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

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

diffuse knapweed


Agapeta zoegana

Agapeta zoegana, or the sulfur knapweed moth, is a biological control agent that attacks knapweed plants by tunneling into the cortex of the root. Adults are 0.4 inches long and usually emerge over a 12-week period from mid-June to early September and live for only several days in the field.

Females mate during the first 24 hours after emergence and begin laying eggs the following day. The adult moths are characterized by bright yellow wings with brown markings. Eggs are laid in small groups or singly on the surface of stems and leaves of knapweed, hatching in seven to 10 days. Larvae hatch and migrate to the crown where they mine the root and pupation then occurs within the root.

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.

Cyphocleonus achates

Cyphocleonus achates, or the knapweed root weevil, is a robust biological control agent that can attack spotted (preferred host) and diffuse knapweeds. The insect overwinters as larvae in the root where the larvae mine and gall the vascular tissue. This larval feeding reduces knapweed density and can result in death of small plants.

Photo credit: Cornell UniversityAdults emerge from June to mid-September and feed on knapweed leaves. The adults are 0.5 to 0.6 inches long and generally live 8 to 15 weeks.

Females mate several times and deposit more than 100 eggs during their lifetime. Eggs are laid singly in a notch excavated by the female on the root crown just below the soil surface, hatching in 10 to 12 days.

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.

Photo credit: USDA ARSAfter 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.

Sphenoptera jugoslavica

Sphenoptera jugoslavlica, or the bronze knapweed root borer, adults emerge in July and are characterized by their metallic, dark reddish-brown appearance. Eggs are placed on the leaf axils of knapweed rosettes during July and August.

Newly hatched larvae feed in the leaf axils, and second instar larvae mine into the root. The larvae overwinter in the root and resume feeding the next year. Galls and tunnels are created by the larval mining and these are often filled with sawdust and frass from the larvae.The larval feeding significantly reduces knapweed biomass, seed output, and density. These beetles can be collected en masse in Oregon and Washington using a sweep net in mid-July in the early evening.