Canada Thistle Biological Control Program
In spite of its common name, Canada thistle (Cirsium arvense (L.) Scop.) is an introduced, invasive, plant species native to southeastern Europe and the eastern Mediterranean. Canada thistle (also called Californian thistle, Canadian thistle, creeping thistle, corn thistle, perennial thistle, and field thistle) is a member of the sunflower family (Asteraceae) and was likely introduced into the eastern Canadian maritime provinces of North America during the 1600s as a contaminant of crop seed and/or ship's ballast. The plant has the dubious distinction of being one of the United States’ first noxious weeds with control legislation enacted by Vermont in 1795 followed by New York in 1831. By 1918, Canada thistle was on the noxious weed lists of 25 northern tier states. Today, the species had been declared noxious in 33 U.S. states and 6 Canadian provinces with introductions throughout Europe, northern Africa, South Africa, central and western Asia, northern India, China, Japan, southeastern Australia, Tasmania, and New Zealand.
Habit: Canada thistle is a herbaceous perennial with erect, branching stems that grow 50 to 120 cm tall. The seed produces a smooth, fleshy cotyledon (seed leaf) upon germination and forms an initial basal rosette of oval-shaped, soft-spined leaves after germination. The seedlings of Canada thistle begin to appear in mid-May and grow slowly after germination, at which time they are susceptible to competition and over-shading from other plants. Also during this time, new shoots emerge from the stem bases and root buds of last season’s plants. As seedlings develop they utilize most of their energy into producing an extensive, rhizomatous root system. The root system of Canada thistle is capable of producing new shoots and roots approximately 3 to 5 weeks after germination. Once developed, this root system produces horizontal or lateral roots some 4.5 meters or more away from the parent plant and vertical roots penetrating 1 to 3 meters deep. The plants bolts (sends up a flowering stalk) in mid- to late-June through July and develops a slender, grooved, smooth stem that becomes covered with fine hair as they age. The stems die back to the ground during late fall and winter, with new shoots produced each spring from old stem bases or root buds.
Leaves: The mature rosette of Canada thistle is composed of numerous irregularly lobed, spine-tipped, leaves that are approximately 5 to 13 cm long by 3 to 5 cm wide, and vary greatly in the amount of lobing, spine length, and hairiness. Leaves of the flowering stem are alternate, irregularly lobed, with prickly margins, green on both sides, and have a smooth to slightly downy upper surface and a smooth to densely gray-hairy lower surface. The lower stem leaves are sessile or have a winged petiole (leaf stalk) and the upper are sessile and decrease in size towards the tip of the stem.
Flowers: Like other members of the sunflower family, the “flower head” of Canada thistle is an oval shaped aggregation of small, individual flowers, 10 to 20 mm long, called a capitulum or head inflorescence. However, unlike other introduced thistles, the flowers of Canada thistle have traditionally been considered dioecious, that is the flowering shoots of a single plant are either male or female. In actuality, research has found that as many as 26% of the "male" plants in a population of Canada thistle are self-fertile hermaphrodites (male and female flowers on the same plant) capable of producing seeds. The incidence of hermaphrodism in a population of Canada thistle varies significantly by locality with great care needing to be taken during plant identification as hermaphroditicflowers closely resemble typical male flowers.
The small, individual flowers of Canada thistle, called florets, are attached to a shallowly depressed base called a receptacle. The receptacle and florets are surrounded by a collection of modified leaves (or bracts) called an involucre. The bracts of Canada thistle are in 6 to 8 rows or series, oval to linear in shape with a purple tinge, and strongly overlap one another. The florets of the “male” flowers are purple colored (rarely pink or white), 12 to 18 mm long and remain longer than the ring of white bristles (pappus) surrounding the seed at maturity. The florets of the female flowers are also purple colored however they are 14 to 20 mm long and are overtopped by the pappus bristles in fruit. Interestingly, Canada thistle is a long-day plant that requires 14 to 16 hours of daylight to produce flowers. As such, plants bloom from June to August at the tips of the main stem and branches with 32 to 69 (up to 100) flowering heads per shoot and up to 100 florets per head.
Seeds: Seeds of Canada thistle are pale brown, slightly tapered, 3 to 5 mm long, smooth, with a ring of white bristles on one end. Seed set is highest when male and female plants are intermixed, with decreasing fertility rates when female plants are more than 50 m from male plants. The average female plant produces 40 to 59 seeds per flowerhead while hermaphroditic male plants average 2 to 10 seeds per head. Seed production is much more reliable with insect pollination (40 to 85 seeds per flowerhead) as opposed to wind pollination (1 to 2 seeds per flowerhead) with single flowering shoots capable of producing 1,500 seeds (up to 5,300 seeds) per year.
Current Distribution and Status
Canada thistle is widely distributed within North America and occurs throughout Canada and in 43 of the 50 states within the U.S. (absent from Florida, Georgia, Hawaii, Louisiana, Mississippi, Oklahoma, and South Carolina). The plant is listed in 33 states as a regulated weed including Alaska (noxious weed), Arizona (prohibited noxious weed), Arkansas (noxious weed), California (B list - noxious weeds), Colorado (B list - noxious weed), Connecticut (potentially invasive, banned), Delaware (noxious weed), Hawaii (noxious weed), Idaho (noxious weed), Illinois (noxious weed), Indiana (noxious weed), Iowa (primary noxious weed), Kansas (noxious weed), Kentucky (noxious weed), Maryland (noxious weed), Michigan (noxious weed), Minnesota (prohibited noxious weed), Missouri (noxious weed), Montana (Category 1 noxious weed), Nebraska (noxious weed), Nevada (noxious weed), New Mexico (Class A noxious weed), North Carolina (Class B noxious weed), North Dakota (noxious weed), Ohio (prohibited noxious weed), Oklahoma (noxious weed), Oregon ("B" designated weed, quarantine), Pennsylvania (noxious weed), South Dakota (noxious weed, regulated non-native plant species), Utah (noxious weed), Washington (Class C noxious weed), Wisconsin (noxious weed), and Wyoming (noxious weed).
Within Kansas, Canada thistle has been documented throughout the state. Historically, northwestern Kansas has been most impacted by this invasive plant. Canada thistle was declared a noxious weed by the Kansas legislature in 1961 and is one of 14 plants species regulated under the Kansas Noxious Weed Law (K.S.A. 2-1314 et seq.). In 2009, over 16,000 acres of land was reported as being infested with Canada thistle within Kansas.
Habitat: Throughout North America, Canada thistle can be found in most upland herbaceous communities within its range and over a wide array of elevations from sea level to elevations in excess of 2,500 m (8,000 feet). The plant favors prairie communities and riparian corridors that have deep, well-aerated, mesic soils with fluctuating water levels but also invades meadows, old fields, pastures, croplands, and waste places. In eastern North America the plant will also rarely invade xeric (very dry) habitats, such as sand dunes, sandy fields, barrens, and glades, in addition to the edges of wetland habitats such as stream banks, lakeshores, cleared swamps, muskegs and ditches. Canada thistle is shade intolerant but will grow in all but the most waterlogged, poorly aerated soils, including clay, clay loam, silt loam, sandy loam, sandy clay, sand dunes, gravel, limestone, and chalk. Research has documented that Canada thistle can tolerate soils with up to 2% salt content. The plant achieves maximum growth between 0 - 32 °Celsius and 400-750 mm of precipitation per year but tolerates a wide range of moisture conditions (ranging from 305-1,015 mm per year).
Spread and Establishment Potential: Canada thistle reproduces both sexually by seed and vegetatively by creeping rootstalks. In general, vegetative reproduction provides for spread and colonization of local plant populations while seeds provide a means of long distance dispersal. Dispersal into new sites is mostly accomplished by windblown or water scattered seed, or by seed and/or root contamination in crop seed, hay, cattle and horse droppings, or machinery. Canada thistle seeds mature quickly and are capable of germinating 8 to 11 days after the flowers open, even if the plants are cut when flowering. Seed bank survival is related to the depth upon which the seeds are buried with seeds surviving up to 22 years when they are covered more than 20 cm deep by soil. Under more shallow conditions or with periods of soil disturbance the seeds of Canada thistle are more short lived (<5 years) with most being lost from the seed bank by germination during the 1st year. However, viability of seeds during the 1st season after dispersal may be as high as 90%.
Canada thistle spreads primarily by vegetative growth of its extensive roots system, growing horizontally as much as 6 m in one season. The greatest emergence of root-originating sprouts (adventitious root buds) occur during the spring, however an additional surge of growth also takes place during the fall months. Furthermore, anytime during the growing season when soil moisture levels are adequate the plant has the potential for an eruption of root-originating growth (adventitious shoots). Most plants spread at the rate of 1 to 2 m per year with individual plants capable of reaching 35 m in diameter. Studies have documented that one plant of Canada thistle is capable of producing 26 adventitious shoots, 154 adventitious root buds, and over 110 m of roots after 18 weeks of growth. If the main shoot of the plant is removed during the growing season, such as by mowing, the adventitious buds along the entire length of the root system are stimulated and new shoots emerge rapidly, especially under high humidity conditions. If plants are plowed, root fragments as short as 6 mm long and 3 mm in diameter that are more than 6 weeks old (but less than 2 years) can regenerate entire plants, regardless of whether they have identifiable root buds at the time of separation. Additionally, these root sections can survive at least 100 days without nutrient replenishment from photosynthesis. As a result, cultivation by disking or plowing often exacerbates the weed problem in most locations, assisting infestations in spreading further.
Canada thistle has been reported to accumulate nitrates that cause poisoning in animals. The plant has also been reported to produce phytotoxins that inhibit the growth (allelopathic) of other plants nearby. Canada thistle also has a unique chemical process (Fructan metabolism) that allows it to grow at relatively cool temperatures, thereby adding to its competitive advantage.
Damage Potential: Canada thistle is an extremely aggressive plant species that has the ability to adapt to a wide range of habitats. Dense stands of Canada thistle growing in corn, soybeans, and wheat fields have been shown to reduce crop yields by 80%, 95%, and 60% respectively. In alfalfa stands grown for seed production, Canada thistle can reduce yields by 48 percent. Within the Canadian Prairie Provinces of North America, Canada thistle occurs on 40 percent of cultivated lands with the loss of wheat production in Saskatchewan alone estimated to be worth $4 million in 1980. Estimation of lost production within Nebraska crop and pasture lands due to Canada thistle infestation is over $2.5 million each year. Additionally, Canada thistle has been shown to be a host for bean aphid, stalk borer, and sod-web worm which damage corn and tomato plants as well as cucumber mosaic virus.
In pastures, livestock tend to dislike and avoid the prickly leaves of Canada thistle thereby reducing their consumption of desirable plants around Canada thistle colonies. Native prairie forage and hay production has been shown to be reduced by as much as 60 percent in dense infestations of Canada thistle. Within non-cropland ecosystems, Canada thistle can crowd out and replace native grasses and forbs limiting an areas ecological value, species richness, and threatening endangered species. In horticultural gardens and turf lawns, Canada thistle’s extensive root system makes it particularly difficult to control. Mowing and/or pulling are ineffective due to the large number of adventitious root buds formed by the plant.
Biological control offers the ability to provide long term control for exotic weed species. The goal of biological control is to reduce the pest population, not to eradicate it. In Kansas, the Canada thistle stem weevil (Ceutorhynchus litura) is being utilized as a biological control insect for reduction of Canada thistle infestations.
Canada Thistle Stem Weevil (Ceutorhynchus litura)
Description: Canada thistle stem weevil is a small, gray and black mottled, weevil approximately 4 mm long, with a long snout. The larvae are white grubs, approximately3 mm long, with a tapered body and a tan, rounded head.
Lifecycle: Canada thistle stem weevils have one generation per year. Adults emerge from soil litter during the spring months (late March to mid-June) with peak emergence during late April. When adult female weevils emerge they deposit their eggs in the middle vein of the emerging Canada thistle rosettes. The larva hatch and bore their way into the main vein of the leaf and ultimately into the stems and crowns of infected plants as the plant bolts during the spring to summer months. The larvae burrow their way out at the base of the plant, leaving a large exit that opens the stem up to secondary parasitism and fungal infection. The larvae then pupate in the soil and emerge as adult weevils during mid-August through September. The newly emerging adults overwinter in the soil litter near the parasitized host plant.
Control: Originally introduced into the United States in 1972, the Canada thistle stem weevil provides some measured impact on populations of Canada thistle. Stunting and swelling of stems is often observed in parasitized plants, however most damage is caused by secondary pathogens that enter the plant via holes made by exiting larvae. With the difficulty in controlling Canada thistle due to the plants ability to reproduce sexually by seeds and asexually by its extensive root system, studies have suggested that the greatest control success may be achieved with a combination of Canada thistle stem weevil and other biological control agents such as musk thistle crown weevil (Trichosirocalus horridus) or Canada thistle stem gall fly (Urophora cardui). Additional mechanisms such as stewardship practices, mechanical control, and herbicides should also be investigated for an integrated management strategy in reducing Canada thistle infestations.