In its homeland in Eurasia, Rush skeletonweed is a minor, inconspicuous component of the local ecosystem held in check by a large complex of natural enemies. When introduced into North America, it escaped the controlling effect of these natural enemies, which allowed it to become an aggressive, vigorous, rapidly spreading weed. Biological control is an effort to return to its homeland, select from its complex of natural enemies those that would be effective and safe for release in the U.S., and go through the necessary steps to import, test, and approve their release in North America.

Biological control has already been attempted on Rush skeletonweed in Australia and North America and resulted in the release of three agents, a small fly, a mite, and a plant rust fungus in both areas. In Australia the program was highly successful, and in the U.S. the three agents are credited with successfully controlling Rush skeletonweed in California and possibly some wheat-growing areas of Washington. However, in the higher elevation, cooler, interior mountainous areas of much of Idaho, the three biocontrol agents, while established, are having little or no effect on Rush skeletonweed. They are either not adapted to the Idaho climate or not capable of attacking the varieties of skeletonweed found in Idaho. A new program to find additional biocontrol agents suitable for Idaho is underway.

1. Review the literature and herbarium specimens to identify the total natural range of Rush skeletonweed and identify areas climatically similar to Idaho.
   
   
2. Visit the natural range of Rush skeletonweed and inventory its associated natural enemies.
   
   
3. Select the most promising natural enemies for biocontrol agents.
   
   
4. Determine the life history and ecological requirements of the most promising candidate biocontrol agents.
   
   
5. Conduct host testing to determine which plants other than Rush skeletonweed each target agent can feed on.
   
   
6. Select the most promising biocontrol agents and work with the USDA-APHIS to obtain approval for their release in North America.
   
   
7. Release the biocontrol agents in Idaho, set up a program for mass rearing, and redistribution of the agents throughout Idaho.
   
   
8. Evaluate the program to determine the degree of success or reason for failure of each biocontrol agents.

Since Rush skeletonweed is a serious pest on national forest lands in Idaho, the U.S. Forest Service Intermountain Research Station has undertaken an effort to initiate a new biocontrol program aimed at finding agents which would be successful at the higher, cooler elevation forested lands. Work to date covers mostly objectives 1 and 2 above. A review of the literature, plus visiting herbariums in North America and Europe is underway to determine more accurately the range of this weed in Eurasia is about complete. Through contacts with entomologists involved in the earlier program in Australia, an effort has been made to compile a complete history of their biocontrol program. Contracts have also been arranged with the USDA-ARS European Biological Control Laboratory (EBCL) at Montpellier, France to oversee the search for new biocontrol agents. To date, exploratory entomologists from EBCL have visited or are searching for new biocontrol agents in China, Greece, Turkey, Uzbekistan, Kazakhstan, and the Republic of Georgia. Agreements are being developed with scientists in the Republic of Georgia and Uzbekistan to conduct more long-term searches for the agents and eventually to conduct the biological studies of the more promising natural enemies encountered. Studies to date have indicated a root-feeding moth as a very promising natural enemy. Shipments of this moth have already been made to the insect quarantine facility at Bozeman, Montana, where studies are being made on how to rear the insect in captivity so that further testing of its feeding behavior can be conducted to prove conclusively that it can feed and exist only on its natural host, Rush skeletonweed. Besides the search for insect natural enemies of Rush skeletonweed, the new program is focusing upon finding new and more effective strains of the rust fungus. Under an agreement funded by the U.S. Forest Service, a plant pathologist in France is screening strains of the rust fungus to try to find more virulent strains that will attack the varieties of Rush skeletonweed found in Idaho.

Skeletonweed (Chondrilla juncea) is a typical composite (Asteracae) is closely related to both dandelions and chicory. It is a long-lived perennial that in the mid summer produce numerous small yellow flowers that are self-pollinating (see photo 2) and soon produce abundant dandelion like seeds that disperse by the wind (see photo 3). Skeletonweed is deep-rooted which allows it to thrive in dry or arid areas, but most of the year is only an inconspicuous rosette that resembles the common dandelion. However by mid summer, the plant begins to bolt and the rosette at its base dies, leaving a leafless multi-branched stem three to four feet tall, justifying its name, "skeletonweed" (see photo 1).

The native range of skeletonweed stretches from the west coast of southern Europe along both sides of the Mediterranean and through central Asia as far east as western China. The plant has been accidentally introduced into both Australia and New Zealand where it has become a major problem in wheat growing areas. It has also been introduced to the eastern United States, where it is not a weed just a minor inconspicuous exotic plant. On the West Coast of the United States, however, it found a suitable climate and became a major problem in northern California. It was also introduced around 1963 to a forested area north of Boise, Idaho, where it has now become the most widespread and rapidly spreading weed west in southern Idaho and is spreading into adjacent areas of Oregon, Washington and Montana.

Once established, the deep root system of skeletonweed makes it exceptionally difficult to control with conventional herbicides. Efforts have therefore concentrated on trying to develop biological control for this weed. The initial program was conducted by the Australians who found and introduced three agents: a small gall forming fly (see photos 5 & 6); a small eriophyid mite that attacks the reproductive buds, galling them, both preventing flowering and stunting the growth of the plant (see photo 7); and Puccinia rust (photo 8). In Australia, in combination these agents gave excellent control of this weed. However while the three agents have also been introduced into the Pacific Northwest and are well established in Idaho, they are not adapted to our interior, mountainous climate or to the local strain of skeletonweed and have little or no measurable effect.

Beginning in 1996 a new cooperative program, jointly supported by the U.S. Forest Service, USDA ARS European Biological Control Laboratory (EBCL) and the Idaho Department of Agriculture was implemented in an effort to see if new biocontrol agents, or at least more effective strains of the existing agents could be found that would give us effective control of this weed in the higher elevation and cooler interior regions of the Pacific Northwest and Northern Rockies. To find strains of biocontrol agents that are more climatically adapted to our interior climate, the new program shifted its emphasis from the Mediterranean, where the earlier Australian program conducted its search for natural enemies, to the areas surrounding the Black Sea, particularly areas of higher elevation with cool, long winters with snow pack.

Using cooperative agreements and grants, funneled through the ARS EBCL local researchers in the Republic of Georgia, Greece, Ukraine and Bulgaria have been funded to conduct surveys of the natural enemies of rush skeletonweed in their respective countries. As new natural enemies are found that appear to have potential as biocontrol agents, additional studies are conducted to learn more about their biology, life cycle and impact on the plant. To date, a series of new insect natural enemies, not previously recorded or at least used in the Australian programs have been identified. (see photos 9-13). The two previous strains of the disease Puccinia chondrillina introduced to the U.S. are not adapted to our varieties of skeletonweed and do not significantly impact the plant, even though they occasionally attack it. We are presently screening to try and identify strains of this rust that are better adapted to our variety of plant.

The search for new insect biocontrol agents has progressed to the point that one new agent, the root feeding moth, Bradyrrhoa gilveollela (see photos 13 & 14), has been tested in quarantine in Bozeman, Montana, and a petition submitted to the USDA APHIS with a request that we be allowed to release this agent in North America. (For more information see "Petition"). Once this petition is approved, the initial release of this insect will be made on lands in Boise National Forest, north of Boise, Idaho. Release sites have been selected and are presently being monitored to obtain base line information on the weed population, prior to the actual release of this agent.

• 2000 Annual Report (PDF 1.4MB)
• 2001 Annual Report (PDF 1.1MB)
• Petition for Release of Bradyrrhoa gilveollela

Click on each thumbnail photograph for an enlarged version.

Photo 1: Mature rush skeletonweed just beginning to flower.

Photo 2: The flower of rush skeletonweed contains six prominent petals that are actually a composite of 20-50 flowerettes (diameter approximately 1 inch). A single mature plant can produce hundreds and even thousands of these flowers over a one to two month period during the summer.

Photo 3: The flowers are self-pollinating and within two weeks of forming, produce 25-50 dandelion-like seeds which the wind readily distributes.

Photo 4: The deep root system of rush skeletonweed allows it to thrive in dry areas where it rapidly replaces native vegetation and forms monocultures such as this in the foothills north of Boise, Idaho.

Photo 5: Blister like galls on the leaves of rush skeletonweed formed by the introduced biocontrol agent, Cystiphora schmidti, a midge. (Courtesy of Washington State University)

Photo 6: Newly formed skeleton gall midge. Insect is approximately 3-4 millimeters in length. (Courtesy of E. Coombs, Oregon Department of Agriculture)

Photo 7: Skeletonweed plant heavily attacked by the gall mite, Eriophyes chondrillae. The knobby growths at the end of the branches are galls formed by the mite, which prevent the plant from flowering and often stunt its growth as well. Unfortunately its attack on the plant is very sporadic and seldom lasts for more than a season before the mites disappear and they have significantly hurt the plants.

Photo 8: Spore-forming bodies of the rust Puccinia chondrillina on rush skeletonweed. While two strains of this rust are established in North America, they usually have a very minor impact on the varieties present in the Pacific Northwest. Infestations as heavy as this are quite rare. (Courtesy E. Coombs, Oregon Department of Agriculture)

Photo 9: Shown here in its homeland in Eurasia, the flower buds and green seed heads of rush skeletonweed are often heavily attacked by both small flies and caterpillars. Several of these are being studied as potential biocontrol agents.

Photo 10: An unidentified stem mining beetle found attacking skeletonweed in northern Greece.

Photo 11: Mine of unidentified beetle larvae in the roots of skeletonweed. These larvae are found in abundance attacking and often killing plants in Bulgaria.

Photo 12: Larvae of the two species of beetles that are known to attack skeletonweed in Eurasia, mining the crown in the upper roots of the plant.

Photo 13: Adult of a root mining beetle of the genus Sphenoptera. Length approximately ¼ inch.

Photo 14: A root of Chondrilla juncea from northern Greece that has been heavily attacked by larvae of the caterpillar Bradyrrhoa gilveollela. The larvae feed on the outside of the root, inside of a protective case they make of soil particles glued together with rubber-like latex that is exuded by the plant. Heavy feeding like this will girdle the plant and kill the attacked stem.

Photo 15: Adult of the moth Bradyrrhoa gilveollela.