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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. If horses consume too much, it can cause neurological disease, equine ni- gropallidal encephalomalacia.
Russian knapweed forms dense monocultures in disturbed lands. It is a bushy 1-3 ft. tall plant with clonally spreading roots. Near the root crown, roots are covered in a black scaly sheath that separates it from other knapweeds. 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 rangeland, and causes reduced biomass of native vegetation, negatively affecting the ecology of our wildlands. 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. The two biocontrol agents are the gall midge, Jaapiella ivannikovi (Cecidomyiidae), and the other is a gall wasp, Aulacidea acroptilonica (Cynipidae). Both agents decrease shoot length, biomass and flowering/seed production. Gall midges have been established in Colorado since 2009 and gall wasps since 2016. The goal of Russian knapweed biocontrol is to reduce competitive ability and spread of the host plant.
Size of sesame seeds the stem gall wasps (A) are early season insects emerging from stem galls (B) in mid to late Spring. The female adults lay eggs in the main and lateral shoots causing pea-like swellings (C-D) as larvae hatch within stems. Developing larvae (E) deprived the nutrient source of the plant. The wasps overwinter in stems as late instar larvae (F) and pupate inside galls in early Spring. This agent has one generation a year. A small number of larvae can pupate after a second winter. The wasps will not sting people or wildlife.
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,E). Larvae develop in silky webs between the growing leaves of the gall (F). There can be up to 14 larvae protected inside each gall where they will hibernate in the pupal stage (G) and emerge once again in the spring.
Gall Midge vs. Wasp
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