Restoration Potential: With proper management, affected areas can be restored to more desirable vegetation. Recommended management practices are to reduce grazing or remove the source of disturbance and introduce a native replacement.
Preserve Selection and Design Considerations: An otherwise high quality natural area with localized infestations of SILYBUM may be suitable for acquisition or management as a preserve. Livestock and other management practices appear to contribute to high nitrogen or disturbed areas which encourage growth of this weed. SILYBUM may not represent a threat to the rest of the area's ecological quality.
Management Requirements: This weed does require active management once it is established in dense groves. Researched methods of control are listed below.
Mechanical: In Wheatley's opinion, "The ideal control (for SILYBUM MARIANUM) is to establish a vigorous perennial grass-legume sward before an area is invaded. By providing permanent ground cover, perennial pastures can prevent thistles from becoming dominant." Where spring sowing of perennial grasses or lucerne is feasible, competition from milk thistle will be greatly reduced because they are not competitive in the fall (Parsons 1973).
Cultivation can also be a useful method of controlling seedlings as a preliminary measure to sowing competitive perennial species. Mowing can be effective before seeds are produced although there is a danger of poisoning if there are any grazers in the vicinity. If there is no threat to stock, it is important to mow before the flowers are fully developed because fertile seeds may still form in the heads (Parson 1973).
In California, a mowing regime should begin in May, and mowing should be repeated four to six times during the spring and summer to suppress seed development. This program could take two years before control is visible (pers. comm., R. Lesco, Resource Management Specialist, Pt. Reyes National Seashore, 1985). This method has not been proven successful for erradication unless forbe seed is introduced to the affected area.
Physical removal of thistle in troublesome spots has had limited success in Pt. Reyes National Seashore (pers. comm., R. Lesco, Resource Management Specialist, Pt. Reyes National Seashore, 1985). However, Lanny Waggoner, Chief Ranger at Marin Area Headquarters, California State Parks, suggested that digging the individuals out by hand was creating ideal conditions for germination (pers. comm., L. Waggoner, Chief Ranger at Marin Area Headquarters, California State Parks, 1985).
Opinions about burning as a form of controlling milk thistle differ. Mr. Waggoner mentioned that he observed a remarkable percentage of mortality in a thistle complex that was burned in an "accidental fire" in January 1980. Other individuals have explained that seed germination may be enhanced by the disturbance created by burning, thereby encouraging SILYBUM establishment (pers. comm., R. Lesco, Resource Management Specialist, Pt. Reyes National Seashore, and N. Havlick, Resource Analyst, East Bay Regional Park District, 1985).
Herbicidal: SILYBUM MARIANUM is most readily killed in the seedling and rosette stages of growth. The more mature the plant, the more resistent it becomes to treatment. One method of herbicide application is spot spraying with ester 2,4-D (80% a.i.) at the dilution of one part in 1,600 parts water, increasing to one part in 400 as the plants near maturity. For boom spraying Parsons recommends 1/2 pint per acre increasing to 1 1/2 pints per acre. Because germination occurs over a period of several months in the fall, autumn spraying will have to be followed up in the winter to deal with plants germinating later.
In an experiment of herbicide use on SILYBUM MARIANUM in wheat, authors R. Meissner and C. Mulder found that when sprayed selectively at the two to three leaf stage, the thistle was controlled 100% by picloram and methabenzthiazuron in combination with phenoxyacetic acid compound, "at rates normally recommended." After forming a rosette of three whorls or so, they discovered that S. MARIANUM was not adequately controlled by methabenzthiazuron alone. They mention that spot treatment with decamba may prove successful at later stages.
Biocontrol: The seed weevil, RHINOCYLLUS CONICUS may attain densities sufficient to destroy the seeds of thistles in amounts necessary for effective biological control. In June 1971, 316 weevils were collected near Rome, Italy and experimentally colonized on S. MARIANUM at a site near Santa Barbara. In September of the same year 9% of 154 dissected mature flower heads had been attacked (Goeden 1971). By 1973 94% of 208 flower heads examined were infested.
The eggs of R. CONICUS are found on the lower surface of the bracts of the newly formed flower heads (Hawkes et al. 1972). When the larvae hatch they bore directly through the bracts and into the flower where they feed upon and destroy the immature seeds and pupate in cells constructed among seed fragments. Adults emerge from these cells after the mature flowerheads open to shed their seeds (Goeden and Ricker l974).
Recently, the introduced seed weevil has been discovered on a variety of native Cirsium species (pers. comm., C. Turner 1985). The United States Department of Agricultures (USDA) Biocontrol Station in Albany, California, is now advising against its release because of the potential harmful effects of its spread. Research concerning this information is now in progress.
Goeden reports that the list of biological control agents effective on S. MARIANUM has the potential to expand given further understanding and research on the effects of introducing new insect species.
Management Programs: Management of SILYBUM MARIANUM has been underway at Kaweah Oaks Preserve since 1984. In February of that year an experimental application of 2,4-D was applied to SILYBUM at its maximum rosette stage of development. The treatment has focused on dense stands under oak trees where cattle had traditionally congregated. This method of control proved to be successful. In 1985 however, herbicidal methods of control will not be employed. This year a 40 acre infested area will be mowed with a "bushwacker" before flowerheads mature. Contact: Rob Hansen, Preserve Manager, 3450 Avenue 144, Corcoran, CA 93212. Phone: (209) 992-5308
Monitoring Programs: Qualitative observations of the changes in a thistle complex which occurs on the Marin headlands are in progress by Terri Thomas, a plant ecologist working for the Golden Gate National Recreation Area (GGNRA).
Rob Hansen, preserve manager for Kaweah Oaks in California, has been monitoring SILYBUM growth patterns in an area from which cattle have been removed. In the spring of 1984 he set out six transect lines, 100 meters each. From the data he gathered he was able to compare the radius of the existing growth to the radius of the growth from 1983. This data expressed a marked decrease in the SILYBUM population over time. The average decrease ranged from 20-80 percent. Follow up data will be gathered in the spring of 1985. Three intercept lines will be set out on original transect corridors.
Management Research Programs: Ms. Thomas of the GGNRA is conducting informal management-related research on a milk thistle complex in the Marin Headlands. To test the effect of mowing on thistle inhibition, a three acre thistle-infested stand was mowed during the winter of 1983. The following spring the mowed area had recovered, with the thistle growing back in what appeared to be an even greater density than the adjacent areas that had not been mowed the previous winter. Although the three acre thistle stand seemed to recuperate quickly and even benefit from mowing in 1983, in the spring of 1984 seedling establishment in the mowed area was poor and large patches of bare ground were visible. At this time Ms. Thomas prepared two 10' X 20' plots within the three acre area to test and monitor thistle regrowth. In one plot, the remaining few young thistles were pulled by hand and FESTUCA seed planted. In the other plot Ms. Thomas sowed FESTUCA on the bare patches among thistle seedlings. By winter 1985, the two experimental plots appeared quite different. Both supported much denser grass than the surrounding thistle-covered areas, and the occurrence of FESTUCA on the cleared quadrat was much greater than on the uncleared plot. The percentage of thistle occurring on the uncleared site was twice that of the cleared plot. Although the evidence is still inconclusive, it appears that thistle mowing reduces thistle seedling establishment and that FESTUCA introduced over areas, both cleared and uncleared of thistle, helps maintain lower thistle regeneration (pers. comm., T. Thomas, Plant Ecologist, GGNRA, 1985). Contact: Terri Thomas, Plant Ecologist, National Park Service, Fort Mason, Building 201, San Francisco, CA 94123. Phone: (415) 556-1838
Management Research Needs: The following are specific questions that need study to improve control efforts: (1) Could mowing, followed by covering the area with a thick layer of mulch, be an effective control for S. MARIANUM? (2) Could burning be considered an effective method of control for this weed? (3) Is S. MARIANUM germination enhanced by the disturbance created by mechanical removal to an extent that would render this method unacceptable? (4) Is grazing exclusion an acceptable method of control?
