Control of Tansy Ragwort Invasion in the Pacific Northwest

Senecio jacobaea, more commonly known as tansy ragwort, is an invasive, toxic biennial weed that has been a serious weed problem in western Oregon, Washington, and northern California.  It was first reported in British Columbia in 1912, and in Portland in 1922.  In the Pacific Northwest it is often found in pastures and along roads and trails.  Ragwort has been declared a noxious weed in many areas and control is required in many selected counties.  This invasive weed is poisonous and lethal to native plants and animals, detrimental to soil quality, is a big hindrance to local economies, and poses a threat to the health of people in the community.

Figure 2. Tansy Ragwort once the flowering stems bolt

Figure 1. Tansy Ragwort in the rosette stage

Tansy ragwort is classified as a biennial herb.  As a biennial, the first year is spent in a rosette stage.  During the second year, the flowering stems bolt.  Initial infestation is by seed.  A large plant may produce as many as 150,000seeds, and the seeds can lie dormant for up to 15 years, which is why tansy ragwort is a very difficult plant to control and eradicate.  Furthermore, physical disturbances such as mowing or grazing often cause the plant to behave as a perennial [E. Coombs et. al].

A major issue with tansy ragwort is that it is noxious and lethal to many native plants and species in the communities that it invades.  The entire plant is toxic, but the most toxic part of the herb is the leaves, which generally are eaten while an animal forages for grasses and other target plants that grow near and around the tansy ragwort.  If the leaves dry out they still maintain their toxicity but not the bitter taste, and when mixed with hay or other silage it is not possible for the animal to detect or avoid the tansy ragwort when grazing or feeding.  Thus they unknowingly continue to ingest the tansy ragwort.  Tansy ragwort contains six different pyrrolizidine alkaloids that are converted to toxic pyrroles in the liverduring metabolism.  These pyrroles are generally both toxic and carcinogenic.

Figure 3. Chemical structure of pyrrolizidine alkaloids. Jacobine is one of the major pyrrolizidine alkaloids found in ragwort.

The damage to the liver is both irreversible and cumulative, and will kill the animal. Poisoning can also occur in young animals that are still feeding on the mother's milk, as alkaloids can accumulate in the milk and be passed from mother to young [E. Coombs et al].  Cattle and horses are the most vulnerable to poisoning but tansy ragwort has also negatively impacted population numbers of deer, pigs and goats. Ragwort fed  to goats at 1 percent of their body weight for 25 days, during lactation (for a total of 125% of the goat's body weight) caused abortions and subsequent death of  the does [Bedell et al]. Livestock are highly affected by tansy ragwort as the weed is very prevalent in pasture land.

Figure 4. Oregon Department of Agriculture Regional Survey. Average flowering plant density and number of cattle deaths diagnosed as pyrrolizidine poisoning at the OSU Veterinary Diagnostic Laboratory.

Figure 5. Cinnabar Moth larvae 

To get rid of ragwort, ideally biological control is preferred to spraying herbicides.  Use of cinnabar moths and adult flea beetles to control tansy ragwort in tandem has worked wonders in Oregon.  The cinnabar moth (Tyria  jacobaeae), was introduced into the Pacific Northwest in 1960 and is now widespread throughout the ragwort-infested areas west of the Cascade mountains [Bedell]. 

Figure 6. Flea Beetle

Figure 7. Cinnabar Moth

The larvae feed on the foliage and flowers and can completely defoliate the plant in large numbers.  However they only provide partial control of the tansy ragwort as they are relatively useless in controlling the rosettes.  The ragwort flea beetle (Longitarsus jacobaeae) eat the foliage during the fall, and the larvae mine the roots of the rosettes and kill plants in the winter and spring.  The beetles augments the cinnabar larval damage, coming at a time when the cinnabar moth is inactive [Bedell].

Ragwort control has significant economic benefits.  During the 1980s when the infestation was extremely bad, Oregonians reported an annual estimated loss of $4 million dollars from their livestock grazing on infested pastures [Radtke et. al].  Oregon legislature funded a measure that included a biological control program for tansy ragwort, which eventually curbed animal losses by $3.73 million annually [Coombs et al].  The benefit of biological control is reflected in a much lower livestock mortality rate, healthier livestock from purer feed, better crop yields, and a reduced expenditure on reactionary weed control methods, which accounted for another $2.12 million annually in savings [Coombs et al].  The biological control programs have cost around $240,000 annually in recent years [Coombs et al], and there clearly is a net benefit by saving farmers from millions of dollars in losses.  This has a positive effect on the entire community - market prices of local produce are a lot lower than they would be if farmers continued to suffer $4 million annually.  Furthermore, based on the dollar estimates over the 19 years, the calculated benefit-to-cost ratio for the entire state varied from 13:1 to 15:1, depending on the interest rate used.  The annual rate of return to the state of Oregon is greater than 80%, [Coombs et. al] which shows that this is a very cost-effective and necessary measure. 

Figure 8.  Percentages of annual savings due to control of tansy ragwort in Western Oregon.

Of course, there are many nonmonetary factors that justify this measure in itself. Reduction in the use of herbicides, improving wildlife habitat, and improving quality of life of livestock are all important reasons to take into account that cannot be quantified fiscally.  Control of tansy ragwort is essential for both economic and ecological reasons, as the entire community is adversely affected through the biotic and abiotic effects that the invasive weed has.

For More Information See

Stevens County Noxious Weed Control Board

King County Noxious Weeds

Pacific Northwest Weed Control Handbook

Image Sources

1. http://www.co.stevens.wa.us/weedboard/htm_weed/tr.htm
2. http://www.kingcounty.gov/environment/animalsAndPlants/noxious-weeds/weed-identification/tansy-ragwort.aspx
3. http://www.biomedcentral.com/1746-6148/4/30/figure/F1?highres=y
4. http://whatcom.wsu.edu/mgtemp/classes/weeds/PNW175.pdf
5. http://www.treknature.com/gallery/photo640.htm
6. http://www.agf.gov.bc.ca/cropprot/tansy.htm
7. http://www.bugsandweeds.co.uk/moths%20p2.html
8. http://whatcom.wsu.edu/mgtemp/classes/weeds/PNW175.pdf

References

1. Bedell, Thomas E. et al. (1981) Print. Pasture Management for Control of Tansy Ragwort. Pacific Northwest Cooperative Extension Bulletin.
2. Coombs, E., et al. (1997).  Tansy Ragwort. Pacific Northwest Weed Control Handbook 175
3. Radtke, H et al.  1993.  An Economic Evaluation of Biological Control of Tansy Ragwort.  Oregon Department of Agriculture State Weed Board.  PP 31
4. Sweeney, S.J. and K.E. Neiman, Jr., K.A. Lakey.  1992.  Alternative Control of Tansy Ragwort.  Prepared for Seattle City Light, Environmental Affairs Division, Parametrix, Inc.  Final Report of 1986 - 1991.
5. Washington State Noxious Weed Control Board. (Dec. 1997). Web. Written Findings of the Washington State Noxious Weed Control Board.