United States Department of Agriculture 

NATURAL RESOURCES CONSERVATION SERVICE 

Invasive Species Technical Note No. MT-24 

June 2009 

 

Ecology and Management of Tansy Ragwort (Senecio jacobaea L.) 

By 

Jim Jacobs, Invasive Species Specialist and Plant Materials Specialist 

NRCS, Bozeman, Montana 

Sharlene Sing, Research Entomologist 

USFS Rocky Mountain Research Station, Bozeman, Montana 

Figure 1. A tansy ragwort infested pasture. Photo by Eric Coombs, Oregon Department of 

Agriculture, available from Bugwood.org 

Abstract 

Tansy ragwort, a member of the Asteraceae taxonomic family, is a large biennial or short-lived 

perennial herb native to and widespread throughout Europe and Asia. Stems can grow to a 

height of 5.5 feet (1.75 meters), with the lower half simple and the upper half many-branched at 

the inflorescence. Reproductive stems produce up to 2,500 bright golden-yellow flowers. 

Capitula (flowerheads) arranged in 20-60 flat-topped, dense corymbs per plant are composed of 

ray and disc florets; both produce achenes containing a single seed. Rosettes formed of 

distinctive pinnately-lobed leaves attain a diameter of up to 1.5 feet (0.5 meter). First reported in 

Montana in 1979 in Mineral County, tansy ragwort has since spread into Flathead, Lincoln, and 

Sanders Counties. Soils with medium to light textures in areas receiving sufficient rainfall (34 

inches or 860 millimeters/year) readily support populations of tansy ragwort. This species is a 

troublesome weed in decadent pastures, waste areas, clear-cuts and along roadsides. Tansy 

ragwort produces pyrrolizidine alkaloids - these can be lethal if ingested by cattle, horses and 

deer, but are less toxic to sheep and goats. Unchecked infestations can result in significant 

livestock losses, decreased pasture yields and increased management costs (see Figure 1). 

Auxinic herbicides such as 2,4-D and picloram can provide effective control of tansy ragwort 

when applied to actively growing immature plants in the spring or fall. Grazing should be 

deferred at least three to four weeks after herbicide application to prevent livestock poisoning 

because spraying increases tansy ragwort’s palatability. The tansy ragwort flea beetle 

NRCS−Montana−Technical Note−Invasive Species−MT-24 1
(Longitarsus jacobaeae) and the cinnabar moth (Tyria jacobaeae) reportedly provide regional 

biological control of tansy ragwort. Mowing is ineffective in controlling tansy ragwort because 

it stimulates vegetative growth and leaves below-ground roots intact. Prescribed burning during 

the reproductive stage may reduce populations. Grazing management and nutrient management 

promoting dense and continuous turf are important for control and prevention because tansy 

seedlings cannot establish on closed vegetation sites. 

Figure 2. Leaves, inflorescence, and flowers of tansy ragwort. Diagram or Graphic; Britton, N.L., 

and A. Brown. 1913. Illustrated flora of the northern states and Canada. Vol. 3: 542. Available 

from USDA PLANTS Database. 

PLANT BIOLOGY 

Identification 

Tansy ragwort is a winter annual, biennial, or short-lived perennial typically growing between 

8-36 inches (20-80 centimeters) tall but can attain heights greater than 6 feet (175-200 

centimeters) under optimal conditions. Rigid stems grow singly or in groups from an upright 

caudex (a short, woody, persistent stem at or just below the ground surface) with only the upper 

half of the stems branching at the inflorescence. Tansy ragwort additionally perennates via 

fragments of the numerous (50-100/crown) soft, fleshy roots that grow out horizontally and 

penetrate the soil to a maximum depth of 12 inches (30 centimeters). 

 

Rosettes, either seedling or root bud in origin, are formed from distinctive stalked basal leaves 

2.7-7.9 inches (7-20 centimeters) long, deeply and pinnately lobed with ovate, obovate, or 

narrow coarsely-toothed segments (see Figure 2). Leaves are either glabrous (without hairs) or 

with loose wooly down on the underside, especially during early development. Rosette leaves 

generally die back at or before flowering. Stem leaves are deeply bi- or tri-pinnatifid, arranged 

alternately on the stem and are reduced in size higher on the stem. Middle and upper stem leaves 

lack a petiole and slightly clasp the stem. 

 

Bright yellow, showy flowers arise only on the pedicel (branch) ends; outer pedicels are 

progressively longer than inner pedicels and form flat- or round-topped, dense and compact 

corymbs (see Figure 3). Capitula (flowerheads) 12-25 millimeters in diameter and numbering 

NRCS−Montana−Technical Note−Invasive Species−MT-24 2
20-60 per corymb have 13 dark-tipped bracts 3-4 millimeters long, and are composed of both ray 

and disc florets. There are 13 achene (small fruit) producing ray florets with rays (petals) 8-2 

millimeters long. Disc florets are numerous and produce both pollen and achenes. The number 

of capitula and achenes produced per plant is highly variable. 

Single seeds are contained within each achene; achenes produced from ray florets shed their 

pappus, are glabrous, and have a thick pericarp (skin), whereas achenes produced from disc 

florets retain their pappus, are pubescent with short hairs (trichomes) along prominent ribs and 

are lighter in weight than ray achenes. The pappus, composed of numerous white 'hairs' 6 

millimeters in length, is attached to the top of the achene; its resemblance to a white beard 

explains the origin of the genus name, Senecio, derived from the Latin senex for "old" or "old 

man". 

Tansy ragwort can be distinguished from native Senecio congeners by its comparatively larger 

size and exaggerated pattern of leaf dissection. Although tansy ragwort superficially resembles 

common tansy (Tanacetum vulgare), it differs significantly in flower composition, producing 

capitula with both ray and disc florets whereas common tansy flowers are composed solely of 

disc florets. 

 

Other common names for tansy ragwort include stinking willy and staggerwort. In the United 

Kingdom Senecio jacobaea has formed hybrids with Senecio aquaticus, Senecio cineraria, and 

Senecio alpinus. 

Figure 3. Tansy ragwort flowers. Photo by Michael Shephard, USDA Forest Service, available 

from Bugwood.org 

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Life History 

Tansy ragwort’s life history is typical of biennial plant species: it reproduces for the most part by 

seed; it overwinters in the seed or rosette stage; and it passes through one growing season in the 

vegetative rosette stage before becoming reproductively mature. However, because both the root 

and the caudex of this species have the capacity to form perennating buds under environmental 

or mechanical stress, tansy ragwort has the vegetative regenerative capacity of a perennial. A 

study of 179 plants in Australia found 2% were annuals, 45% biennials, and 39% were 

perennials. A separate study in England found 8% of the study plants were annuals, 39% were 

biennials, and 53% were perennials. Tansy ragwort has been observed as a true perennial by 

continuing to grow after flowering, particularly after disturbance. An Oregon study reported 

20% of the tansy ragwort plants in a population were true perennials. 

Tansy ragwort has four distinct life history stages: seeds, seedlings, rosettes, and flowering 

plants. The viability of a tansy ragwort seed crop is about 80%. The seeds have no mechanisms 

of innate dormancy other than the relatively thick pericarp on ray floret achenes which is 

believed to delay germination by a few days. The maximum potential for germination is reached 

when achenes leave the flowerhead: however, experimentation showed this potential did not 

significantly decrease after three years of storage under field conditions. Most seeds germinate 

in the late summer or early autumn, but seeds lying dormant throughout the winter will 

germinate at the beginning of the growing season in the spring. Ideal temperatures for 

germination range between 41-86o

 F (5-30o C). Soil moisture influences germination: at any 

given temperature an increase in soil moisture increases germination and the optimum 

temperature for germination changes as soil moisture changes (maximum germination is reached 

at a lower temperature when soil moisture is high). Vegetative cover may inhibit germination 

and frost, drought, or burial may induce dormancy. Seeds buried in soil deeper than 0.75 inch (2 

centimeters) remain dormant. Light is required for germination. Studies showed 24% of seeds 

buried for six years maintained their viability. Also, seeds buried up to 2 centimeters deep had a 

higher germination rate than seeds left on the soil surface, most likely because moisture is 

consistently greater below the soil surface than on the surface. Seeds subjected to drought or 

cold temperature shock had delayed germination; the longer the shock, the longer the delay. 

Seedling establishment and survival are variable, the largest determinant of survival being the 

amount of vegetative cover. In experimental garden plots two months after seeding tansy 

ragwort, no seedlings survived in plots with long grass or in plots with short but continuous turf, 

whereas seedling survival was greater in cleared or woodland areas than grassy areas. Grazed 

pastures on Prince Edward Island, Canada, had about eight times more tansy ragwort plants than 

ungrazed pastures. 

 

Once established, seedlings form flattened rosettes that are effective competitors. The large 

rosette leaves are able to cover neighboring plants, thereby suppressing their growth. When the 

rosette leaves die, sites are opened for tansy ragwort seed germination. One study found 

seedling establishment was over four times higher on sites opened by dying rosettes than on sites 

in surrounding vegetation. Alkaloids produced by rosettes may have the allelopathic effect of 

suppressing other plants. Rosette leaves die when plants flower. 

Plants typically flower during the second year of growth. Flowering has been reported to begin 

as early as June and to last as late as mid-November. It is believed rosettes must achieve some 

minimum size before flowering can begin, and the probability of flowering increases as rosette 
NRCS−Montana−Technical Note−Invasive Species−MT-24 5

size increases. Floral expansion is rapid and florets are receptive to pollination as soon as the 

floret is fully expanded. Hymenopteran (bee/wasp) and dipteran (fly) insect species are the 

primary pollinators of tansy ragwort. Seeds disperse throughout the fall. Rosettes that do not 

flower continue to grow into the fall accumulating storage carbohydrate important for winter 

survival. Carbohydrate content of the plant is highest going into winter. 

 

Habitat 

Tansy ragwort tolerates a wide range of habitats and environmental conditions but is generally 

found on mesic sites with cool, wet, cloudy weather. It grows on many soil types but usually on 

lighter, well-drained loamy or sandy soils. It normally does not grow where there is a high water 

table or on soil with high acidity. In Europe it naturally occurs in sand dune, woodland, and 

grassland communities. In North America it is found in pastures, forest clearings, and waste 

places and is often associated with Canada thistle (Cirsium arvense), common St. Johnswort 

(Hypericum perforatum), and yellow toadflax (Linaria vulgaris). 

Spread 

The pappus on the achene of tansy ragwort facilitates wind dispersal. However, achenes are 

generally considered to be poor wind dispersers and studies suggest 60% of tansy ragwort seeds 

fall within seven feet (2 meters) of the parent plant with only 0.5% of seeds produced actually 

becoming wind borne. One dispersal study found that all achenes fell within 46 feet (14 meters) 

of the parent plants. Seeds can also disperse by water movement, through the digestive tract and 

adhering to livestock and other animals, and viable seeds have been collected from bird 

droppings. Original infestations in North America are believed to have arrived in soil used as 

ship’s ballast. Tillage equipment used in fields where tansy ragwort occurs can spread root and 

caudex fragments within the field and to fields where the equipment is subsequently used. In 

Montana, tansy ragwort has established on logging sites and clearings, implicating equipment 

used for logging as a long-distance spread vector. The showy flowers may encourage people to 

transplant or seed tansy ragwort into gardens from which it can spread to pastures and native 

plant sites. 

 

Impacts 

Tansy ragwort can reduce forage yields by as much as 50% in pastures. Pyrrolizidine alkaloids 

are present in all plant parts. Cattle, deer, horses and goats consuming either growing plants or 

tansy ragwort in silage and hay store these alkaloids in their livers. Even if symptoms are not 

evident or are minor in nature, the cumulative storage of alkaloids can result in reduced weight 

gain, liver degradation, reduced butterfat content of cow’s milk, and sudden death in apparently 

healthy animals. Alkaloids in tansy ragwort pollen also taint honey, making it bitter, off-color 

and unmarketable. 

 

MANAGEMENT ALTERNATIVES 

Herbicide 1/

 

Tansy ragwort can be controlled using auxinic herbicides (mimics of auxin, a naturally-occurring 

plant growth regulator). The best timing of application is when tansy ragwort is actively 

growing in the rosette stage either in the spring or mid-fall. Herbicides are less effective after 
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plants have bolted to produce flowers. In most cases reapplication of herbicide or integration 

with other control methods will be needed for sustained population reductions. The amine, lowvolatile ester, or emulsifiable acid formulations of 2,4-D are effective when applied at 2 lb. 

a.e./ac. (two pounds acid equivalent per acre). Always check the herbicide label to confirm 

formulation and proper usage of the product before applying. When using 4 lb. a.e./gal. (four 

pound acid equivalent per gallon) formulation, apply the mixed product at a rate of 2 qt./ac. (two 

quarts per acre). 

 

Tansy ragwort is listed on the following herbicide labels at rates in parentheses: picloram (1-2 

qt./ac.), aminopyralid (5-7 oz./ac.), metsulfuron (0.5-1 oz./ac. with nonionic surfactant at 0.5%), 

and chlorsulfuron (1-3 oz./ac. with nonionic surfactant at 0.5% volume to volume). Metsulfuron 

and chlorsulfuron should not be applied where soil pH is greater than 7.9 because under these 

conditions chemical breakdown is slow resulting in extended residual activity that increases the 

risk of off-site movement of the chemical and non-target plant injury. Consult individual 

herbicide labels for other possible soil pH restrictions. 

Herbicides are most effective where competitive desired plants are present to fill voids in the 

plant community following successful tansy ragwort control. However, herbicide injury to nontarget desirable broadleaved plants and some grasses should be expected; consult product labels 

for further information on potential non-target injury. To avoid non-target injury, apply 

herbicides in the fall after desired plants are dormant for the winter. This tactic is most effective 

when using herbicides with low residual activity. Always follow label instructions to reduce 

toxicity or other unintended risks to humans and the environment, and to confirm potential 

grazing and replanting restrictions. Consult your local County Extension agent or weed 

coordinator for herbicide recommendations in your area. Herbicide treatments typically increase 

the concentration of water-soluble carbohydrate in tansy ragwort, making it more palatable to 

livestock and thereby increasing the risk of alkaloid poisoning. Grazing should therefore be 

deferred for 3-4 weeks after herbicide application. 

 

Hand Pulling 

Hand pulling and digging that extracts all of the caudex and fleshy rootstock is an effective 

method to temporally reduce tansy ragwort on small-scale infestations and scattered plants either 

as new invaders or those persisting after herbicide treatments. Pulling rosette and flowering 

plants will reduce seed set. Follow-up management will be needed to eliminate plants 

regenerating from root fragments or seed. Protective gloves are recommended to be worn as a 

precautionary measure by anyone handling tansy ragwort. 

 

Mowing 

Mowing is not an effective control for tansy ragwort. Rosettes are flat to the ground and may be 

missed by the mower blade, and when clipped, vegetative reproduction is stimulated. Mowing 

when plants bolt to flower may temporarily reduce seed production; however, plants will survive 

to flower again. A dispersal study found achenes dispersed as much as two and a half times 

1/ Any mention of specific products in this publication does not constitute a recommendation by the 

NRCS. It is a violation of Federal law to use herbicides in a manner inconsistent with their 

labeling. 
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farther on mowed sites than where the natural vegetation remained. However, in Switzerland, 

frequent mowing promoted fast-growing grass species and was associated with reduced 

occurrence of tansy ragwort. Mowing to maintain vigorous grassland communities may help 

prevent tansy ragwort invasion. 

Tilling 

The roots and caudex of tansy ragwort have the ability to regenerate after they are broken-up by 

tilling. Therefore, tillage has the potential to spread tansy ragwort and is not recommended by 

itself. The disturbance of tillage can create a favorable environment for tansy ragwort growth 

and reproduction by reducing competitive perennial plants. Tillage should be combined with 

herbicide management and followed by revegetation with desired, competitive plants. 

Irrigation 

Tansy ragwort thrives under mesic conditions and therefore irrigation is not recommended as a 

control by itself. Where tansy ragwort invades irrigated pastures and hayland, carefully planned 

irrigation management will stimulate the competitiveness of the forage crop and when combined 

with nutrient, forage harvest, and grazing management practices will help prevent the reestablishment of tansy ragwort after other control practices are applied. 

Fertilization 

Tansy ragwort is found on pastures of low to moderate nutrient status in Switzerland, which is 

part of its native range. A Swiss study found the amount of plant-available nitrogen was one of 

the most important factors predicting the occurrence of tansy ragwort. The model developed 

from that study predicted a fivefold reduction in the risk of tansy ragwort occurrence by doubling 

the application of nitrogen from about 50 lb./ac. to 100 lb./ac. On cultivated pastures and hay 

meadows, nutrient management is important to maintaining the competitiveness of desired 

perennial grasses. Nutrient management combined with judicious use of herbicides and crop 

rotation is recommended where tansy ragwort invades non-native pastures and hay meadows. 

Prescribed Burning 

Fire is reportedly effective in killing reproductive tansy ragwort plants and achenes. Fire is also 

used to maintain the vigor and density of grassland communities by burning excess plant litter 

and possibly increasing soil fertility. Fire can therefore be used as a preventative measure or in 

combination with other control methods to reduce tansy ragwort populations. On forested sites 

and clearings, the disturbance caused by fire may create openings favorable to tansy ragwort 

invasion. 

Grazing Management 

The way grazing is managed is an important factor influencing localized presence and 

distribution of tansy ragwort in its native range. In the Swiss study referred to in the 

Fertilization section above, continuously grazed pastures were 11 times more likely to be 

infested by tansy ragwort than pastures managed under rotational grazing because the former 

management regime resulted in more openings in the grass canopy where tansy ragwort could 

establish. Pastures that had greater than 25% uncovered soil had a 40-fold greater risk of tansy 
NRCS−Montana−Technical Note−Invasive Species−MT-24 8

ragwort occurrence than pasture with less than 25% bare ground. Prescribed grazing that 

maintains a continuous grassland community is recommended both for the prevention of tansy 

ragwort invasion and in pastures where it is already being controlled. 

 

Tansy ragwort leaves and flowers exceed the standard protein and digestibility requirements for 

sheep. In addition, sheep seem immune to the plant’s toxic alkaloids, and are willing grazers of 

young plants. In New Zealand, intensive sheep grazing is the predominant management 

approach for tansy ragwort; the weed generally does not occur where sheep are regularly 

stocked. In an Oregon grazing study, tansy ragwort seed production was prevented and plant 

mortality was attributed to sheep grazing in the summer following cattle had grazing in the 

spring. A separate study found tansy ragwort plants subjected to 75% defoliation were able to 

compensate fully five months after treatment when grown without competition. When grown in 

competition with Richardson’s fescue (Festuca rubra), there was no re-growth after 75% 

defoliation. These studies suggest sheep grazing is effective in controlling tansy ragwort where 

it grows in a competitive grassland community. 

 

Biological Control 

Cinnabar moth 

Biological control of tansy ragwort in the U.S. was initiated in 1959 with the California release 

of the cinnabar moth, Tyria jacobaeae L. (Lepidoptera: Arctiidae). This agent is now wellestablished in California, Oregon, Washington, and in northwestern Montana. The cinnabar 

moth flourishes in lower elevation (below 3,000 ft./900 meters) open canopy areas with warm, 

sunny summers and large, high-density infestations of tansy ragwort. Adult moths are 0.60-0.88 

in (15-22 millimeters) long with a wingspan of 1.08-1.40 inches (27-35 millimeters) and have a 

distinctive appearance: the black forewing is marked by a few irregularly-shaped crimson spots 

and a crimson line along the outer edge of the wing while the hind wing is entirely crimson (see 

Figure 4). Cinnabar moths typically emerge by late spring, mate and finish depositing eggs by 

mid-summer. Moths can be roused into flight during daylight hours when host vegetation is 

disturbed. Eggs are laid on the underside of rosette leaves in small groups. The eggs hatch after 

several weeks’ development and emerging larvae begin feeding on foliage closest to the hatch 

site. The damaging larval stage of this agent is easily recognized by its alternating bands of 

bright orange and black (see Figure 5). Pupation begins in late summer when the fully-grown 

fifth-instar larvae seek out suitable sites in debris or soil, or under bark; the cinnabar moth 

overwinters as a 0.80-1.00 inch (20-25 millimeters) long dark reddish-brown pupa. 

Roving groups (10-30 individuals) of the gregarious larval stage of this agent aggressively feed 

on the leaves, flowers and apical meristems of bolting tansy ragwort, leaving plants stripped of 

all foliage. Larval feeding can lead to significant reductions in seed production and stand 

density, although impact varies considerably. The larval stage of this agent is optimal for 

redistribution onto uncolonized infestations of tansy ragwort. Caterpillars are easily collected by 

shaking plants over collecting pans (such as kitty litter trays); retain larvae under cool, dry and 

uncrowded conditions in ventilated (with pin holes) containers provisioned with fresh host plant 

material for as short a period as possible before releasing. Because endogenous toxic alkaloids 

sequestered from tansy ragwort are present in both the larval and adult stage of this agent, 

implications for livestock health and management should be carefully considered when choosing 

sites for releasing the cinnabar moth. 
The efficacy of the cinnabar moth alone in controlling tansy ragwort is limited with successful 

control reported only when larval defoliation was accompanied by favorable environmental 

factors (frost). Although this agent is highly effective in combination with the ragwort flea 

beetle, its deployment has been surrounded by controversy. Non-target attack on the exotic weed 

common groundsel Senecio vulgaris, two native species Senecio triangularis and Packera

pseudaurea (formerly Senecio pseudaureus) and the ornamental silver ragwort, Senecio bicolor, 

have been reported when moth population densities outstrip local tansy ragwort resources. 

Figure 4. Cinnabar moth. Image: Eric Coombs, Oregon Department of Agriculture. Available at 

Bugwood.org 

 

Figure 5. Larval stage of cinnabar moth. Image: Eric Coombs, Oregon Department of 

Agriculture. Available at Bugwood.org 

NRCS−Montana−Technical Note−Invasive Species−MT-24 9
Ragwort seed fly

The ragwort seed fly Botanophila seneciella Meade (Diptera: Anthomyiidae) was first 

introduced to the U.S. in California in 1966 and was the first tansy ragwort agent to become 

well-established east of the Cascades. Botanophila seneciella was released to additively improve 

tansy ragwort biocontrol by bolstering the low levels of control realized with the cinnabar moth. 

Adult flies (see Figure 6), 0.20-0.28 inch (5-7 millimeters) in length, emerge in spring and 

deposit eggs on tansy ragwort flower buds through early summer. The 0.16-0.24 inch (4-6 

millimeters) long creamy-white larva tunnels into the flowerhead and throughout the receptacle, 

eventually moving back up to the seed head. Evidence of infestation by this agent is obvious: 

larval feeding generally destroys all seed within mined seed heads, and flowers under attack are 

typically marked by frothy spittle. Mature larvae exit host seed heads in late summer to pupate 

in the soil. The ragwort seed fly overwinters as a dark brown 0.2 inch (5 millimeters) long pupa. 

Because this agent shares the same habitat preferences as the cinnabar moth, their joint release is 

often marked by resource competition for favorably located host plants. The seed fly usually 

loses out because the moth’s phenology allows it to begin ovipositing earlier so that caterpillars 

have generally stripped all flowerheads from plants in open canopy areas by the time the seed fly 

is ready to begin oviposition. Reproductive success is significantly reduced when resource 

competition forces the fly to lay eggs on host plants located in moist, shaded areas. This agent 

has self-distributed comprehensively throughout the Pacific Northwest; redistribution is 

generally unnecessary but if desired is best accomplished by transplanting fly-infested plants in 

late spring. 

Figure 6. Ragwort seed fly - adult. Image: Eric Coombs, Oregon Department of Agriculture. 

Available at Bugwood.org 

Ragwort flea beetle 

The light gold-colored ragwort flea beetle (see Figure 7), Longitarsus jacobaeae Waterhouse 

(Coleoptera: Chrysomelidae), first introduced to the U.S. in 1969, is now established in Montana, 

Washington, Oregon, and California. Intensive foliar feeding by adult flea beetles, termed "shot 

holing," is a clear indicator of whether or not a tansy ragwort stand has been colonized by this 

NRCS−Montana−Technical Note−Invasive Species−MT-24 10
agent. Ragwort flea beetles introduced to the U.S. before 2002 were collected in Italy; the socalled Italian strain of this agent is credited with exceptional control and continued suppression 

of tansy ragwort infestations west of the Cascades. A Swiss strain of this agent with a 

completely different phenology than the Italian strain and believed to be better adapted to higher 

elevations, colder winters and shorter growing seasons typical of tansy ragwort infested areas 

east of the Cascades was released in Montana in 2002. 

Adult Italian-strain ragwort flea beetles emerge briefly in spring before returning to the soil to 

estivate (summer dormancy) until prompted by late summer/early fall rains to begin mating. 

Eggs laid from October through November produce slender white larvae 0.08-0.16 inch (2-4 

millimeters) in length that feed throughout the winter on the host plant roots. Larvae leave the 

roots in the spring to pupate in the soil. Pupae are white and 0.08-0.16 inch (2-4 millimeters) 

long. 

Swiss-strain adult flea beetles emerge from their pupation sites in the soil in late spring to early 

summer and begin feeding, mating and laying eggs immediately. Development in the eggs of 

Swiss-strain beetles is delayed through the rest of that summer, fall and winter and does not 

begin until the following spring. Larvae feed on and in tansy ragwort roots until pupating in the 

fall. The Swiss strain of the ragwort flea beetle is unique in that it overwinters both in the egg 

and pupal stage. Adults of both strains are 0.08-0.16 inch (2-4 millimeters) in length with the 

males approximately 0.04 inch (1 millimeter) shorter than the females. 

Tansy ragwort rosettes appear to be the most susceptible stage of the weed to flea beetles as 

significant control is realized from both larval root mining and adult foliar feeding that 

specifically targets vegetative (pre-reproductive) plants. The adult stage of the flea beetle is best 

suited to successful collection for redistribution, either by vacuum collection or sweeping, 

October-November for the Italian strain and mid-summer for the Swiss strain. 

 

Figure 7. Ragwort flea beetle - adult. Image: USDA ARS European Biological Control 

Laboratory. Available at Bugwood.org 

Revegetation 

The information presented above strongly suggests tansy ragwort is not competitive in a closed 

plant community. Control of tansy ragwort regardless of method will most likely be short term 

where competitive plants are absent. In western Montana, tansy ragwort commonly invades in 

forest communities after logging because the disturbance associated with logging creates 

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favorable establishment sites and the logging equipment often serves as a vector of spread. 

Seeding rapidly establishing perennial plants after logging is therefore extremely important to 

preventing or reducing the severity of tansy ragwort infestations. In forage crop fields, crop 

rotation can facilitate tansy ragwort control while keeping the crops vigorous and competitive. 

 

Plant species selected for revegetating disturbed sites likely or known to be infested by tansy 

ragwort should be appropriate for management objectives, adapted to site conditions, and 

competitive with the weed. Management objectives will determine if forage species or native 

species should be seeded and specific species mixture components. The environmental 

conditions of the site, including precipitation, soil texture and depth, slope and aspect, will affect 

species establishment. On native rangeland, a diversity of perennial grass and forb species that 

occupy many niches over time and space will most fully utilize available resources and compete 

effectively with tansy ragwort. Refer to Montana Plant Materials Technical Note 46, 'Seeding 

Rates and Recommended Cultivars,' and Extension Bulletin EB19 'Dryland Pasture Species for 

Montana and Wyoming' for seeding rate guidance and revegetation species selection. State, area, 

and field resource specialists can help determine the most appropriate, site-specific species mix 

and timing of seeding for local conditions. 

Integrated Pest Management (IPM)

Tansy ragwort thrives on disturbed ground with open sites in the plant community. On hay 

ground, IPM practices include crop rotation, nutrient management, irrigation management 

(where available), and forage harvest management to maintain a competitive closed plant 

community. New infestations of tansy ragwort should be aggressively controlled using hand 

pulling and/or broadleaf herbicide. On pastures and rangeland, herbicide application should be 

combined with prescribed grazing. Multi-species grazing with cattle and sheep will provide 

more uniform utilization than single-species grazing. In the spring, grazing sheep first to target 

tansy ragwort before grazing with cattle will prevent accidental poisoning of cattle. When tansy 

ragwort plants are mature, cattle will avoid them. Severe infestations should only be grazed by 

sheep. Forest clearings should be revegetated and monitored for new infestation after logging 

operations. Roadways, trails, and irrigation ditches should be maintained weed free. 

References 

Bain, J.F. 1991. The biology of Canadian weeds. 96. Senecio jacobaea L. Canadian Journal of 

Plant Science 71: 127-140. 

Coombs, E.M., J.K. Clark, G.L. Piper and A.F. Cofrancesco, Jr. (Eds). 2004. Biological Control 

of Invasive Plants in the United States. Corvallis, OR: Oregon State University Press. 

467 pp. 

Coombs, E.M., H. Radtde, D.L. Isaacson and S.P. Snyder. 1997. Economic and regional 

benefits from the biological control of tansy ragwort, Senecio jacobaea, in Oregon. 

Proceedings of the IX International Symposium on Biological Control of Weeds, pp. 489-

494. V.C. Moran and J.H. Hoffmann (Eds). 19-26 January 1996. Stennenbosch, South 

Africa. University of Cape Town. 
NRCS−Montana−Technical Note−Invasive Species−MT-24 13

Crawley, M.J. and R. Pattrasudhi. 1988. Interspecific competition between insect herbivores: 

asymmetric competition between cinnabar moth and the ragwort seed-head fly. 

Ecological Entomology 13: 243-249. 

Harper, J.L. and W.A. Wood. 1957. Senecio jacobaea L. Ecology 45: 617-637. 

McEvoy, P.B. and C. S. Cox. 1987. Wind dispersal distances in dimorphic achenes of ragwort, 

Senecio jacobaea. Ecology 68(6): 2006-2015. 

Mitich, L.W. 1995. Intriguing world of weeds - tansy ragwort. Weed Technology 9: 402-404. 

Sharrow, S.H. and W.D. Mosher. 1980. Sheep as a biological control agent for tansy ragwort. 

Journal of Range Management 34(4): 440-482. 

Suter, M., S. Siegrist-Maag, J. Connolly, and A. Lüscher. 2007. Can the occurrence of Senicio 

jacobaea be influenced by management practice? Weed Research 47: 262-269. 

del-Val, E. and M.J. Crawley. 2004. Interspecific competition and tolerance to defoliation in 

four grassland species. Canadian Journal of Botany 82: 871-877.