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How to identify it
- Adults are up to 6 mm long with a short snout, dark brown or black and covered in spring with tufts of yellowish hair that give them a mottled look. Older adults lose many of the hairs and turn brownish black.[1]
- The eggs are the easiest sign: laid in spring on the bracts of the flower heads and on stems, each is capped with chewed plant material that turns tan with age, looks like part of the plant, and is plainly visible to the naked eye.[1]
- Larvae are white, C-shaped grubs with a brown head, up to 4 mm, hidden inside the seed heads, usually several to a head. Attacked heads turn straw-colored later in the season and do not shed their seed normally.[1]
- Adult feeding leaves rounded holes scattered over the whole leaf surface, though this damage is usually minor.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Musk thistle crown weevil (Trichosirocalus horridus) | The other weevil released against musk thistle in North America: smaller (about 4 mm), round, brown with white mottling, with obvious spines on the thorax and a long snout. Its larvae feed at the root-stem junction rather than in the seed heads, and the two are often credited together with thistle declines.[1] |
Where it lives
Native to Europe, the weevil was released in Canada from 1968 and across the United States from 1969 using three strains collected from thistles in France and Italy. It is now abundant on musk and plumeless thistle through much of North America and has spread on its own to bull and Canada thistle in the West, plus marsh thistle in western Canada. It does best in meadows and moist places with moderate temperatures; where summer comes fast it cannot use the later flower heads.[1]
Biology and life cycle
Overwintered adults emerge in early spring and lay eggs on the bracts of the developing seed heads and on the stems, covering each egg with chewed plant tissue.[1]
Larvae hatch in late spring and early summer and pass through four stages, burrowing through the bracts or mining the stem until they reach the seed head, where they feed on the receptacle and the developing seeds.[1]
They pupate inside the heads in late summer as the seed ripens. The new adults are out only briefly before overwintering in sheltered spots. There is one generation a year.[1]
It prefers Carduus thistles (musk, plumeless, Italian and slenderflower) over other thistles. On Canada, bull and milk thistle it does little, because those plants keep making flower heads after the weevil has finished laying, and Canada thistle spreads by its roots anyway.[1]
Damage and signs
- Larval feeding on the receptacle and seed cuts the amount of viable seed a thistle produces. It does not kill existing plants; the benefit is a slower rate of spread.[1]
- The weevil also keeps seed from leaving the plant. Research on musk thistle found that attacked heads release seed poorly and that in the United States the seeds that do leave fall faster, so wind carries them less far, an effect on spread that had gone unrecognized.[2]
- Results have been uneven. In some states substantial declines in musk and plumeless thistle are credited at least partly to this weevil, alone or with the musk thistle crown weevil; elsewhere densities have not fallen because some seed escapes inside attacked heads and late-season heads escape altogether. Recent studies find it effective on musk and plumeless thistle only where competing plants are dense.[1]
- Its host range includes native thistles: at least 22 of the 90 Cirsium species native to North America are attacked. A review of weed biocontrol worldwide found that more than 99 percent of agents have caused no known significant harm to nontarget plants, and that the two largest exceptions are the cactus moth on native cacti and this weevil on native thistles, introductions that would not be allowed today.[1][3]
Health risk
A plant-feeding weevil; none of these sources describes any risk to people.[1][2]
Control
Prevention
Do not collect or move this weevil. Because of its attacks on native thistles its redistribution permits in the United States were revoked in 2000, and redistribution is no longer permitted.[1]
Biological control
Where it already occurs on musk or plumeless thistle, it is a weed biocontrol agent and needs no control; its effect on those thistles is greatest where competing vegetation is dense and the growing season gives it time to reach the flower heads.[1]
Common questions
- What are the tan bumps on my thistle flower heads?
- Egg caps of the thistle seedhead weevil: each egg is laid on a bract or stem and covered with chewed plant tissue that dries tan and looks like part of the plant. Later the attacked heads turn straw-colored and hold onto their seed.[1]
- Can I move seedhead weevils to a new thistle patch?
- Not in the United States. Permits to redistribute the weevil were revoked in 2000 because it attacks native thistles.[1]
- Will it get rid of my thistles?
- No. It reduces seed and slows spread but does not kill plants, and it has little effect on Canada, bull or milk thistle. Declines in musk and plumeless thistle have come where the weevil works alongside dense competing vegetation, sometimes with the crown weevil as well.[1]
Sources
- North American Invasive Species Management Association (PDF hosted on Bugwood), “Introduced Thistle Biocontrol Agents: History and Ecology in North America. In: Winston, ed., Biological Control of Weeds in North America, NAISMA-BCW-2024-8-INTRODUCED THISTLES-A (2024).” bugwoodcloud.org/resource/files/29169.pdf (accessed 2026-11-10)
- Ecological Applications (Ecological Society of America), “Marchetto, Shea, Kelly, Groenteman, Sezen and Jongejans. Unrecognized impact of a biocontrol agent on the spread rate of an invasive thistle. Ecological Applications 24: 1178-1187 (2014), doi:10.1890/13-1309.1.” doi.org/10.1890/13-1309.1 (accessed 2026-11-10)
- PLoS ONE, full text in PubMed Central (PMC3890286), “Suckling and Sforza (USDA-ARS European Biological Control Laboratory). What magnitude are observed non-target impacts from weed biocontrol? PLoS ONE 9: e84847 (2014), doi:10.1371/journal.pone.0084847.” doi.org/10.1371/journal.pone.0084847 (accessed 2026-11-10)
- iNaturalist, “Rhinocyllus conicus taxon page (order and family).” www.inaturalist.org/taxa/229899 (accessed 2026-11-10)