Agriculture

Dr. Lincoln Smith, Research Entomologist, USDA