Gorse is a spiny shrub that can form fast growing impenetrable thickets up to 4 m tall in New Zealand. Even-aged stands can live up to 29 yr. Plants can produce 34,000 seeds per m-2 per annum, and the seeds can be long-lived in the soil. Gorse is highly invasive. Blaschke recorded it in varying densities on 700,000 ha of New Zealand. This is approximately 5% of the land area not occupied by indigenous forest, or alpine or sub-alpine vegetation. It covers land suitable for agriculture. It shades out tree seedlings in new plantation forests, and then reduces growth of surviving trees, and invades vulnerable natural habitats. Gorse is combustible, and increases the fire risk in forests and peri-urban areas.

The first attempt to control gorse biologically was one of the earliest undertaken worldwide. The value of gorse as an inexpensive live fence, and shelter plant was taken into account, and the initial search for agents in Europe was restricted to those insects that damaged flower buds, flowers, and pods. Davies recorded that 92% of the pods that he examined in England were infested with gorse seed weevil Apion ulicis Forster, and recommended this species as a control agent. The weevil was duly imported into New Zealand in 1928, and was widely released from 1931 to 1947. Soon the proportion of pods infested in spring approached the levels observed in England by Davies, and Miller intimated that successful control was expected. However, the high levels of pod infestation recorded were misleading. Gorse can form seeds in both spring and autumn, while E. ulicis is only active in spring. Later studies have revealed that where the bulk of annual seed production is in autumn, infestation of the small number of pods formed in spring often exceeds 90%. However, where most seed develops in spring, production swamps the weevil, and the rate of seed destruction is lower. Cowley found that although infestation of pods in spring was high in the area she studied, seed weevil only reduced the annual seed crop by about 35%. Markin and Yoshioka observed that feeding by adult E. ulicis caused significant damage to gorse foliage.

A population of gorse spider mite was introduced to New Zealand in 1989, but failed to establish in the warmest and wettest regions. A temperature-driven population model gave excellent prediction of mite phenology, and indicated that mites would complete 6 generations in the north of New Zealand, and 4-5 in the south. However, the model suggested that temperature alone was not the determinant of establishment success . Millar showed that the reproductive performance of strains varied under artificial rain. Further populations were imported from NW Spain and Portugal and released. Comparative trials in a region where the mites from England originally failed to establish indicated that the new populations performed better. Gorse spider mite is now established throughout New Zealand.

Bull thistle, Cirsium vulgare (Savi) (Asteraceae), is a widespread exotic biennial weed that is associated with a high degree of disturbance, such as overgrazed permanent pasture and woodland clearings. The bull thistle seedhead gall fly, Urophora stylata (Fabricius) (Diptera: Tephritidae), is a host specific biological control agent that has been released into California from Europe for the biological control of bull thistle. The bull thistle seedhead gall fly has one generation per year. The adult flies emerge from overwintering seedhead galls from late May through early July and may live up to two months. Oviposition occurs on top of developing flower buds, and the eggs hatch after about one week. After hatching, the larvae migrate to the receptacle where they induce gall tissue formation which the larvae feed on. A multi-chambered gall forms on the receptacle of the seedhead with individual larvae occupying separate chambers of the gall. Each gall may contain from five to 20 larvae. Seedheads infested with the gall flies produce less seed due to the limited amount of receptacle area for seed production.

Although no economic assessments have been conducted for yellow starthistle, millions of dollars in losses probably occur from interference with livestock grazing and forage harvesting procedures, and lower yield and forage quality of rangelands. Because of the spiny nature of yellow starthistle, livestock and wildlife avoid grazing in heavily infested areas. Thus, yellow starthistle can greatly increase the cost of managing livestock. Although the nutritional component of yellow starthistle leaves is high during the growing season, its nutrient value declines as the plants mature. Yellow starthistle in the pre-spiny stage contains between 8 to 14% protein. However, an analysis of the nutritional status of cattle manure in the fall indicated that yellow starthistle-infested pastures contain considerably less crude protein and total digestible nutrients compared to uninfested pastures.

Starthistle weevils are univoltine, meaning they complete one generation per year. The adult overwinters in protected areas on the ground and become active the following spring. Thus, the first weevils seen in the spring are from the overwintering generation and the new weevils that emerge mid- to late summer are from the new generation. As adults, weevils generally cause little damage to the plant. They may do some feeding of the foliage, but the amount is usually negligible. it is the internally-feeding juvenile stage, or larva, that causes damage. Larvae undergo three molts (or instars) during their development inside the starthistle head.

Yellow Starthistle Gall Fly adults are black with a yellow triangle on the back of the thorax. Wings are marked with dark crossbands. The adults are approximately 0.2 to 0.24 inch (3 to 5 mm) long. The female is typically longer than the male and has an obvious ovipositor.

Urophora sirunaseva overwinters in galls in the yellow starthistle seedhead as a mature larva and pupates inside the gall in the spring; pupation lasts 4 to 5 weeks.

First generation adult flies emerge from galls in the spring, about 2 to 4 weeks after C. australis, and begin mating within 3 to 4 days. Females then begin oviposition. Eggs are laid on the top of BU-2 or BU-3 buds where the points of the smaller bracts emerge. The eggs are white and spindle-shaped, and about 0.006 inch (0.15 mm) long, and hatch within 9 or 10 days.

After hatching, the larvae eat through the florets in the head, and can be found on and between the florets. When the larvae reach the receptacle, the gall begins to form. Tissues begin to grow and change consistency, forming a hard gall around the larva. There is one larva per gall and up to four galls per seed head. Galls of the summer generation larvae are thin and delicate compared with the thicker galls formed by the overwintering generation.

Larvae of this generation feed and develop for about 3 weeks. Once the larvae reach maturity, they enter diapause and overwinter in the gall formed in the yellow starthistle seedhead for approximately 7 months. Internal larval feeding reduces the number of developing seeds in the head by approximately 50 percent. Heads infested with this gall fly produce fewer seeds than heads infested with Chaetorellia flies.

The Yellow Starthistle Hairy Weevil, Eustenopus villosus, is our best new insect for this poisonous weed. Adult weevils feed externally on the flowers and larvae feed within, destroying seed production. A good disperser, this agent can become widespread and have a significant impact on starthistle seed production. These flies can have 2 or 3 generations depending on the length of the growing season.

Yellow Starthistle Peacock Flies are straw-colored with several black spots on the thorax and light brown wing bands. They are about 0.12 to 0.24 inch (3 to 5 mm) long; females are typically longer than the males and have an ovipositor. C. australis can be distinguished from C. succinea by the number of black spots in the thorax

Chaetorellia overwinters in yellow starthistle seedheads as a mature larva and pupates when yellow starthistle is in the rosette stage. The new generation adults emerge in the spring during the yellow starthistle bolting stage and feed on plant nectar.

First generation adults develop on bachelor’s button (C. cyanus). Summer generation adults generally emerge at the BU- 3 stage. Female flies oviposit during the BU-3 or BU-4 bud stage. Eggs are laid singly at the lateral walls of the closed capitulas beneath an bract of a flower head. The eggs are white, spindle-shaped and have a long characteristic filament thickened at the distal end, which can extend beyond the margins of the bract. Eggs hatch within 2 to 4 days. The development from egg to adult takes about 4 weeks.The fly larvae tunnel into the center of the head, where they feed on the ovaries and developing seeds during the BU-4 through flowering stages.

Internal larval feeding reduces the number of developing seeds in the bud by 80 to 100 percent. This agent disperses very well, thus it is widespread in most areas it was established.

Although admired for its yellow blooms, the Scotch broom shrub has become an invasive species throughout the Pacific Northwest, where it competes with native plants and forms dense stands that are difficult to manage and remove.

Scotch broom was introduced from Europe as a garden ornamental by early settlers of the Pacific Coast. Later it was used to prevent erosion and stabilize banks and sand dunes. The woody shrub establishes quickly in disturbed areas.

Two USDA approved insects, a stem boring moth, Leucoptera spartifoliella, and a seed beetle, Apion fusciostre, were introduced in the 1960s as biocontrol agents, but have had limited success in California. New insect biocontrol agents are being tested in England and France for use on broom in Australia and New Zealand. If proved safe and effective in California, these insects may ultimately become available for use as biocontrol agents in California.

The ash whitefly invaded California in 1988 and quickly became a serious pest of several commonly planted urban trees. Adults filled the air in some neighborhoods to such an extent that they posed a health threat due to breathing impairment. A single species of parasitic wasp was imported from Europe, the origin of the ash whitefly. Within two years of its release, this wasp greatly reduced the ash whitefly population. Today, although both insects are still present in California, they are difficult to detect in ash and ornamental pear, trees previously heavily infested by the ash whitefly. The California Department of Food and Agriculture has an ongoing monitoring program that is tracking the long-term impact of the Encarsia inaron, the wasp imported from Europe.

Italian Thistle has no known beneficial economic value, but it has invaded range and pastures. Native vegetation is threatened by this aggressive weed that tends to form dense stands.

The seed weevil Rhinocyllus conicus Froelich was found to attack thistles in the genera Carduus, Onopordum, Silybum, and Cirsium in central and southern Europe. Intensive studies by the CIBC demonstrated the weevil’s noneconomic status, restricted host range, and potential effectiveness as a biological control agent. It was first imported into Canada and liberated in mid-1968 in Saskatchewan for biological control of musk thistle and in Ontariofor control of plumeless thistle.

Puncturevine (Tribulus terrestris) is an aptly named summer annual found widely in California. Native to southern Europe, it can grow under a wide range of conditions, but its success is likely due to its ability to thrive in hot and dry conditions where other plants cannot. It can be a major problem in orchards, pastures, turf, and along roadsides and ditch banks. Although it is known to be toxic to sheep, its main weedy characteristic, as indicated by its common names of puncturevine or caltrop, is its spiky seedpods (A caltrop is a metal device, used to deter passage by vehicles with pneumatic tires or the hooves of horses; it has four projecting spikes so arranged that when three of the spikes are on the ground, the fourth points upward to poke a tire or hoof.) The seeds of puncturevine are enclosed in a hard caltrop-like case that can injure livestock, people, and pets when stepped on and can even puncture bicycle tires. Another common name is "goathead."

Female Seed Weevils chew into the side of a young puncturevine bur, deposit eggs into the seed and seal it with fecal material. Females may deposit up to 324 eggs. Weevils grubs develop inside the seed and pupate therein. Each seed may produce 1-3 weevils. The life cycle from egg to adult requires about 25 days. Adult weevils may feed on the plant but do not cause appreciable damage to the plant. The number of generations per year depends on the climate. Adults overwinter.

The first release of Microlarinus lareynii for the biological control of puncturevine in the United States occurred in 1961. In the early summer months of 1961, Microlarinus lareynii seed weevils were released in Clark County, Nevada and Stanislaus County, California. Subsequent releases were made in Arizona, California, Colorado, Utah and Washington. The weevil became established in Arizona, California and Nevada but failed in Colorado, Washington and Utah. Today, puncturevine weevils are widely established throughout the United States.

Like yellow starthistle, purple starthistle is extremely aggressive and poses a major threat to annual rangelands. In California, from San Diego County in the south to Humboldt County in the north, this noxious weed has dominated native plants and has choked out a fragile ecosystem. The plant is unpalatable to both wildlife and livestock due to its poor taste and its spine-covered flowers and seedheads. Unmanaged populations of the plant threaten the quality of grazing lands, thrive in healthy, moist soils, reduce forage production, and decrease the overall health of native plants. Animals avoid purple starthistle infestations, thereby reducing grazing potential and causing a reduction in the cattle carrying capacity of rangelands. In recreational areas, such as parks and public lands, the weed degrades the usefulness of the land and inhibits proper management.

the Seed Head Weevil Bangasternus orientalis was introduced in 1985. It is now widely established in California, Oregon, Washington and Idaho and is abundantly available for collection and redistribution. Yellow starthistle is also a host for this weevil.

B. orientalis is a good flier and disperses well.

source: http://www.nwcb.wa.gov/weed_info/Written_findings/Centaurea_calcitrapa.html
source: http://www.globalrestorationnetwork.org/wp-content/uploads/cipm_pdfs/res%20883.pdf
source: http://www.invasive.org/weeds/starthistle/chapter2.html

Klamath weed (Hypericum perforatun L.) is known in Europe, where it originates, as St. Johnswort. It was first reported in California about 1900, when it was discovered in the vicinity of the Klamath River. It is an aggressive plant, and since its discovery along the Klamath River it has taken over about 400,000 acres of California open range grazing lands, greatly cutting their carrying capacity.

Klamath weed is detrimental to the thrift of sheep and cattle, particularly cattle. Though the weed is not palatable, livestock will graze it when forced to do so by the absence of better forage. The eating of the weed causes the white-skinned portions of the animal's body to become photosensitized-sensitive to the sunlight. Blisters form on these areas when the animal is exposed to the direct rays of the sun with the result that the animal becomes scabby, soremouthed and underweight.

After the initial North American release in 1946, the klamoth weed beetle C. quadrigemina multiplied so quickly that at one location two years later, 5000 beetles, a small fraction of the total population, were collected for redistribution. By 1951, populations were established in all 21 California counties in which Klamath weed was a problem, and the plant was already under control in some of the original locations. By 1957, the plant was reduced by 99% in Northern California.

Canada thistle (Cirsium arvense) is a perennial, developing from deep and extensive horizontal roots. Stems are 1 to 4 feet tall, ridged, branching above. Leaves are alternative, lacking petioles, oblong or lance-shaped, divided into spiny-tipped irregular lobes. Flowers are unisexual, on separate plants, flowers purple (occasionally white) in heads 1/2 to 3/4 inch in diameter. These male and female flower heads make the Canada thistle unique from other true thistles. The weed is a native of southeastern Eurasia. It was introduced to Canada as a contaminant of crop seed as early as the late 18th century. This aggressive weed is difficult to control.

In the earlier part of the period 1961 to 1984, host specificity testing for agents attacking Canada thistle was focused on assessing potential risks to economic species of Cardueae, of which the two most important are safflower (Carthamus tinctorius L.) and globe artichoke (Cynara scolymus L.). In later studies, some native North American Cirsium species also were tested, but potential impacts of most agents on native non-target Cirsium species were not assessed in detail.

Plumeless thistle, Carduus acanthoides L., is an introduced Eurasian noxious weed in pastures, rangelands, croplands, and along highways in 19 of the contiguous states in the United States.

Plumeless thistle prefers fertile soils developed over limestone, but it is highly adaptable and can even grow in shallow soil, emerging from stone quarries. Infestations of plumeless thistle reduce productivity of pastures and rangeland by suppressing growth of desirable vegetation and preventing livestock from eating plants growing in the vicinity of thistle stands. It is very persistent and has the ability to regenerate because of the longevity and large number of seeds that it produces.