Integrated Weed Control Project

Larinus planus* Larinus planus

* listed for information only. This insect is known to attack native thistles, is not an approved agent, and is NOT distributed by IWCP.

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Common name

Canada thistle bud weevil

Weed impacted

Larvae impact Canada thistle flower buds by feeding within developing buds and preventing seed formation.

Insect description

Adult weevils are 5 mm to 10 mm long, dark in color with hair tufts creating a mottled yellowish appearance. Its rostrum (nose or snout) is long and narrow. Larvae are white and segmented with brown heads.

Life cycle

Adults emerge from overwintering in plant litter in late spring and early summer, before Canada thistle plants set bud. When buds are set, adult weevils lay eggs on buds. Larvae feed in buds and developing flowers, consuming all seed head material. Pupation occurs in the seed heads and adults emerge in late summer to overwinter in the litter.

Field identification

Adult weevils can be found on flower buds early in the summer. They can be differentiated from a similar seed thistle weevil, Rhinocyllus conicus, by the long narrow rostrum (snout) of L. planus.

Availability

This insect is NOT a USDA approved biological control agent and is not distributed by IWCP.

Comments

The Canada thistle bud weevil is known to attack native thistles and its intentional distribution is discouraged. Additionally, because Canada thistle vigorously reproduces by vegetative means, using this weevil to limit seed production will not control or limit the spread of Canada thistle.

References

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.

Louda, S.M. and C. O’Brien. 2002. Unexpected ecological effects of distributing the exotic weevil, Larinus planus (F.), for the biological control of Canada thistle. Conservation Biology 16:717-727.

http://www.for.gov.bc.ca/hfp/biocontrol/agents/Larinus_planus.htm

Photo Credit

IWCP staff

 

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