Centaurea solstitialis

Description

Centaurea solstitialis is an annual herb from the Asteraceae family. During the vegetative stage it 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.

Ecology

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.

Invasive species

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.

Impacts

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.

Toxicity in horses

Grazing of the plant by horses can cause igropallidal encephalomalacia or "chewing disease", a neurological condition. The disease generally follows consumption of 60-200% of the horse's body weight over an extended period of a month or more, or 2.3-2.6 kg of starthistle per 100 kg body weight per day. Though starthistle is most dangerous when it is the only plant available or is delivered as a contaminant in dried hay, horses may develop a taste for it and seek it out. Many other species of grazers, including mules and burros, are not affected.

Holistic Perspective

Star thistle is a valuable source of pollen, thus nectar for pollinators. Yellow star thistle, a noted pest plant, is a major nectar source for many central valley and foothill butterflies. Star Thistle populates ground that has been abused: dry, compacted, or scraped clean. A plant with a tap root system, it has a crucial role in restoring the soil by bringing up vital micronutrients. Similar to many plants classified as 'weeds', they "quickly establish in, protect, and restore soil that has been left exposed by natural and human-caused disturbances".

Species introduction

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. In addition to the United States, starthistle has been introduced throughout Africa, Europe, and North and South America.

Range increase

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 15,000,000 acres (61,000 km²) 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.

Chemical control

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.

Biological control

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.

Insects

Fungi

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.

Grazing

Grazing by goats, cattle, or sheep can be effective in controlling Yellow star-thistle. Goats will eat star-thistle even in its spiny stage.Because Yellow starthistle growth is particularly difficult to inhibit in canyon rangelands since its remoteness limits control options, goats and other herbivores have become an excellent option to curb the plant's spread. According to one study, grazing has decreased yellow starthistle presence by 58% when compared to the study's controls. Subplots also showed a 94% decrease in seed heads after only three years of experimentation.

Integrated Methods

California researchers (Thomsen et al., 1996) tested mowing, controlled sheep grazing and subterranean clover plantings to control starthistle growth. According to the researchers, subclover would help fill the void left by starthistle populations.

They compared subclover seeding, grazing, two mowings, grazing + mowing without subclover - and an untouched control without grazing, subclover, or mowing. Thistle seed production was 130 times higher where only one mowing was done, and 1,720 times higher where nothing was done, as compared to the area that had been grazed and mowed twice. Excellent yellow starthistle control can be achieved with the combination of competing plants, mowing, and rotational grazing.

Research

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).

Similar species

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).