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Russian knapweed (Rhaponticum repens) is a clonal perennial forb in the Aster family. It was accidentally introduced in the United States during the late 1800s via alfalfa seed. Russian knapweed originates from countries bordering the Caspian Sea in western Asia; also China and Mongolia. It is now found throughout the United States west of the Appalachians and is most problematic in the semi-arid rangelands of the Great Basin and Rocky Mountains. Russian Knapweed seed is often transported to new locations in the stomachs of grazing animals.
Russian knapweed forms dense monocultures in disturbed lands. It is a bushy 1-3 ft. tall plant with clonally spreading roots. The roots are covered in dark paper-like sheathing, especially near the root crown. Solitary pink/purple flowers grow at the tip of leafy branches. Lower leaves are 2-4 in. long and lobate while upper leaves are about 1 in. long and lanceolate. In contrast, other knapweeds have more deeply lobed (pedate) lower leaves.
Russian knapweed is an aggressive, non-native plant that can dominate the landscape, due in part, to US populations producing nearly four times more seed (1200 seeds per plant) than in native populations. It has no agricultural or ecological value. Russian knapweed out-competes crops, degrades range-land, and causes reduced biomass of native vegetation, negatively affecting the ecology of our wildlands. It can cause a lethal neurological disease in horses if consumed in substantial amounts. In many instances, Russian knapweed is so widespread on low-value land that the use of herbicides is uneconomical and detrimental to the environment.
Biological control of weeds is the use of natural enemies such as insects, mites, pathogens or other animals to reduce the spread, reproductive ability or reduce the density of the target weed. The state of Colorado is fortunate to have a large and active program that works with biological control agents on many problematic pest species in the state and operates out of an insectary in Palisade. It is at this location that Russian knapweed insects are collected, reared, and distributed throughout the state.
There are currently two biological control agents approved for release on Russian knapweed in the US. Our main agent is a gall midge, Jaapiella ivannikovi (Cecidomyiidae), and the other is a gall wasp, Aulacidea acroptilonica (Cynipidae). The stem galling midge was recently approved by the USDA for release and has been successfully established in Colorado since 2009, however, the stem galling wasp has proven more difficult to work with and is not yet available for distribution. CDA is planning on releasing other bio-control agents on Russian knapweed as they become available.
The midge originates from countries neighboring western China. It can complete up to five generations in a growing season, each generational cycle requiring 28 days. Females mate soon after becoming adults in spring (A) and seek out young, tender knapweed plants to lay eggs. Eggs are placed on the surface of growing bud tips (B). Developing larvae feed on the plant through three larval stages (C). This feeding causes the stem to stop growing and form a rosette gall of fused leaves (D). Larvae develop in silky webs between the growing leaves of the gall (E). There can be up to 14 larvae protected inside each gall where they will hibernate in the pupal stage (F) and emerge once again in the spring.
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