About Larinus obtusus Gyllenhal, 1835
Larinus obtusus Gyllenhal, 1835 is a species of true weevil, commonly called the blunt knapweed flower weevil. This weevil is used as a biological pest control agent against noxious knapweeds, particularly spotted knapweed (Centaurea maculosa).
Adult blunt knapweed flower weevils are dark brown with a large, bulbous snout, and measure 5 to 7 millimeters (0.19 to 0.27 inches) in total length. They are active all summer, when females lay yellow eggs inside opened knapweed flower heads. When the larva emerges, it feeds on the plant's developing seeds inside the flower head. The larval stage lasts 17 days; after this period, the larva builds a cocoon from seed remnants and pupates inside it for around nine days.
Nearly all of the damage the weevil causes to the host plant comes from larval seed feeding. While adult weevils do feed on the plant's foliage, this causes much less severe damage to the plant. The average lifespan of this species is around 94 days for males, and 58 days for females.
Larinus obtusus is native to Europe and the Middle East. It was first introduced to the United States for knapweed biocontrol in the Boulder, Colorado area in 1991. Today it is established across much of the northwestern United States, though its current total abundance is unknown.
This weevil is very similar to its close relative Larinus minutus, another knapweed specialist that is also used for knapweed biocontrol. The two are currently classified as separate species, though this classification may be updated with further research.
