Cyphocleonus achates 
<<back to diffuse knapweed
<<back to spotted knapweed
Common Name
Knapweed root weevil
Weeds Impacted
This beetle attacks both spotted and diffuse knapweed. Larvae feed within the roots (root mining) and destroy the plant’s stored energy reserves reducing plant biomass and density.
Insect Description
Adults are mottled, dark brownish-black-white weevils and are 14 to 15 mm (approximately 1/2 inch) long. Larvae are whitish C-shaped grubs found within galls in the roots.
Life Cycle
Adults emerge from June to mid-September and can live for up to 15 weeks. During late summer and fall, females mate several times and lay eggs singly in root crowns just below the root crown. Larvae mine the upper parts of the roots causing a gall-like enlargement of the roots. Larvae overwinter and pupate within the root galls until the following year.
Field Identification
Several insects have been released for control of knapweed. Cyphocleonus achates adults can easily be distinguished from other weevils by their much larger size and white and gray mottled color. If C. achates occurs with other root-feeding insects (Sphenoptera jugoslavica and Agapeta zoegana), C. achates larvae will be larger and obviously C-shaped.
Availability
This beetle has established in Washington, Idaho, Oregon and a few other states, but availability is currently limited. To inquire about availability and to request this biocontrol agent, please download, complete and submit this form.
General Comments
Cyphocleonus achates is reducing knapweed biomass and density at several sites in western Montana. The weevils do best in hot, dry, well-drained areas with low, scattered vegetation.
Using the Agent: Integrated Weed Management (IWM)
Releases of C. achates should only be made at sites where knapweed infestations are large and immediate eradication of the weed is not the primary objective. Knapweed seed head insects and/or other root feeders may already be present at a given site and C. achates can be used effectively in combination with all available agents. Most treatments are not directly compatible with the use of this biocontrol agent. Any control methods that directly kill the plant will reduce or eliminate biocontrol populations. Biocontrol agents can be used within large patches and satellite patches (which lead to increased spread) can be controlled using other management tools.
Small or satellite knapweed populations can either be hand-pulled or sprayed. Consult with your county weed office, Extension office or an IWM practitioner to determine which herbicide treatments are safe and appropriate. Herbicide applications on larger infestations should only be made after insect agents have definitively established, and then should be timed according to the life cycles of both the plant and the insect.
Mowing will not control knapweed. While it may not affect C. achates or other root-feeders, mowing when plants are flowering interferes with the success of seed head insects. Repeated grazing in the spring by sheep and goats (not horses) can weaken plants; but like mowing, grazing while plants are flowering can be detrimental to seed head insect populations.
Prescribed burning can destroy above-ground parts, promote germination of existing seeds and facilitate earlier treatment of developing plants. However, it also increases the susceptibility of sites to new invasions by knapweed or other weeds.
Any management of knapweed should involve revegetation with native or other desirable plants, especially grasses. Revegetation promotes competition between plants, which stresses weeds and allows for more effective treatment. To learn more about revegetation and IWM, click here.
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.
Seastedt, T. R., D. G. Knochel, M. Garmoe, and S. A. Shosky. 2007. Interactions and effects of multiple biological control insects on diffuse and spotted knapweed in the Front Range of Colorado. Biological Control 42(3):345-354.
Waldo, A. J. 2003. Identification and Control of Knapweed Species in Central and Eastern Oregon. Oregon State Univ. Extension Publication EC 1559. URL http://extension.oregonstate.edu/catalog/html/ec/ec1559/
Photo Credit
IWCP staff
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