Centaurea solstitialis

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Centaurea solstitialis

Yellow Starthistle

Kingdom:	Plantae
Division:	Magnoliophyta
Class:	Magnoliopsida
Order:	Asterales
Family:	Asteraceae
Genus:	Centaurea
Species:	C. solstitialis

Binomial name

Centaurea solstitialis
L.

Yellow starthistle (Centaurea solstitialis) is a member of the Asteraceae family, native to the Mediterranean region. The plant, also known as golden starthistle, yellow cockspur, and St. Barnaby’s thistle^[1] is a thorny winter annual in the knapweed genus.

It is a grayish-green plant with multiple rigid stems that extend in all directions from the base, forming a bushy-looking cluster that can reach two meters in height and more than that in diameter. It produces bright yellow flowers ringed with long, sharp spines. The plant grows quickly and is very competitive. It bears a taproot that can reach a meter deep into the soil, allowing it to thrive during dry, hot summers. It is versatile in its growth patterns, and can adapt to drought or low soil moisture content by producing smaller plants with fewer seeds during dry years.

Since its introduction to California in the mid-nineteenth century^[2], it has become a large-scale noxious weed throughout the United States, currently dominating over 15 million acres in California alone ^[3].

1 Invasive species
2 Biological control
3 References
4 External links

[edit] Invasive species

The plant grows as a normal part of the ecology in Eurasia, where it is kept in check by an assortment of natural enemies and other plants that have co-evolved with it in its native habitat.

Its introduction in North America probably occurred in California sometime after the start of the California Gold Rush, as a seed contaminant in Chilean-grown alfalfa seed, also known as Chilean clover (Trifolium macraei).^[2] After the turn of the century, Spain, France, Italy, and perhaps Turkestan were also likely sources of invasion.^[2]

It was dispersed into agricultural fields in California, and immediately took hold in the area's Mediterranean-type climate. Human factors such as mowing, domestic animal grazing, and cultivation of wildlands contributed to the success and spread of the plant. It is now a very common sight in pastures, fields, and vacant lots, and along roadsides.

By 1970^[4], yellow starthistle had reached 23 states.^[2] 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).^[5] The plant is considered invasive in six of the 41 states: California, Oregon, Washington, Idaho, Utah, and New Jersey.^[5]

The yellow starthistle plant has the ability to create monotypic stands in fields that prevent other species from growing there. Whole fields of solid yellow starthistle are not uncommon. Its growth plasticity, competitiveness, preference for the Mediterranean climate, and lack of natural enemies and co-evolved species make it a very successful invader. The plant is a pest in field crops, prevents domestic animals' grazing in rangelands, and acts as a physical barrier to wild animal movement in wildlands. It is toxic to horses.^[6]

[edit] Biological control

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C. solstitialis in California, autumn 2007.

Yellow starthistle is sometimes resistant to removal methods such as mowing, because of its long root system, and burning, due to the seeds' ability to withstand fire. The plant has been the target of biological pest control programs with positive results. Six types of seed-feeding insect have been found to be effective against the plant.

Three species of weevil in the beetle subfamily Cleoninae effectively reduce seed production in the yellow starthistle.
- Yellow starthistle 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 starthistle 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.
Three species of tephritid fruit fly also attack the seedheads of yellow starthistle.
- Yellow starthistle peacock fly and false peacock fly (Chaetorellia australis and C. succinea, respectively) are small nectar-feeding flies that deposit eggs into the seedheads, where their larvae consume the seeds and flower ovaries.
- Banded yellow starthistle gall fly (Urophora sirunaseva) produces larvae that pupate within a woody gall within the flower and disrupt seed production.

Additionally, a variety of the rust fungus Puccinia jaceae, first released in July 2003 on a ranch in the Napa Valley, has shown promise as an agent against yellow starthistle.^[7] The rust causes widespread pathology in the leaves of the plant and slow its dispersal.