|Impact of Biological Control Agents on Musk Thistle Populations|
Impact of Biological Control Agents on Musk Thistle Populations at the Indiana and Radford Army Ammunition Plants
by Phillip A. Lewis
U.S. Army Industrial Operations Command, Rock Island, IL
The musk thistle head weevil (Rhinocyllus conicus) and rosette weevil (Trichosirocalus horridus) were released at INAAP in 1987 and at RFAAP in the spring of 1978 to quell populations of musk thistle (Carduus nutans). Details of the release scenario, current thistle infestation and head weevil preponderance at each installation is reported. Impact on native thistle species will be assessed in future trips.
Musk thistle or nodding thistle (Carduus nutans) is an aggressive, invasive weed accidentally introduced from Europe into the U.S. in the 1800’s. This weed was first recorded in 1853 from Harrisburg, Pennsylvania and has since spread rapidly throughout North America, degrading pastures and rangelands. The head weevil Rhinocyllus conicus (Coleoptera: Curculionidae) was the first insect approved for the control of musk thistle in the U.S., with releases made in 1969 in Montana, Virginia and Nebraska. The rosette weevil Trichosirocalus horridus was imported from Italy, with the first field releases made in Virginia in 1974.
The U.S. Army Industrial Operations Command (IOC) is located in Rock Island, Illinois. The IOC is a U.S. Army Command that is responsible for the management of the U.S. Army Ammunition Plants, Depots and Arsenals located throughout the continental United States. The IOC has 36 installations in 29 states, covering over 650,000 acres. These installations have natural resources programs and projects that include fish and wildlife, forestry, agricultural outleasing, and wetlands.
Musk thistle is present at several IOC installations and estimates of chemical and mechanical control of the thistle range from $15-$60 per acre, depending on frequency of control required. In order to reduce control costs and comply with state noxious weed laws and avoid costly fines, releases of biological control agents have been made at IOC installations. Following are details of the original releases and current status of thistle and head weevil populations.
Indiana Army Ammunition Plant
This inactive ammunition plant, located in Clark County on the north shore of the Ohio River near Louisville Kentucky, once had a serious musk thistle problem which arose from the construction of a railroad spur in the late 1970’s. Having no native enemies, this aggressive weed flourished and infested 3,500 acres (1,000 heavily) of this 9,800 acre installation. Head and rosette weevils were introduced in 1987 to control the thistle (about 400 of each species) and 800 acres of the worst infested areas were sprayed with herbicide in 1989 (Figure 1).
Survey Methods and Results
Four areas were selected that had populations of musk thistle in the vicinity of the original release sites (Figure 1). Twelve plants were randomly inspected at each of these areas for the presence of the head weevil during the third week in July, 1998 (Table 1). Area 3 had a heavy, isolated infestation
(cattle grazing lease) of thistle and two areas north and south of the road were assessed, counting eight plants on either side of the road (Figure 2).
Eleven years after their release, musk thistle head weevils are well distributed at INAAP and have effectively reduced thistle populations to manageable levels (Figure 3). Problem areas of musk thistle are now confined to disturbed soils (due to grazing, erosion, etc.) and only mechanical control efforts (mowing) are needed to satisfactorily control the thistle in these areas.
Radford Army Ammunition Plant
RFAAP is located near Blacksburg, Virginia and is an active ammunition plant producing propellants and explosives. The New River Unit of RFAAP is a 3,000 acre storage facility which once had a serious musk thistle problem that arose from importation of hay by area farmers in the late 1960’s. Head weevils were introduced in the late 1960’s at a nearby farm and were found to have migrated to thistle plants at New River by the early 1970’s. A release to augment this population was made by Dr. L. T. Kok of Virginia Tech in the spring of 1978. Additionally, the rosette weevil was released at New River in 1985.
Survey Methods and Results
Four areas were selected that had populations of musk thistle. Twelve plants were randomly inspected at each of these areas for the presence of the head weevil during the first week in September, 1998 (Table 2). Areas 2 and 3 were plowed up and planted in sedge grass in the spring of this year and were heavily infested with a number of thistle species. Area 4 was planted in sedge grass
2 years ago, and Area 1 was a patch of thistles along the roadside.
Weevil infestation at Areas 1 and 4 were comparable, indicating that thistles in an area disturbed as recently as 2 years previous could be colonized at a high level by the head weevil. Efficient colonization is further evidenced at Areas 2 and 3 where new populations of musk thistle had sprung up from ground disturbing activities, but blossoms were adequately infested with weevils.
Release of these biological control agents at INAAP and RFAAP has resulted in acceptable populations of musk thistle with substantial cost savings to the Army. Environmental impacts from mowing and herbiciding were also avoided. Weevils are able to quickly colonize new populations of thistle once they are established in an area. Rosette weevils are assumed to be present at these installations, however spring surveys will have to be conducted in order to assess population levels and their impact on musk thistle. Besides bull thistle, native thistle species were not noted at either installation. Comprehensive surveys of native thistle species, and the impact of rosette and head weevils on these species, will help to determine any negative impact by the weevils.
This project was supported in part by the Army Environmental Center and an appointment to the Environmental Management Participation Program for the U.S. Army Environmental Center administered by the Oak Ridge Institute for Science and Education through an agreement between the U.S. Department of Energy and USAEC.