|The examples and perspective in this article deal primarily with North America and do not represent a worldwide view of the subject. (June 2013)|
Centaurea solstitialis, yellow star-thistle, is a member of the family Asteraceae, native to the Mediterranean Basin region. The plant is also known as golden starthistle, yellow cockspur and St. Barnaby's thistle (or Barnaby thistle) The plant is a thorny winter annual species in the knapweed genus.
Centaurea solstitialis is an annual herb from the Asteraceae family. During the vegetative stage if forms a rosette of non-spiny leaves (5–20 cm diameter). As the summer approaches, it produces a flowering stem (1 m) which will produce numerous spinous capitula containing numerous (10-50) yellow flowers. Flowers within capitula are pollinated by insects and each capitula will produce a mix of (10-50) pappus and non-pappus seeds. It is an annual semelparous species, and will die after reproduction is completed, normally by the end of the summer.
Centaurea solstitialis is a weed also on its native European range (e.g., Turkey, Greece, Italy, France, Spain), consequently, it inhabits highly disturbed ruderal habitats, being typically found on roadsides and cereal crop margins. After introduction in several parts of the world as an exotic species (e.g., Australia, Argentina, Chile, USA), it has developed local adaptations to the different habitats colonized, and an incipient level of reproductive isolation between native and non-native ranges has been detected. This makes this species a candidate case for incipient ecological speciation.
Centaurea solstitialis grows as a balanced part of the ecosystems in Eurasia, where it is kept in check by an assortment of natural herbivore enemies and other plants that have co-evolved with it in its native habitat. However, it has been introduced in several parts of the world, including Australia, Argentina, Chile, and the USA. In many of these non-native regions, where the particular array of natural biological controls found in the native regions do not exist, it has become an invasive species and noxious weed.
The Yellow star-thistle plant has the ability to create monotypic stands and habitats in the cultivated soil of fields, graded dirt sites, and disturbed natural ecosystem lands. Its colonization eliminates and prevents other plant species from growing, terminating the habitat's biodiversity. Extensive spreading monotypic fields of yellow starthistle are not uncommon. Its growth plasticity, competitiveness, preference for the Mediterranean climate, and a lack of natural herbivore enemies and co-evolved species, make it a very successful invader. The plant is an invasive pest in field crops, degrades native plant habitats and natural ecosystems, prevents the grazing of domestic animals in rangelands, and is a physical barrier to indigenous animal movement in wildlands.
The introduction of Centaurea solstitialis in North America probably occurred in California sometime after the start of the California Gold Rush, as a fodder seed contaminant in imported Chilean-harvested alfalfa seed, also known as Chilean clover (Trifolium macraei). Introduction in other parts of the world is poorly known.
In California, Yellow star-thistle was dispersed into agricultural fields, and immediately took hold in the state's Mediterranean-type climate areas. Human factors, such as mowing, land grading for development and roads, domestic animal grazing, and disturbance of the soil surface for agricultural tillage and wildland firebreaks have and continue to contribute to the successful thriving and spread of this plant. Yellow starthistle is now a very common sight in vacant lots and fields, along roadsides and trails,in pastures and ranch lands, and in parks, open-space preserves and natural areas.
After the turn of the 20th century, Spain, France, Italy, and perhaps Turkestan were also likely sources of the invasion's seed in California. Since its introduction to California in the mid-nineteenth century, it has become a large-scale invasive species (noxious weed or invasive exotic) throughout twenty-three states. It currently dominates over Script error: No such module "convert". in California alone.
By 1970, yellow star-thistle had reached 23 U.S. states. According to the USDA Forest Service, as of 2006 the plant has been reported present in 41 of the 48 contiguous U.S. states, with the only exceptions being Maine, Vermont, and five of the Deep South states (Arkansas, Louisiana, Mississippi, Alabama, and Georgia). The plant is considered an invasive species in six of the 41 states: California, Oregon, Washington, Idaho, Utah, and New Jersey.
Most herbicides used for controlling Yellow Star-thistle are registered for range lands, right-of-way, and other non-crop areas. Many auxin-like or growth-regulator herbicides are used for post-emergence control, including 2,4-D, aminopyralid, clopyralid, dicamba, picloram and triclopyr. Alternatively, glyphosate (e.g., Monsanto "Roundup") may be used. Pre-emergence herbicides used for yellow star-thistle control include chlorsulfuron and sulfometuron. Pre-emergence and post-emergence herbicides may be used effectively together to kill growing plants as well as any new seedlings that may emerge from the copious soil seed bank often produced by yellow starthistle. Controlled burning may also be used in conjunction with clopyralid application as an effective integrated approach to yellow star-thistle management.
A yellow star-thistle biotype resistant to picloram was discovered in a pasture near Dayton, Ohio in 1988. This biotype was determined to have cross resistance to other auxin-like herbicides, especially clopyralid. Resistance was discovered to be conveyed by a single nuclear recessive gene.
Yellow star-thistle is sometimes resistant to removal methods such as mowing and burning, because of its long root system and the seeds' ability to withstand fire. The plant has been the target of biological pest control programs with positive results. Seven types of seed-feeding insects have been released (one accidentally) to control the plant.
- Three species of weevil in the beetle subfamily Cleoninae effectively reduce seed production in the yellow star-thistle.
- Yellow star-thistle bud weevil (Bangasternus orientalis) is a fuzzy brown weevil that lays its eggs in the flowers, and when its larvae hatch, they feed on the developing seed.
- Yellow starthistle hairy weevil (Eustenopus villosus) is a long-snouted, hairy-looking weevil that lays a single egg inside each flower bud. The larva then consumes the seeds within.
- Yellow star-thistle flower weevil (Larinus curtus) is a brownish weevil that lays eggs in the flowers as it feeds on the pollen. The larvae then eat the seeds when they hatch. This insect has failed to established dense populations in the United States.
- Four species of tephritid fruit fly also attack the seedheads of yellow star-thistle.
- Yellow star-thistle peacock fly and false peacock fly (Chaetorellia australis and Chaetorellia succinea, respectively, the latter released unintentionally) are small nectar-feeding flies that deposit eggs into the seedheads, where their larvae consume the seeds and flower ovaries.
- Banded yellow star-thistle gall fly (Urophora sirunaseva) produces larvae that pupate within a woody gall within the flower and disrupt seed production.
- Another gall fly Urophora jaculata was released in 1969 but never established.
A variety of the rust fungus Puccinia jaceae var. solstitialis, first released in July 2003 on a ranch in the Napa Valley, has shown promise as an agent against yellow star-thistle. The rust causes widespread pathology in the leaves of the plant and slow its dispersal. The fungus Synchytrium solstitiale (Synchytrium of phylum Chytridiomycota) is also being considered as an agent of biological control. 
Although these biocontrol agents have proven somewhat effective at controlling Centaurea solstitialis, there is interest in finding additional agents to further control the species. Two weevils, including Larinus filiformis, a flowerhead feeding weevil, and Ceratapion basicorne (Illiger) (Coleoptera: Apionidae), which develops in the root crown of rosettes have been evaluated and proposed for introduction. Also under evaluation are a stem-feeding flea beetle Psylliodes chalcomera (Illiger) (Coleoptera: Chrysomelidae) (Cristofaro et al. 2004a), a lacebug Tingis grisea Germar (Heteroptera: Tingidae), and a mite Aceria solstitialis de Lillo (Acari: Eriophyidae). (de Lillo et al. 2003).
Species similar to Centaurea solstitialis are purple star-thistle (Centaurea calcitrapa), sulphur star-thistle (Centaurea sulphurea), Maltese star-thistle (Centaurea melitensis), and rough star-thistle (Centaurea aspera).
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- USDA - " Larinus filiformis as a Biological control agent for Centaurea solstitialis (flowerhead feeding weevil)
- Balciunas, J. K., and B. Villegas; "Unintentionally released Chaetorellia succinea (Diptera: Tephritidae): Is this natural enemy of yellow starthistle a threat to safflower growers?"; Environ. Entomol 30: 953-963; pub. 2001.
|40x40px||Wikimedia Commons has media related to Centaurea solstitialis.|
- Species Profile--Yellow Star Thistle (Centaurea solstitialis), National Invasive Species Information Center, United States National Agricultural Library. Lists general information and resources for Yellow Star Thistle.
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- U.C. Jepson Manual treatment - Centaurea solstitialis
- California Integrated Pest Control - Biocontrol of Centaurea solstitialis -Yellow star-thistle
- Centaurea solstitialis - U.C. CalPhoto gallery
- Texas AgriLife Extension - Toxic Plants of Texas - Starthistles - Yellow Starthistle is toxic to horses