BACKGROUND: Russian knapweed is native to Eurasia. It infests rangelands, field edges, pasture, roadsides, and other disturbed soils. Knapweeds release chemical substances into the soil that inhibit the growth of competing vegetation. It can cause “chewing disease” in horses that consume it.
OTHER SCIENTIFIC NAMES: Centaurea repens, Rhaponticum repens
DESCRIPTION: A creeping herbaceous perennial, Russian knapweed grows 2 to 3 feet tall. Roots are black and may go 8 feet deep or more. Basal leaves are lobed and are 2 to 4 inches in length. Flowers are pinkish to purple, and flower bracts have membranous cream-colored tips. Bloom is early summer through late summer.
CONTROL: Biocontrol is available, but limited. Select herbicides can offer good to excellent control when applied between pre-bloom to the killing frost.
BIOLOGY: Gall flies are very small and look similar to a tiny mosquito. Adults are difficult to find and are better identified by the gall formation on plants. Adults emerge in early spring around April – May. They lay eggs on stems. Microscopic larvae hatch in stems to form webby terminal galls. Gall midges undergo many generations per year. They emerge as a new generation every two weeks throughout the summer.
DESTRUCTIVE STAGE: Larvae (galls in stems)
IMPACT TO HOST: Gall formation can impact plants by reducing resources to kill plants. It is still too early to determine overall impact to Russian knapweed. However, early results show high establishment and distribution rates of insects. Galls can reduce flowering. Early data shows 20% reduction to Russian knapweed stands.
REDISTRIBUTION: Collect galls infested with larvae in June – August. Hatch gall wasps in lab or quarantine and collect adult midges for redistribution into the field.
BIOLOGY: Gall wasps are small and black. They do not have stingers. Adults emerge in early spring around April – May. They lay eggs in stems. Microscopic larvae hatch in stems to form hard, pea-pod shaped galls. Larvae overwinter inside stems.
DESTRUCTIVE STAGE: Larvae (galls in stems)
IMPACT TO HOST: Gall formation can impact plants by reducing resources to kill plants. It is still too early to determine overall impact to Russian knapweed. However, early results show high establishment and distribution rates of insects.
REDISTRIBUTION: Collect dry galls infested with larvae in early spring. Hatch gall wasps in lab or quarantine and collect adult wasps for redistribution into the field.
