Centaurea solstitialis

Centaurea solstitialis, yellow starthistle, golden starthistle, yellow cockspur, or St. Barnaby’s thistle, is a member of the Asteraceae that is native to the Mediterranean region. Since its introduction to North America in the mid-nineteenth century it has become a large-scale noxious weed there, currently dominating over 18 million acres (73,000 km²) in the United States alone.

Yellow starthistle 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.

Yellow starthistle as an 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. It was imported to the United States in the mid-1800s as a contaminant in a batch of alfalfa seed. It was dispersed into agricultural fields in California, and immediately took hold in this new environment with its own ideal 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. Most of the western United States has yellow starthistle infestations, with California and the Pacific Northwest being the most heavily affected areas.

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, acts as a physical barrier to wild animal movement in wildlands, and is toxic to horses.

Biological control
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. Additionally, a variety of the rust fungus Puccinia jaceae has shown promise as an agent against yellow starthistle. The rust causes widespread pathology in the leaves of the plant and slow its dispersal.
 * 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.