BACKGROUND: Native to Eurasia, diffuse knapweed inhabits dry rangeland, roadsides, field edges, and waste areas. Knapweeds release chemical substances into the soil that inhibit the growth of competing vegetation.
OTHER COMMON NAMES:
DESCRIPTION: It is an annual or a short-lived perennial averaging 1 to 2 feet tall. Leaves have finely divided lobes. Flowers are white to rose in color. Diffuse knapweed differs from squarrose knapweed in that the terminal spine of the toothed flower bracts is straight rather than arched outward. It blooms throughout summer.
CONTROL: Several biocontrol agents are available and provide fair to good control. Select herbicides can offer good to excellent control when applied from rosette to pre-bud stages. Tillage offers good control.
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.