J.M. DiTomaso, The Regents of the University of California

J.M. DiTomaso, The Regents of the University of California

Squarrose Knapweed


Bangasternus fausti

Bangasternus fausti, or the broad-nosed seed head weevil, emerges after overwintering as adults when plants are in the bud to mid-flowering stage, usually from May until late July. Adults are 0.14 inches long and are grayish-black with a much shorter snout than the Larinus spp. weevils.

Eggs are laid from May to mid-July under leaflets below developing flower heads or on the ends of stems and leaflets. New larvae either mine into the midrib of the leaflet or into the stem before tunneling into the flower head. Larvae can consume up to 100% of the seeds in a flower head and the viability of unattacked seeds is also reduced. The weevil prefers hot, dry areas and has limited effectiveness at high elevations and areas with prolonged rain. The weevils are usually collected with a sweep net in late spring when plants begin to bloom.

Larinus minutus, L. obtusus

Larinus spp., or the knapweed flower weevils, are abundant biological control agents that can utilize spotted, diffuse, and squarrose knapweeds. These insects overwinter as adults, and emerge in the spring when they begin to feed on knapweed foliage.

Females produce between 28 and 130 eggs which they lay in clusters in open flowers. The hatching larvae feed on seeds and receptacle tissues and later construct cocoons within the seedheads using pappus hairs.

After four weeks, pupation occurs within this cocoon. Emerging adults chew a characteristic round hole in the top of the cocoon that is visible when viewed from above.