Bangasternus fausti 
<<back to diffuse knapweed
Common Name
Broad-nosed seed head weevil
Weeds Impacted
This weevil attacks diffuse, spotted, and squarrose knapweed. Larvae inhibit plant reproduction by feeding on seeds within flower heads. Up to 100% of the seeds in a single flower head may be consumed and the viability of any seeds not attacked is reduced.
Insect Description
Adults are small, hairy, grayish-black weevils. They are 3.5-4 mm (approximately 1/8 inch) long and have a short, blunt snout.
Life cycle
Females lay eggs from May to mid-July on stems and leaflets directly below the developing flower head. Larvae hatch and tunnel through to the flower heads, where they begin feeding on developing seeds. Newly emerged adults feed on knapweed leaves and flowers, then overwinter either in soil litter near the plants or within seed heads (in warmer climates).
Field Identification
Bangasternus fausti is one of several small, grayish-brownish-blackish weevils that attack knapweed. However, it is typically smaller and has a much shorter and broader snout than either Larinus minutus or Larinus obtusus. It is substantially smaller, darker in color and generally more abundant in the field than the knapweed root weevil, Cyphocleonus achates.
Availability
Bangasternus fausti is available for mass redistribution in Washington and Oregon. Adults are typically collected from late spring to early summer. To inquire about availability and to request this biocontrol agent, please download, complete and submit this form.
Comments
These beetles prefer hot, dry areas and are not effective at high elevations or in areas that may receive rain for prolonged periods of time.
Using the Agent: Integrated Weed Management (IWM)
Several seed head agents have been released for control of knapweed. Because B. fausti larvae attack any other insects occupying the flower heads, beetles are best released at sites where other seed head agents are not present. Releases should be made only at sites where knapweed infestations are large and immediate eradication of the weed is not the primary objective. Most treatments are not directly compatible with the use of this biocontrol agent. Any control methods that prevent flowering and seed production 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, and mowing while plants are flowering interferes with the maintenance of strong agent populations. If it is necessary to mow, plants can be mowed in the fall when beetles have emerged from the flower heads (and if seed head flies are not present - see Chaetorellia acrolophi and Terellia virens).
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.
Photo Credit
Bangasternus fausti adult - USDA archive photo, bugwood.org (insectimages.org) http://www.insectimages.org/images/768x512/0022006.jpg
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