Rhinocyllus conicus* 
* listed for information only. This insect is known to attack native thistles and it is no longer legal to move it across state lines. It is NOT distributed by IWCP.
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Common name
Thistle head weevil
Weed impacted
Larvae impact Canada thistle (and other thistle) seed production by feeding within developing buds and preventing seed formation.
Insect description
Adult weevils are 3 mm to 6 mm long, dark in color with hair tufts creating a mottled yellowish appearance. The rostrum (nose or snout) is short and blunt. Larvae are white and segmented with brown heads.
Life cycle
Adults emerge from overwintering sites in protected areas (caves, tree hollows, etc.) in late spring and early summer. Eggs are laid on bracts of flower buds. Larvae feed in buds consuming receptacle and maturing seed tissue. Pupation occurs in the seed heads and adults emerge in late summer or autumn to overwinter.
Field identification
Adult weevils can be found on flower buds early in the summer. They can be differentiated from a similar looking seed thistle weevil, Larinus planus, by the long narrow rostrum (snout) of L. planus.
Availability
This insect is NOT distributed by IWCP due to known nontarget effects.
Comments
Rhinocyllus conicus is known to attack native thistles and its intentional distribution is discouraged. It was first released in the U.S. in 1969. Pre-release testing determined that this weevil would attack many thistle species but because native thistle species were not valued at the time, it was approved for release. Social and environmental value shifts have led to changes in the criteria required to approve biocontrol agents for release. In 2000, USDA-APHIS revoked all permits for interstate shipment of R. conicus. 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
Arnett, A. E. and S. M. Louda. 2002. Re-test of Rhinocyllus conicus host specificity and the prediction of ecological risk in biological control. Biological Conservation 106:251-257.
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.
Gassmann, A. and S. M. Louda. 2001. Rhinocyllus conicus: Initial evaluation and subsequent ecological impacts in North America. Pages 147-183 in E. Wajnberg, J. K. Scott, and P. C. Quimby, editors. Evaluating Indirect Ecological Effects of Biological Control. CABI Publishing, Oxon, UK.
Zwölfer, H. and P. Harris. 1984. Biology and host specificity of Rhinocyllus conicus (Froel.) (Col., Curculionidae), a successful agent for biocontrol of the thistle, Carduus nutans L. Zeitschrift für angewandte Entomologie 97:36-62.
Photo Credit
Rhinocyllus conicus - Loke T. Kok, Virginia Polytechnic Institute and State University, Bugwood.org http://www.ipmimages.org/images/768x512/0580019.jpg
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