Canada thistle is most likely to occur in moist environments with previous disturbances, such as in pastures, range land, crop land, ditch banks, road sides, mud flats, lake shores and stream banks. Approximately 450,000 such areas are infested with Canada thistle across Nebraska, with the majority being in the panhandle region. We have implemented an intensive thistle control program at Agate.
Canada thistle can reproduce in two ways, which makes it very difficult to control and is the reason it has become so wide spread. This perennial forb spreads through seed production as well as by creeping roots known as rhizomes. Canada thistle is dioecious, meaning each plant contains all male or all female flowers. However, a population of all males or all females can sustain itself and spread through the extensive lateral root system. Cultivation (tilling, ploughing) only causes the thistle population to increase due to the plants ability to reproduce from as little as 1/2" of root.
Plants appear at Agate in early to mid-May as small rosettes of spiney-tipped, wavy leaves. The plant grows vigorously until flowering in July or August. At this time stems are one to four feet tall and rigid with several branches. Plants remain green until the first frost, when the above ground portion dies, but the roots remain until the following spring.
Agate Fossil Beds National Monument has about 3,000 acres within its boundaries but only 2,270 acres are federally managed. The Niobrara River winds eleven miles through the parks' four-mile width, creating extensive riparian and wetland habitats. These areas provide excellent growing conditions for many plants, including the Canada thistle that thrives in the moist environment. Canada thistle is competitive with native plants, making it difficult for them to reproduce and survive. The thistle disturbs the natural, scenic qualities of the park and spreads to neighboring pastures. Canada thistle is believed to have entered Agate around 1970. It spread through out the wetland and lower terraces of the monument. In 1996 it was estimated that approximately 100 acres where infested with thistle. This signaled the need for action.
Agates' first biological thistle control agents were released in May 1997. One hundred and forty-eight stem mining weevils (Ceutorhynchus litura) were released near the Bone Cabin, west of the Visitors Center and Museum. In early spring the weevils inject eggs into the young thistle shoots, and the eggs hatch into larvae by mid-May. The larvae mine the thistle stem, root crown, and roots, weakening the plant by using nutrients. This inhibits the thistles' ability to produce seeds and grow roots. The larvae then chew an exit hole near the base of the plant and move into the soil to pupate. The exit hole leaves the plant susceptible to secondary infestation of ants, fungi and other insects. The weevil overwinters as an adult in the soil near the base of the plant. It emerges in March and April to again lay eggs in the thistle until mid-May when the plants are too large to be easily penetrated by the weevil. A female can lay 120 eggs in young shoots. Six to seven larvae are needed in the shoots to effectively weaken the plant.
In 1998, Agate located a second release site near the boardwalk on the south side of the river. At each the new site and the original site 125 stem mining weevils were released. A new species, the stem and shoot gall fly (Urophora cardui), was also released at both sites. These were still in the larval stage, contained in galls. A gall is a round growth or tumor on a stem caused by irritation from larvae. Galls can contain zero to ten living gall fly larvae. The larvae begin to pupate in warm, spring temperatures and emerge from the galls as adults in late spring or early summer. The flies then lay one to 30 eggs in young shoots. After the egg hatches, larvae tunnel into the stem, creating the gall. The fly over winters in the gall as a larvae. In 2000, five additional insect release were created in the wetland areas between the Bone Cabin and the west park boundary with 2,000 gall flies and 440 stem mining weevils released at those sites.
In addition to biological control of Canada thistle, Agate uses mowing and herbicides. Areas of dense infestations are mowed from late July to mid-August to prevent the plant from seeding. Herbicides are then applied mid-September after the first hard frost. The park is currently using Telar µ, containing chlorosulfuron, which is approved for non-cropland use, and is used in small amounts. It does not leach deep into the soil, is practically nontoxic to most fish and aquatic invertebrates, and does not bioaccumulate (build up) in fish. Telar is also shown to be nontoxic to birds and mammals. Agate has had a contract with the Sioux County Weed Superintendent to spray the Telar µ annually since 1999. Telar is applied after the first frost, when the thistle begins to go dormant for the winter and draws moisture and nutrients into the roots which brings the herbicide with it, reducing the roots ability to spread the following spring. Approximately 40 acres are mowed and have herbicides applied annually.
Agate Fossil Beds National Monument has experienced success in its efforts to control its exotic Canada Thistle populations. Staff estimates that from 1996 though 2002 there was a 70% reduction in Canada Thistle population across the park. The project is documented in the parks Geographic Information System (GIS) and in reports on file. New methods of control are being evaluated including new biological control agents, prescribed burns, and different herbicides.
Text and photo by Kimberly Howard, Biological Technician, Agate Fossil Beds National Monument, August 7, 2002. For further information go the Related Document link and select References.