Centaurea diffusa, also known as diffuse knapweed, white knapweed or tumble knapweed, is a member of the genus Centaurea in the family Asteraceae. This species is common throughout western North America but is not actually native to the North American continent, but to the eastern Mediterranean.
Description Diffuse knapweed is an annual or biennial plant, generally growing to between 10 and 60 cm in height. It has a highly branched stem and a large taproot, as well as a basal rosette of leaves with smaller leaves alternating on the upright stems. Flowers are usually white or pink and grow out of urn-shaped heads carried at the tips of the many branches. Diffuse knapweed often assumes a short rosette form for one year, reaching maximum size, then rapidly growing and flowering during the second year. A single plant can produce approximately 18,000 seeds.Harris, P., and R. Cranston. 1979. An economic evaluation of control methods for diffuse and spotted knapweed in western Canada. Canadian Journal of Plant Science 59:375-382. Synonyms Centaurea microcalathina Tarassov Centaurea cycladum Heldr. Centaurea parviflora Sibth. & Sm., non Desf. Centaurea comperiana StevenIn Tutin & alt., Flora Europea
Distribution It is native to Eastern Europe and Western Asia, specifically the nations of Turkey, Syria, Bulgaria, Greece, Romania, Ukraine, and southern Russia.
Invasive species Diffuse knapweed is considered an invasive species in some parts of North America, having established itself in many areas of the continent. C. diffusa was first identified in North America in 1907 when it was found in an alfalfa field in Washington state. The seeds had presumably been transported in an impure alfalfa seed shipment coming from somewhere in the species native range. Now present in at least 19 states in the United States, it has naturalized in all contiguous states west of the Rockies and additionally in Connecticut, Massachusetts, and New Jersey. Portions of western Canada have also been colonized by this plant.
Areas in which diffuse knapweed has been established generally are plains rangelands or forest benchlands. Land that has recently been disturbed is commonly colonized. It grows in semi-arid and arid environments and seems to favor light, dry, porous soils. Areas with large amounts of shade or high levels of water discourage diffuse knapweed growth.
C. diffusa can be dispersed in multiple ways, such as contamination of food, wind dispersal, and water dispersal however wind is the primary dispersal method. Effects By 1998 diffuse knapweed had naturalized over in the western US, and was increasing its range at a rate of 18% annually. Diffuse knapweed can establish itself in grassland, scrubland and riparian environments. It has little value as feed for livestock, as its thistles can damage the mouth and digestive tract of animals that attempt to feed on it. A study in 1973 concluded that ranches lost approximately US$20/km2 (8 cents per acre) of diffuse knapweed due to decreased grazing area. In an agricultural setting, it can greatly reduce crop yield and purity.
Control Effective control of diffuse knapweed requires a fusion of well-executed land management, biological control, physical control, chemical control and reestablishment of the native species. Any method of control must ensure that the root is removed or the plant will grow back. Additionally, native plant growth in areas where diffuse knapweed has been removed should be encouraged to prevent reestablishment.
Biological control Biological control involves the introduction of organisms, usually natural competitors of the invasive species, into the invaded environment in order to control the invasive species. Since 1970, 12 insects have been released to control diffuse knapweed. Of these 12, 10 have become established, and 4 are widely established (Urophora affinis and Urophora quadrifasciata, the root boring beetle, Sphenoptera jugoslavica, and the weevil Larinus minutus). Research based on simulation models have shown that for biocontrol agents to be effective, they must kill their host, otherwise plants can compensate by having increased seedling survival.Myers, J.H., Risley, C., 2000. Why reduced seed production is not necessarily translated into successful biological weed control. In: Spencer, N. (Ed.), Proceedings X. International Symposium Biological Control of Weeds. Montana State University, Bozeman, MT, pp. 569–581.
Some of the more commonly utilized biocontrol agents are the Lesser knapweed flower weevil and the Knapweed root weevil. Individuals of these species lay their eggs on the seed heads of both diffuse and spotted knapweed. When the larvae emerge from the eggs, they feed upon the seeds of their host plant. As the females of this species can create from 28 to 130 eggs and each larva can consume an entire seed head, an adequate population of Larinus minutus can devastate entire stands of knapweed. The adult weevils feed upon the stems, branches, leaves and undeveloped flower buds. It is native to Greece and is now found in Montana, Washington, Idaho and Oregon.Groppe, K. 1990. Larinus minutus Gyll. (Coleoptera: Curculionidae), a suitable candidate for the biological control of diffuse and spotted knapweed in North America. In: Final Report C.A.B International Institute of Biological Control. Delemont, Switzerland, p. 30. Insects are also used for biocontrol, such as the Yellow-winged knapweed root moth (Agapeta zoegana), and several species of Tephritid flies, mostly Urophora affinis and Urophora quadrifasciata. Physical control Physical control of diffuse knapweed primarily comprises cutting, digging or burning to remove the plants.
Cutting
While cutting the aboveground portion of diffuse knapweed will greatly decrease the spread of seeds, it does not remove the root. With only its root still intact, diffuse knapweed can survive and continue to grow. For a program of cutting to be effective, it must be long-term so that the effect of reduced seed spreading can be realized.
Digging
this removes both the portion above ground and the root of diffuse knapweed and has shown to be very effective; if the plant is properly disposed of, it can neither regrow nor spread its seeds. The largest problem with digging knapweed is that it is extremely labor-intensive. Additionally, the recently vacated soil should be planted with a native species to avoid knapweed reintroducing itself in the disturbed soil.
Burning
setting fire to a crowd of knapweed, if the fire is sufficiently severe, can successfully destroy the above ground and belowground sections of diffuse knapweed. However, precautions must be taken to first ensure that the fire is properly contained and that a new plant community is established to prevent the reintroduction of diffuse knapweed. Chemical control Chemical control involves the use of herbicides to control diffuse knapweed. The herbicide Tordon (picloram) is recognized as the most effective, but it is common to use multiple herbicides in order to reduce strain on local grasses. The herbicides 2,4-D, dicamba, and glyphosate are also effective for control. In order to be most effective, it must be applied before the knapweed plants have released their seeds, regardless of which herbicide is used. Ongoing research at the University of Colorado suggests that Tordon treatment does not contribute to long-term reductions of exotic species cover and may contribute to recruitment of other invasive species, such as redstem filaree and Japanese brome, which quickly take the place of herbicide-treated diffuse knapweed.
Human influence on invasion One of the first influences humans had on diffuse knapweed was to inadvertently introduce it to North America.
Diffuse knapweed is known to establish more easily and effectively in recently disturbed environments. Disturbed environments generally present low environmental stress because more resources are available than are being used. These available resources often allow the establishment of an invasion in an ecological community. The concentration of diffuse knapweed in such an area is often linked to the level of soil disturbance. Human disturbances often lead to less species diversity in a community. In turn, less species diversity can lead to unused resources, which allow invasive species to more readily establish. Areas such as fallow land, ditches, rangelands, residential and industrial districts and roadsides are all disturbed habitats where diffuse knapweed frequently establishes. Additionally, the removal of foliage and other ground cover increases the likelihood that seeds will come in contact with the soil and germinate.
The largest impact of humans on diffuse knapweed is efforts in controlling and eradicating its invasive populations. Besides reducing the spread of diffuse knapweed, efforts are also providing selective pressure against the individuals that cannot withstand a certain method of control. Selective pressure, given sufficient time, can cause the adaptation or evolution of invasive species such as diffuse knapweed. If an individual diffuse knapweed plant survives control efforts because of a trait it possesses, its progeny will make up a greater portion of the population than the plants that succumbed to the control.
Toward an integrated control strategy To successfully control diffuse knapweed, an understanding of the mechanism that allows it to be invasive must be developed. Isolating the reason for its invasiveness would allow control methods designed to specifically target the effectiveness of that mechanism to be developed. Additionally, precautions designed to minimize the invisibility of at-risk environments could be carried out.
Summary The success of diffuse knapweed must be attributed to a combination of several mechanisms. Its invasiveness is due to a mix of allelopathy, the enemy release hypothesis (ERH) and superior resource competition. However, the most importance must be attributed to the ERH because diffuse knapweed, while a very effective invasive species in its novel environment, is non-invasive and doesn't establish monocultures in its native range. It is the differences, biotic and abiotic, between its novel and native surroundings that cause it to be invasive.
To demonstrate that the ERH applies to diffuse knapweed, it is essential to show that the absence of natural enemies has a significant positive effect on its success. One way to show this is to observe the effect of introducing some of diffuse knapweed's natural enemies into its novel environment. If diffuse knapweed, which generally thrives in its invaded environment, is significantly inhibited through the introduction of natural enemies, it can be concluded that diffuse knapweed is more competitive in the absence of its natural enemies. A recent effort at biocontrol of diffuse knapweed in Idaho's Camas County effectively reduced of knapweed to minimal levels through the release of the lesser knapweed flower weevil and the knapweed root weevil. Since both of the insects released are natural competitors of diffuse knapweed, and since this and other similar efforts at biocontrol have been successful, there is significant evidence that diffuse knapweed benefits from the absence of its natural enemies.
Another aspect of diffuse knapweed's success relies on the effect of its allelopathic chemicals in its novel environment. Although there is still debate concerning the effectiveness of allelopathic chemicals in the field, the evidence of allelopathic effects demonstrated in a laboratory setting and its propensity to establish monocultures support the importance of allelopathy to diffuse knapweed's success.