Larinus obtusus 
<<back to meadow knapweed
<<back to spotted knapweed
Common Name
Blunt knapweed flower weevil
Weed Impacted
Spotted and meadow knapweed are the preferred hosts, although weevils will occasionally attack diffuse knapweed. Beetles are most destructive in the larval stage, which inhibits plant reproduction by feeding on seeds within flower heads. Adults feed on leaves.
Insect Description
Adults are 5-7 mm long (approximately 1/4 inch), dark mottled brown with a large snout. Larvae are white grubs that form cocoons within flower heads.
Life Cycle
Adults are active from May through August. Females lay eggs in newly opened flower heads, and the eggs hatch in about three days. Larvae feed on hairs and new seeds within the flower heads. Adults emerge in three to four weeks from cocoons in the flower heads, begin feeding, and then overwinter in soil litter beneath the plants.
Field Identification
When this beetle shares knapweed with other seed head weevils, identification can be difficult (especially if Larinus minutus is also present). L. obtusus is generally larger with fewer gray hairs than L. minutus, and has a much longer snout than the Broad-nosed seed head weevil (Bangasternus fausti).
Availability
This beetle is becoming increasingly available in Washington, Oregon, Montana, and Wyoming. Adults are typically collected in Washington and Oregon from mid- to late summer. To inquire about availability and to request this biocontrol agent, please download, complete and submit this form.
Comments
Grazing livestock on infested sites should be avoided early in the season, since grazing while knapweed plants are bolting prevents the establishment and maintenance of strong beetle populations.
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.
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