2026 Annual Dues
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BACKGROUND: Originally found in Eurasia, spotted knapweed infests rangeland, pastures, roadsides, or any disturbed soils. Knapweeds release chemical substances into the soil that inhibit the growth of competing vegetation.
OTHER COMMON NAMES: Centaurea maculosa
DESCRIPTION: Spotted knapweed is a short-lived, herbaceous perennial that is 1 to 3 feet tall. The rosette leaves are deeply lobed and may be 6 inches in length. The stems are moderately leaved. Flowers are typically pink with black-tipped flower bracts. Bloom is in early summer.
CONTROL: Several biocontrol agents are available and offer fair to good control. Select herbicides can offer good to excellent control when applied between rosette and pre-bud stages.
BIOLOGY: Adults are active from May to September. They are 4 to 5 mm long. Females produce 30-130 eggs in their lifetime. Up to 5 eggs are deposited in the flower head of each knapweed plant. Larvae hatch in 3 days and then spend 4 weeks in the seed head before pupating. Pupation takes 1 week. Adults drop off of plants in September and overwinter in the soil.
DESTRUCTIVE STAGE: Larvae (feeding on seed head) and adult (leaf and seed feeding)
IMPACT TO HOST: Damage can reduce stand density and provide excellent control. L. minutus will be most effective when released with other knapweed biological control agents.
REDISTRIBUTION: Adult L. minutus can be collected using a sweep net or by picking them off of plants in June and July. A typical release is 150-250 adult insects.
SPECIES OF KNAPWEED ATTACKED: Diffuse knapweed, spotted knapweed, squarrose knapweed.
BIOLOGY: C. achates adults emerge from June to September. They are large (14-15mm long) and live for 2-3 weeks. Females lay one egg at a time in the root crown just below the soil. A female will mate and lay eggs over 100 times. Larvae undergo 4 instars. The fourth instar overwinters and pupates in June.
DESTRUCTIVE STAGE: Larvae (root feeding and root galling)
IMPACT TO HOST: First and second instar larvae reduce root mass. Third and fourth instar larvae form swelling and galls in roots. C. achates can kill knapweed plants in a single year, but populations are often not dense enough to reduce entire stands of knapweed. C. achates will be most effective when released with other knapweed biological control agents.
REDISTRIBUTION: Adults can be collected by picking them
directly off of plants in the middle of the day in July. A release is typically 50-75 C. achates.
SPECIES OF KNAPWEED ATTACKED: Spotted knapweed is the preferred host, but also attacks diffuse knapweed.
