plumeless thistle
Carduus acanthoides

plumeless-thistle photos

Musk and plumeless thistles are often found growing close to each other.  One of the thing that both of these weeds share are some biological control agents, Rhinocyllus conicus and Trichosirocalus horridus. Another name for this weed is a bristly thistle.

Roots:  The plumeless thistle has a stout, fleshy taproot.

this picture from  Plumeless Thistle Identification

Stems and Leaves:  Plumeless thistle grows between 0.3 and 1.2 m tall.  The stems are heavily branched at the top and covered with 6.2 to 18.5 mm spiny wings going upward to about the flower heads.  The leaves are blended into the stem and are alternately arranged.

this picture from  Nodding Plumeless Thistle (Carduus nutans)

Flowers:  The flowers of the plumeless thistle are about 1 to 3 cm in diameter.  They can be found in different colors like a purple-pink or ever white to cream colored in May and July.  The flowers can grow individually or in clusters of about 2 to 5.

Fruits and seeds:  The seeds are about 2-3 mm long and are usually straw-colored to light brown with dark brown striations.

Life duration/habit:  Like musk thistle, plumeless thistle is a winter annual, an annual or a biennial.

Biological Control:  Like musk thistle there are three biological control agents that we can use to control this weed. These are: Rhinocyllus conicus, Trichosirocalus horridus, and Urophora solstitialus.

Rhinocyllus conicusis is a small beetle about 3-7 mm long that is dark brown and oval shaped.  It emerges as an adult in spring and lays eggs in early summer on the underside of the flower bud. The larva develops inside of the flower buds.  Adults can last up to 15 months and winter in the leaf litter. The main attack stages are the larvae and the adults. The larvae attack the seed heads and the adults attack the leaves.  You can collect this insect in the pupa stage prior to the adult stage and all you have to do is clip seed heads.  You can also collect them in the adult stage by just hand picking them.

Trichosirocalus horridus is a weevil about 3-4 mm long that is brown and spherical shaped.  It emerges as an adult in early spring and lays eggs in early spring on vegetative buds. The larva develops in and feeds on the vegetative buds.  The adults can last up to 10 months and winters under the rosettes.  The attack stages are the larvae and the adults.  The larvae attack the seed heads and the adults attack the leaves.  You can only collect these in the adult stage and only in mid to late summer.

Urophora solstitialus is a fly about 4.5 mm long with a black body and a distinctive pattern on the wings. The adult emerges in late spring to early summer.  They usually lay eggs in June or July and laid in floret tubes.  The larva develop in the flower head receptacle.  The adults last about 3 weeks and overwinters in the larva stage in galled seed heads.  The most damaging stage is the larvae in the seed heads. You can collect this fly in the larvae stage in fall and early spring by cliping galled flower heads.

1) Piper G.L., Rees, N.E., et. al,  Biological Control of Weeds in the West, "plumeless thistle, Carduus acanthiodes."

2) Plumleless thistle, Carduus acanthoides L., [website]

mtwow-home   New-Weed-Alerts      plumeless-thistle photos

This page made by Jody Singleton and Alissa Frazer on March 16, 2001.