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How to identify it
- Adults are elongate weevils up to 8 mm long with a long snout. The body is black, covered with mottled tan-yellow hairs that can rub off with age, leaving the beetle looking darker.[1]
- Larvae are white, C-shaped grubs with a brown head, up to 5 mm long, found inside the seed heads of thistles in summer.[1]
- Adult feeding in spring leaves holes in young leaves and deformed bracts and seed heads.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Thistle seedhead weevil (Rhinocyllus conicus) | The approved musk thistle agent is also a dark weevil with mottled yellowish hairs whose larvae eat seed inside the heads, but it is smaller (up to 6 mm) and short-snouted, while the bud weevil is up to 8 mm with a long snout. Oregon researchers at first mistook bud weevils in Canada thistle heads for R. conicus until they reared them out.[1][2] |
| Knapweed flower weevils (Larinus minutus and L. obtusus) | Same genus and same habit of developing in flower heads, but those two were deliberately released against knapweeds and develop in knapweed heads, not thistles.[1] |
Where it lives
Established on Canada thistle in scattered populations across the United States and Canada, and on bull, musk and plumeless thistle in western Canada. In eastern Oregon it was already well established on a wildlife refuge when a Forest Service biocontrol study began there in 1995.[1][2]
Biology and life cycle
Adults emerge in early spring and feed on young thistle foliage. When flower buds form, the female inserts her eggs into the developing heads.[1][2]
Larvae feed on the developing seeds and the receptacle through the summer, passing through three stages, and pupate inside the head.[1]
New adults emerge in late summer and early fall and overwinter in plant litter. There is one generation a year.[1]
In North America it is recorded on Canada, bull, musk and plumeless thistle and on many native thistles.[1]
Damage and signs
- Larval feeding destroys developing seed. On the Oregon refuge, samples of Canada thistle heads in four different years showed 28 to 43 percent attacked by this weevil.[2]
- Over ten years of monitoring at that refuge, with the Canada thistle stem weevil and the stem gall fly also present, Canada thistle fell from about 50 to about 8 plants per square metre, plants grew shorter, and after 2004 the refuge stopped spraying herbicide for it. The authors credited the combined stress from all the agents, which let grasses outcompete the weakened thistles; the bud weevil's share of that result was not separated out.[2]
- The weevil attacks numerous native thistle species, and its damage to natives often exceeds its damage to the introduced thistles. For that reason it is not approved for redistribution in the United States.[1]
Health risk
A seed-feeding weevil; none of these sources describes any risk to people.[1][2]
Control
Prevention
Do not collect or move this weevil. It is not an approved biological control agent in the United States and is not approved for redistribution, because of its attacks on native thistles.[1][2]
Biological control
Where it already occurs in Canada thistle heads, it is one of several insects feeding on the weed and needs no control itself.[2]
Common questions
- Is the bud weevil in my Canada thistle an approved biocontrol insect?
- No. It arrived by accident, was recorded in the United States by 1968, and has never passed the host testing required for approved agents. It is not approved for redistribution in the United States.[1][2]
- Does it hurt native thistles?
- Yes. It attacks numerous native thistle species, and the damage to natives often exceeds the damage to the introduced thistles it is found on.[1]
- How much seed does it destroy?
- On an Oregon refuge, 28 to 43 percent of sampled Canada thistle heads were attacked in the years measured. Each larva develops inside a head and eats the developing seed and receptacle.[2][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)
- USDA Forest Service (Treesearch 44760), “Markin (USDA Forest Service, Rocky Mountain Research Station) and Larson (Ladd Marsh Wildlife Refuge). Effective landscape scale management of Cirsium arvense (Canada thistle) utilizing biological control. In: Proceedings of the XIII International Symposium on Biological Control of Weeds, Session 9, pp. 423-428. USDA Forest Service, Forest Health Technology Enterprise Team (2013).” research.fs.usda.gov/treesearch/44760 (accessed 2026-11-10)
- iNaturalist, “Larinus carlinae taxon page (order and family).” www.inaturalist.org/taxa/768460 (accessed 2026-11-10)