Musk (Carduus nutans), Italian (C. pycnocephalus), and milk (Silybum marianum) thistles are hosts of a small suite of biological control agents. In total, five species of insects have been approved for release by USDA-APHIS: the seed head weevil Rhinocyllus conicus (Curculionidae: Coleoptera), the root weevil Trichosirocalus horridus (Curculionidae: Coleoptera), the rosette beetle Psylliodes chalcomera (Chrysomelidae: Coleoptera), the seed head fly Urophora solstitialis (Tephritidae; Diptera), and the stem fly Cheilosia grossa (Syrphidae: Diptera). However, none of these species are currently permitted for use in California because of risks to nontarget native plants. Some of these agents were approved by APHIS at a time when there was little concern about native species, and they would not meet current standards (Hinz et al. 2014). APHIS has revoked the permit for the seed head weevil.

In California, only the seed head weevil was intentionally released, and it occurs wherever musk thistle is found. The stem fly moved into California on its own and is limited to thistle populations in central Siskiyou County. The root weevil is established in many states, but it has not been reported in California. The rosette beetle and the seed head fly are not known to be established in the USA. In addition, an exotic rust, Puccinia carduorum (Uredinales: Pucciniaceae), which was intentionally introduced under an experimental use permit as a classical biological control agent in Maryland in 1987, eventually spread on its own into California and now occurs wherever musk thistle is known to occur.

The seed head weevil deposits eggs on young flower heads, and the larvae burrow into the head and feed on the developing seeds (Piper and Coombs 2004b; Figs. 1-3). It has one generation per year and overwinters as an adult. High populations of the weevil can reduce seed production by up to 80% in musk thistle and 60% in Italian thistle, but usually less than 20% in milk thistle. Following its release in California this weevil has been found to attack at least 12 species of native thistle (Cirsiumspp.) (Turner et al. 1987). In California, the weevil emerges from winter diapause before many of the native thistle species begin flower production, and this asynchrony limits the amount of seed loss due to the weevil (Goeden and Ricker 1985, Herr 2000). However, on a national level, this weevil has attacked at least 22 species of native thistles, including causing significant damage to a federally threatened plant species (Cirsium pitcheri)(Pemberton 2000, Louda et al. 2005, Hinz et al. 2014). It also attacks thistle species in several other genera, including Carduus, Onopordum, and Silybum, all of which are alien to North America.

The stem fly emerges in the early spring and lays eggs in the growing tip of the young bolt growing from the rosette (Piper and Coombs 2004a; Figs. 4 & 5). Larvae mine the elongating flowering stem during the summer and fall. Pupation occurs in the lower stem near the root crown and lasts through the winter (Fig. 6). In California, the stem fly has been found inside musk thistle, Italian thistle, and bull thistle.

The rust has a complicated life cycle with five life stages (Littlefield et al. 2004; Fig. 3). It is known to infect only musk thistle. The rust produces resting spores that resist cold temperatures during winter. Resting spores germinate in the spring and produce secondary spores that infect plants. Optimal conditions for infection are cool (65-70°F) and humid (at least 8 hours of dew). Infected plants produce pustules in about 2 weeks, which release spores that disperse in the wind. Infection cycles can repeat during the summer, and resting spores are produced when plants senesce in autumn.

Biological Control Agents


Common name

Host plant




Cheilosia grossa [=C. corydon]

stem fly

musk thistle

Italian thistle

(bull thistle)



Moved into CA on its own, first obs. in 2015; not a permitted agent

Puccinia carduorum

musk thistle rust

musk thistle



Moved into CA on its own, first obs. in 1998; not a permitted agent

Rhinocyllus conicus

seed head weevil

musk thistle

Italian thistle

milk thistle

and other thistles


moderate to high

First released in CA in 1969; not a permitted agent

How the Technique Is Employed

The only organism that is known to cause significant damage to populations of these target weeds in California is the seed head weevil, R. conicus, which is not a permitted agent. It is widespread and occurs on almost all stands of musk and Italian thistles. Its presence on milk thistle is patchy, and when found, occurs in lower numbers.

No organisms are permitted for use as biological control agents; however, control methods that complement the seed head weevil damage, such as encouraging competitive vegetation and treating weeds from the periphery of the infestation inward, would work best to control thistle populations.

For additional information, see Winston et al. (2012).

Special Tips

The seed head weevil has performed best in the musk thistle populations found in Siskiyou County where it provides good control of musk thistle. In this area, musk thistle populations usually erupt following disturbance, such as clear cutting of forest plantations. The seed weevil then builds up its populations, and over the next several years causes a steady decline in musk thistle plants. Observations in the Sierra Nevada Mountains near Truckee in Nevada County suggest that weevil populations occur at lower levels (<50%) so the ability of the weevil to reduce musk thistle abundance can vary regionally.

The seed head weevil is present in plants throughout most of the year. Adult weevils emerge in June and are active visiting flower heads in July and August while larvae are present inside seed heads during the other ten months. Dead plants with seed heads laying on the ground have live weevil larvae in the heads, so leaving them in place will likely lead to higher weevil numbers. If the goal is to eradicate musk thistle at the site and protect nearby native thistles from the weevil, then destroy mature seedheads, or use herbicides to prevent their formation.


The seed head weevil has been observed feeding on over 20 native Cirsium species in the U.S. Larvae of the stem fly, C. grossa, have been found inside the exotic thistle Cirsium vulgare but no native Cirsium species to date. There has been no non-target infection by the rust.

Where Can I Get These?

None of the biological control agents for musk, Italian or milk thistle are permitted for use in California.


Goeden, R. D. and D. W. Ricker. 1985. Seasonal asynchrony of Italian thistle, Carduus pycnocephalus, and the weevil, Rhinocylus conicus (Coleoptera: Curculionidae), introduced for biological control in Southern California. Environmental Entomology 14:433-436.

Herr, J. C. 2000. Evaluating non-target effects: the thistle story. In: M. S. Hoddle (ed.) Proceedings of the California Conference on Biological Control, Riverside, CA, pp. 12-17.

Hinz, H. L., Schwarzländer, M., Gassmann, A., and Bourchier, R. S. 2014. Successes we may not have had: a retrospective analysis of selected weed biological control agents in the United States. Invasive Plant Science and Management 7: 565-579.

Littlefield, J.L., W.L. Bruckart, D.M. Woods, and A.B.A.M Baudoin. 2004. Puccinia carduorum. In: E.M. Coombs, J.K. Clark, G.L. Piper, and A.F. Cofrancesco, Jr. (eds.). 2004. Biological Control of Invasive Plants in the United States. Western Society of Weed Science, Oregon State Univ. Press, Corvallis, pp. 363-365.

Louda, S. M., A. Tatyana, F. Rand, L. Russell and A. Arnett. 2005. Assessment of ecological risks in weed biocontrol: Input from retrospective ecological analyses. Biological Control 35: 253-264.

Pemberton, R. W. 2000. Predictable risk to native plants in weed biological control. Oecologia 125:489–494. doi: 10.1007/s004420000477

Piper, G.L. and E.M. Coombs. 2004a. Cheilosia corydon(=Cheilosia grossa). In: E.M. Coombs, J.K. Clark, G.L. Piper, and A.F. Cofrancesco, Jr. (eds.). Biological Control of Invasive Plants in the United States. Western Society of Weed Science, Oregon State Univ. Press, Corvallis, pp. 359-361.

Piper, G.L. and E.M. Coombs. 2004b. Rhinocyllus conicus. In: E.M. Coombs, J.K. Clark, G.L. Piper, and A.F. Cofrancesco, Jr. (eds.). Biological Control of Invasive Plants in the United States. Western Society of Weed Science, Oregon State Univ. Press, Corvallis, pp. 365-368.

Turner, C.E., R.W. Pemberton, and S.S. Rosenthal. 1987. Host utilization of native Cirsium thistles (Asteraceae) by the introduced weevil Rhinocyllus conicus (Coleoptera: Curculionidae) in California. Environmental Entomology 16: 111-115.

Winston, R., R. Hansen, M. Schwarzlander, E. Coombs, C.B. Randall, and R. Lym. 2012. Biology and Biological Control of Exotic True Thistles. USDA Forest Service, Forest Health Technology Enterprise Team. FHTET-2007-05, 3rd edn., April 2012.

Contributing Authors

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

Dr. Lincoln Smith, Research Entomologist, USDA