Integrated Pest Management Manual
Thistles
Thistles include many species of composite plants in the subfamily Cynareae. Although most species in this subfamily are native and are beneficial for wildlife, a number of introduced species are well known as serious weeds of crops and rangelands. The pest status of introduced species results from the lack of population suppression exerted by natural enemies (e.g., insect herbivores and diseases). Thus, long-term goals in thistle management emphasize biological and cultural controls, although emergency intervention with chemical or mechanical controls may be necessary.
IDENTIFICATION AND BIOLOGY OF THISTLES
Five species of thistle are currently considered major pest weed species by the National Park Service and are under chemical, biological, or cultural control programs in North America. Other species, such as Carduus acanthiodes (plumeless thistle), C. crispus (welted thistle), and C. macrocephalus (nodding thistle), may be pest weeds in local areas. The five species listed below were introduced from Europe and North Africa into North America.
Musk thistle (Carduus thoermeri Weinmann). Originally thought to be C. nutans (L), C. thoermeri is only found in small isolated pockets. The musk thistles are a complex of several species or subspecies, the taxonomy of which is unclear. See McCarty and Lamp (1982) for details. Musk thistles are found throughout the United States, and are especially common in Southern California and Midwestern and Appalachian regions. Musk thistles are biennial thistles that flower in May-June.
Italian thistle. (Carduus pycnocephalus [L]). Italian thistle is found in California (mainly in coastal counties) and are rare in other areas of the United States. Italian thistles are summer annuals or may be biennials in dry habitats.
Canada thistle. (Cirsium arvense [L]). Canada thistle is found throughout North America except in Alaska, and is most common in northern tier of states and southern Canada. Canada thistles are perennial thistles which flower in June-October.
Bull thistle. (Cirsium vulgare (Savi) Tenore). Bull thistle is found in southern Canada and throughout the United States. Bull thistles are biennial thistles which flower in July-September.
Milk thistle. (Silybum marianum [L]). Milk thistle is found in California, mainly in the coastal counties and drier areas. Milk thistles are winter annuals or may be biennials in dry habitats.
See Fernald (1950), pages 1538-1542, and Peterson and McKenny (1968), pages 302-306, for descriptive keys and illustrations of thistle species. See also Moore and Frankton (1974) for detailed keys to species.
Thistles are pioneer species and are most often found in sites where the ground cover has been disturbed by grazing, erosion, traffic, or other means. Thistles reduce the use of an area for grazing or recreational purposes because of the prominent spines on leaves, stalks, and blooms. Livestock do not eat thistles and will not graze between thistle plants on more desirable forage (Batra 1982).
Each thistle produces many seeds, often in excess of 10,000 seeds per plant. The fine filaments or pappus (thistle down) of the seed coat permit windborne dispersal over long distances to suitable habitats. Reinfestation occurs from roadsides or other areas where control is not practiced or by long-lived seeds stored in the soil from previous years. Newly germinated thistle seeds require considerable light and usually become established on disturbed areas of pastures or croplands where competition is limited during the seedling stage (Hodgson 1968). Foliar growth occurs during the spring, summer, and fall. The amount of growth and rate of new establishment varies from region to region according to the geographic, ecologic, and climatic characteristics of each region.
Losses in cultivated crops are as high as 60% at usual levels of infestation (25 shoots of Canada thistle per square yard). Losses in productivity of forage grasses from Canada thistle at a density of less than two shoots per square meter are as high as 15% (Hodgson 1968).
The introduced thistles represent a range of life histories, and the life history of each species may vary depending on habitat conditions:
- Summer annuals grow each spring or summer from seed. They grow, mature, produce seeds, and die in one growing season. Seeds generally overwinter before germinating the following spring.
- Winter annuals germinate in late summer or fall from seed, then mature and produce seed the following spring or summer. Seeds are dormant during the spring.
- Biennials germinate any time during the growing season. They usually produce a rosette of leaves close to the soil during the first season, then flower (using energy and nutrients stored during the first season's growth), mature, and die during the next year.
- Perennials become established by seed or vegetative parts (e.g., roots, tubers, or rhizomes). Once established, they live for more than 2 years, and often for many years.
MONITORING AND THRESHOLDS FOR THISTLES
Thistles are relatively conspicuous weeds and in most cases periodic visual inspections should be sufficient to monitor thistle populations. The permanent plot technique is a good way to monitor thistle populations after they have become established and while they are being controlled. A representative section of the field is marked off and thistles are counted and mapped and notes made on their condition (height, flowering, etc.). Monitor on a regular basis (weekly, biweekly, monthly). Keep careful records, note when treatments take place, or when biological controls are introduced (naturally or artificially). Study of records, over time, will show population trends and indicate whether or not control strategies are successful.
A variation on the above is the use of photo plots. Take a series of photographs of the sample plot showing the density of thistles and condition. Include in the photo an object of known size (person or measuring stick) to indicate thistle size. Also include in the frame a sheet of paper with the date in bold letters. All photos should be taken from the same location with the camera pointed in the same direction and with lenses of equal coverage. This method is especially useful in monitoring the effects of control measures over the course of several seasons.
Many states have laws requiring the control or removal of pest thistle species before they flower, whenever and wherever they occur. In these states, and in most other instances, the threshold action level is one or more weed thistles.
NON-CHEMICAL CONTROL OF THISTLES
The primary control strategy for annuals and biennials is seed management while the control strategy for perennials must include depletion of plant reserves. Long-term strategies for thistle control depend on biological and cultural controls. Generally, no one technique will provide adequate control. Currently available biological controls using insects require several years for establishment of the insect, and even longer for control. Most successful programs combine biological control with cultural controls such as timely mowing or reseeding with competitive desirable plants. Suppression of thistles may require altering land use.
Biological Control
Pest species of thistles have been introduced into North America without their compliment of natural enemies. In Europe, Carduus thistles are attacked by approximately 340 species of insects and 7 fungal pathogens. Current research in biological control is an attempt to reunite natural enemy species with their hosts. Biological control agents seldom eliminate pest thistles from an area, but can reduce populations below set economic thresholds.
Imported thistles have been the subject of biological control programs for several years. The following is a brief description of several biological control agents.
Rhinocyllus conicus, a European weevil that feeds on developing seed heads has been introduced into the United States and Canada for control of Carduus thistles, particularly the musk thistle. It has also been introduced for control of Italian and milk thistles in the western United States. In the absence of Carduus thistles, R. conicus will feed on Canada and bull thistles, but control is not as complete as on its primary host. R. conicus deposits eggs on bracts and flower stems. Larvae feed beneath developing seeds, destroying them. Pupation occurs in the flowers, and adults emerge in mid-summer. Adults hibernate in overwintering floral rosettes. There is one generation per year. Release of R. conicus on National Park Service lands is not being allowed at this time due to possible impacts on native plants.
Trichosirocalus horridus, another European weevil, has also been introduced for control of Carduus thistles. This insect feeds primarily on the root crowns of musk and Italian thistles. T. horridus has been released in Canada and most of the United States. It has not been introduced on the west coast of the United States because it has been shown experimentally to feed on artichoke. However, it is not considered to be a pest in artichoke-producing areas of Europe, and further studies are being carried out. T. horridus deposits eggs on leaf ribs and larvae migrate to the root crown where they feed. Pupation occurs in the soil. Adults feed after emergence and overwinter in the rosettes. Weevils from populations in southern Europe and from central Europe have been introduced into the United States. Southern European weevils mate in autumn, oviposit from mid-December to March, and adults emerge in April and June. Central European weevils mate in spring and oviposit in May to June. Adults emerge in September and hibernate until the spring thaw. These two populations are currently undergoing further study to develop more effective control for thistles. Stoyer and Kok (1987) suggest that a combination of T. horridus and sublethal dosages of 2,4-D herbicide could aid in Carduus thistle control.
For control of Canada thistle, Altica carduorum, a European weevil has been imported into North America. Adults feed throughout the summer on leaves, defoliating the plant and weakening it. Although Canada thistle is seldom killed outright by this weevil, the continued stress upon it reduces the number and vigor of vegetative shoots and reduces seed production. Although repeatedly released in North America, this species is not yet well established (Batra et al. 1981).
A second weevil, Ceuthorhynchus litura, that feeds on leaves and root crowns of Canada thistle is established and providing some control in Canada, Idaho, Montana, and California (Rees 1990).
A tephritid fly, Erophora cardui, that feeds on Canada and bull thistles was released in 1973 and is established in British Columbia.
Cassida rubiginosa, a chrysomelid beetle that feeds on leaves of Carduus and Cirsium thistles, has been established in North America since 1927 (Batra et al. 1981).
Several other species of insects, mostly seed-head weevils, are currently being studied for possible importation and release for biological control of thistles in the United States.
Two fungal pathogens that are spread by thistle feeding insects are also being considered for release in the United States. Rust fungi in the genus Puccinia, which attack the leaves of the basal rosette and underground basal parts, have been introduced into Canada. Further studies are required to determine their effectiveness.
Ustilago cardui, a smut fungus, has been observed to attack late maturing seed heads of Carduus thistles in Europe. Seed production is stopped in infected plants, giving full control. This fungus compliments control by R. conicus , which feeds on early flower heads (Bolt 1978).
Consult your National Park Service regional Integrated Pest Management coordinator for further information on biological control for thistles in your area.
Cultural Control
In areas that are grazed, eroded, or subject to heavy traffic, the grass cover may not be dense enough to prevent establishment of thistles. Rotational or deferred grazing, water conservation, erosion control, redirection of traffic, and sound pasture and turf management practices can reestablish heavy grass cover and prevent thistle establishment (Bendall 1973, Trumble and Kok 1982, Kok et al. 1986).
Mechanical Control
Cutting or removing thistles (where feasible) can be effective in reducing thistle populations. Annual and biennial thistles, if mowed within two days of flowering of the terminal blooms, will not produce seed or regenerate significantly. Timing in mowing is important; if mowing occurs four days after terminal bloom anthesis (full flowering), significant amounts of seed are produced. Since thistle stands mature at different times, careful monitoring and proper timing are necessary for mowing to be a viable option in an Integrated Pest Management program. However, even if mowing is done late and seed is produced, mowing the stalks will reduce seed dispersal and seed production, keeping infestations from spreading widely (McCarty and Hattling 1975).
Canada thistle, a perennial, is difficult to control by mechanical methods. Occasional cultivation may increase sprouting from broken roots due to its ability to propagate vegetatively. However, repeated cultivation can significantly reduce infestations if begun when plant reserves are at their lowest stage in early spring (early bud stage), before the shoot leaves can furnish energy to the roots in amounts greater than the roots require for production of new growth. Cultivation should start in early spring by plowing and disking. When new shoots appear, the area should be cultivated 3"to 4" inches deep every 20-21 days to destroy new shoots. Up to 90% or more of a Canada thistle infestation can be eliminated in a single season of cultivation when properly performed. Remaining plants can be eliminated by continuing cultivation in the following spring (Hodgson 1968). Hodgson (1968) reports excellent control of Canada Thistle in alfalfa fields mowed for hay twice a year.
Mechanical controls are compatible with biological controls if the mechanical controls are used early in the season to stress the plants, and natural enemies are allowed to enter the system to further weaken and eliminate thistles. Mechanical controls combined with chemicals may be successful in some cases. In most cases, however, combining a chemical and biological control is a more viable approach to thistle management.
Controlled burning may only damage the above ground portion of the thistle allowing rapid regrowth from the root section or from seed. Fire should be used only in combination with other control measures.
CHEMICAL CONTROL OF THISTLES
Several herbicides are useful for thistle control and your regional Integrated Pest Management coordinator should be consulted for more information on these. Spot treatments, rather than broadcast treatments, are preferred. Chemical control for annuals, biennials, and perennials must be initiated before the plants blossom and produce seeds. Young plants are most susceptible to control with chemicals. Best results are obtained when plants are in their initial and heaviest growth stage. The use of herbicides provides a quick and easy (albeit expensive under largescale operations) method of control, but without a long-term strategy herbicides often lead to greater problems because of their effect on other plant species, the development of resistance, and the lack of susceptibility of certain life stages of thistles.
Trials combining herbicides (usually 2,4-D), and biological control agents (R. conicus and T. horridus) have shown the two to be compatible if precautions are taken (Trumble and Kok 1980). Field and laboratory tests have shown that spring application of 2,4-D (when blooms are beginning) provides the most effective thistle control, and causes the fewest adverse effects on thistle weevils; R. conicus adults and T. horridus pupae, the only life stages likely to be exposed to such spraying, are relatively unaffected by the herbicides. Adults of both species will move to unsprayed plants, thus increasing biological control in nearby areas where herbicide treatment is not feasible or economical. Tests to determine compatibility of biological control agents with herbicides other than 2,4-D are still in the planning stage.
SUMMARY
To summarize, the following steps are recommended to manage thistles:
1. Monitor infestations over time with the use of maps, plots, or photographs.
2. The primary control strategy for annuals and biennials is seed management, while the strategy for perennials must include depletion of plant reserves.
3. Use biological controls in your area if possible. Check with your National Park Service regional Integrated Pest Management coordinator for details.
4. Use cultural controls to reestablish dense grass or ground cover in order to prevent or reduce thistle establishment.
5. Cut, mow, or otherwise remove thistles, if feasible. Thistles should be cut before the flowering of terminal blooms to prevent seed production.
6. Use appropriate herbicides on a spot treatment basis. Time applications to control thistles at prebloom stage and for compatibility with natural enemies.
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