The ragwort species common or tansy ragwort (Jacobaea vulgaris, formerly Senecio jacobaea), marsh ragwort (S. aquaticus), Oxford ragwort (S. squalidus) and hoary ragwort (S. erucifolius) are native members of the Asteraceae family in Europe, which invaded North America, Australia and New Zealand as weeds (Roberts and Pullin ²⁰⁰⁷). Ragwort species contain different groups of secondary plant compounds (Kirk et al. ²⁰⁰⁵; Leiss et al. ^2009a) defending them against generalist herbivores, contributing to their success as weeds. Phenylpropanoids, flavanoids, the benzoquinone jacaranone and mainly pyrrolizidine alkaloids (PAs) are involved (Table 1). Especially the PAs defend ragwort against herbivorous insects. In addition PAs are toxic to grazing mammals like cattle, other livestock and horses (Duby ¹⁹⁷⁵). Both freshly grazed ragwort plants as well as dried plants within feed such as silage and hay cause acute and chronic liver damage (Candrian et al. ¹⁹⁸⁴). Ragwort is one of the most frequent causes of plant poisoning of livestock. It is responsible for over 90% of the complaints on injurious weeds in the United Kingdom (Crews et al. ²⁰⁰⁹). The Australian dairy industry estimated a loss of 4 Million Dollars per year due to ragwort poisoning of cattle (Roberts and Pullin 2006). Due to its carcinogenic properties (Prakash et al. ¹⁹⁹⁹) the World Health Organization (WHO) considers PAs as contaminants in food as a threat to human health. Different European countries prescribed a maximimum level of 0.1 μg as daily oral dose or 0.1 μg per 100 food (Edgar et al. ²⁰⁰²). In feeding experiments with cows PA concentrations of up to 10 μg/100 ml milk and up to 4 μg/100 g liver tissue have been detected (Dickinson et al. ¹⁹⁷⁶; Candrian et al. ¹⁹⁹¹). Based on these experiments the expected possible concentration of PAs naturally occurring in cow’s milk is estimated at a maximum of 0.2 μg/100 ml (Candrian et al. ¹⁹⁹¹). Tests of commercial honeys determined PA concentrations of 0.19–12 μg PAs/100 g (Kempf et al. ²⁰⁰⁸) while honey of common ragwort contained up to 390 μg PAs/100 g (Deinzer et al. ¹⁹⁹⁷). Consequently, control of common ragwort is obligatory in some countries and is as such included in the legislation of the UK: “Weeds Act 1959” and “Ragwort Control Act 2003”, Ireland: “Noxious Weed Act 1936”, New Zealand: “Noxious Plant Act 1978”, Australia: “Weed Management Act 1999”, and Friesland, province of the Netherlands “Verordening Jakobskruiskruid 2007”. However, ragwort species, especially the common and the marsh ragwort, remain a problem and their abundances are actually increasing in west- (Bezemer et al. ^2006a) and central (Suter et al. ²⁰⁰⁷; Suter and Lüscher ²⁰⁰⁸) Europe. In this review the biology of ragwort species is shortly described and the different management practices to control them are discussed.

The most widespread of the ragwort species, as is expressed by its name, is the common or tansy ragwort. It is native to Europe from where it spread to North America, Australia and New Zealand (Roberts and Pullin ²⁰⁰⁷). It is abundant on light disturbed calcareous soils and dunes (Harper and Wood ¹⁹⁵⁷) and poorly maintained pasture (Roberts and Pullin ²⁰⁰⁷). Common ragwort is usually a biennial plant, forming a rosette in the first year, while bolting and flowering in the second year (Harper and Wood ¹⁹⁵⁷). However, it may become perennial when damage or cutting prevent flowering. Common ragwort is an early successional species that establishes during periods of disturbance (van der Meijden and van der Waals-Kooi ¹⁹⁷⁹; Wardle ¹⁹⁸⁷). Flowering is partially controlled by achievement of a certain minimum rosette size (van der Meijden and van der Waals-Kooi ¹⁹⁷⁹). Common ragwort usually reproduces by seeds, whereby each plant can potentially produce thousands of seeds (Wardle ¹⁹⁸⁷). Seed dispersion is usually restricted to short distances not exceeding 15 m (McEvoy and Cox ¹⁹⁸⁷). Seeds mainly colonise spaces close to the mother plant. The horizontally growing rosette inhibits the surrounding vegetation, allowing the left space to be colonised by its own offspring (McEvoy ¹⁹⁸⁴). Seeds just covered with soil germinate faster than surface sown seeds. Burial of seeds enforces seed dormancy (Wardle ¹⁹⁸⁷). Also frost and drought may induce dormancy (van der Meijden and van der Waals-Kooi ¹⁹⁷⁹). Dormant seeds can persist in seed banks for up to 20 years (Crawley and Nachapong ¹⁹⁸⁵). Viability of seeds after 6 years was still high (Thompson and Makepeace ¹⁹⁸³). Germination is rapid (Cameron ¹⁹³⁵) and germination rates are high (60–90%) depending on the season (Wardle ¹⁹⁸⁷ and references therein). However, only a small proportion of seedlings and young plants will survive. Van der Meijden (¹⁹⁷¹) estimated that over 98% of the seed produced in a Dutch dune area failed to germinate, probably due to drought and frost. Also Forbes (^1977a) reported that 57% of all germinating plants of a common ragwort population in Scotland died. Disturbance or injury promotes regrowth through roots and vegetative regeneration by crown buds. If the plant has sufficient food reserves regrowth occurs from the crown of the plant producing secondary flower shoots. However, seed production from these secondary flower shoots is usually less compared to undamaged plants (Cameron ¹⁹³⁵). Regeneration may also occur from root buds. This mainly occurs in young roots of rosette plants, but roots of flowering plants can also regenerate this way (Poole and Cairns ¹⁹⁴⁰).

Marsh ragwort originates in Europe from where it invaded the United States (USDA ²⁰¹⁰), Australia (McLaren et al. ²⁰⁰⁰) and New Zealand (Sullivan ²⁰⁰⁶). It mainly inhabits wet grasslands such as meadows and marshes (Roberts and Pullin ²⁰⁰⁷). However, in Western and Central Europe it has established on farmland and pastures within the last 10 years (Suter and Lüscher ²⁰⁰⁸). It is a biennial species, which can become perennial when flowering is prevented by disturbance (Suter and Lüscher ²⁰⁰⁸). Each plant produces an abundant number of seeds, which are wind dispersed. Non-germinating seeds form long-lasting seed-banks, which can lead to re-colonisation of marsh ragwort (Forbes ¹⁹⁷⁶).

Oxford ragwort is a short-lived perennial native in Europe from where it spread to the United States (USDA ²⁰¹⁰). It is locally distributed along disturbed habitats with open and dry conditions. These comprise cultivated ground as well as public sites such as railway banks, waysides and waste ground (Rag-Fork ²⁰⁰⁶).

Hoary ragwort is a stoloniferous perennial plant deriving from Europe. It has been reported to occur in the United States (USDA ²⁰¹⁰). It grows on grassy waysides, waste ground and wood-margins, generally on clay soils (Rag-Fork ²⁰⁰⁶).

Ragwort species should be controlled whenever they form problems as weeds as well as forming health risks for humans and animals. Therefore, farmers should check the development of ragwort species in agricultural fields and pasture regularly. Based on the potential spread of common ragwort seeds three risk zones have been defined: High risk if ragwort species are present in less than 50 m distance of a field or pasture, medium risk at the presence of ragwort plants between 50 and 100 m distance and low risk at a distance less than 100 m (Neumann et al. ²⁰⁰⁹). Where high risk is identified immediate action to control the spread of ragwort species should be taken. At medium risk a control policy should be established to ensure where a change from medium to high risk of spread can be anticipated, it is identified and dealt with in a timely and effective manner. At low risk no immediate action is required.

Mechanical control of common ragwort is difficult because the plant can regenerate from small root fragments (Wardle ¹⁹⁸⁷). Effective manual control requires removal of the crown and all larger roots. It may, therefore, be only effective for seedlings and rosettes in contrast to bigger plants, which usually develop deep roots. However, disturbance of the soil also exposes buried seed to light which may lead to more germination.

Slashing or mowing is another form of mechanical removal. However, common ragwort has the ability within a few weeks to quickly grow back. The plants that grow back tend to switch to vegetative reproduction forming multiple crowns extending their life-span (van der Meijden and van der Waals-Kooi ¹⁹⁷⁹; Wardle ¹⁹⁸⁷). Thus slashing or mowing per se is not sufficient for control of common ragwort (McLaren and Faithfull ²⁰⁰⁴).

Flaming killed 93% of common ragwort in the seeding stage whereby the burnt plants did not retain their viability (Wardle ¹⁹⁸⁷).

Deep ploughing of non-agricultural land infested with common ragwort followed by cultivations in summer and autumn is another successful method to kill existing plants, regrowth and seedlings (McLaren and Faithfull ²⁰⁰⁴). However, cultivation is only recommended when carried out systematically and when suppressing common ragwort which is germinating from the soil seed bank.

While cattle and horses will generally not feed on common ragwort, sheep do browse it, especially at the rosette stage (Cameron ¹⁹³⁵). Sheep are the most resistant ruminants in regard to PA poisoning due bacterial decomposition of PAs in their rumen (Cheeke ¹⁹⁸⁸). Older sheep eat the crown or growing portion of the rosette, while younger animals feed on the younger leaves only. Sheep may even develop a preference for common ragwort after they have acquired a taste for it (Poole and Cairns ¹⁹⁴⁰). Amor et al. (¹⁹⁸³) implied that sheep might reduce common ragwort reporting a mean ragwort ground cover of 5–6% in ungrazed pasture, of 1.7–2% in sheep grazed pasture and 7.8–13.2% in cattle-grazed pasture. In contrast, Sharrow and Mosher (¹⁹⁸²) could not observe any differences in mortality of common ragwort between sheep- and cattle grazed pasture. However, in the cattle-grazed pasture significantly more common ragwort plants flowered before death compared to the sheep-grazed pasture. Thus sheep grazing leads to a lesser number of flowering and seeding plants. This may lead to a reduction in the seed bank over time. Especially in areas that are difficult to access grazing with sheep may be the best control option (McLaren and Faithfull ²⁰⁰⁴).

Ragwort species can be controlled by a pasture management promoting a dense, continuous and competitive pasture sward. This can be achieved through appropriate stocking densities and grazing regimes and/or irrigation and fertilization of pastures to promote their competitiveness.

The effect of grazing on the pasture cover greatly influenced the number of common ragwort seedlings in experimental sites in England (Cameron ¹⁹³⁵). Continuous pasture inhibited germination of common ragwort seeds. Early in the life cycle the competitive balance between pasture plants and common ragwort is in favour of pasture (Wardle ¹⁹⁸⁷). Later as the rosette of common ragwort establishes it competes well with grasses and clovers (Harper ¹⁹⁵⁸). High species diversity will only suppress common ragwort when accompanied by high productivity (Bezemer et al. ^2006b).

Continuous grazing leads to a significantly higher risk of infestations with ragwort species compared to rotational grazing (Suter et al. ²⁰⁰⁷). Due to the selective preferences of cattle continuous grazing often leads to unevenly grazed pasture (Fehmi et al. ²⁰⁰²). Overgrazed pasture leads to gaps in the sward in which seedlings of ragwort species can germinate and establish (Silvertown and Smith ¹⁹⁸⁹). Indeed the fluctuation of temperature and moisture at these microsites can promote germination (Moretto and Distel ¹⁹⁹⁸), while competition from other grasses is reduced. Overgrazing can also lead to damage of the sward by animal hooves. This especially occurs on steep inclinations and in wet soil conditions (Suter et al. ²⁰⁰⁷, Suter and Lüscher ²⁰⁰⁸). Undergrazed pasture provides conditions for establishment and completion of growth leading to seeding plants.

Fertilization of pastures with superphosphate or urea promoting a dense pasture sward reduced densitities of common ragwort (Thompson and Saunders ¹⁹⁸⁶). Similarly, high nitrogen application, doubling nitrogen from 50 to 100 kg per hectare per year, reduced the risk of occurrence of common ragwort fivefold (Suter et al. ²⁰⁰⁷) and that of marsh ragwort threefold (Suter and Lüscher ²⁰⁰⁸). Together with high mowing frequencies high nitrogen applications promoted fast growing grass species, which resist frequent defoliation and which are strong competitors (Suter et al. ²⁰⁰⁷). Under such conditions the chance of common ragwort to germinate and establish is strongly impaired (Crawley and Nachapong ¹⁹⁸⁵). Indeed, in meadows cut more then twice per year no common ragwort could be observed (Suter et al. ²⁰⁰⁷). Cutting common ragwort at the start or end of anthesis reduced the number of flowerheads by 87% (Siegrist-Maag et al. ²⁰⁰⁸). They recommended at least two cuts of common ragwort per year with the first mowing taking place when 50% of the plants start anthesis and the second mowing when half of the re-established plants start anthesis again. A high mowing frequency though can lead to more mechanical damage, especially at higher inclinations and in wet conditions, resulting in gaps of the sward. Damage of the sward can also transport buried seed back to the surface and may lead to more germination of common ragwort. Marsh ragwort, however, can endure high cutting frequencies (Suter and Lüscher ²⁰⁰⁸). Therefore, the control of marsh ragwort should focus on the maintenance of dense swards and the prevention of sward damage instead.

Cut plants of ragwort species are usually incinerated, which is not environmentally friendly. Alternatively, Crews et al. (²⁰⁰⁹) demonstrated that composting common ragwort in black bin bags in the direct sunlight in the field leads to a breakdown of PAs within 4 weeks and a complete loss within 10 weeks.

Classical biological weed control is based on the introduction of exotic control organisms from the geographical area of the weeds origin. These can be pathogens or insects.

Harper and Wood (¹⁹⁵⁷) published a short list of fungi occurring on common ragwort in the United Kingdom. According to Wardle (¹⁹⁸⁷) there are no fungi, which inflict significant damage on this plant. However, Bezemer et al. (^2006b) suggested that soil organisms, in particular soil fungi, may determine the performance of common ragwort. They showed in a long-term field experiment with sown and unsown plots of common ragwort that the plant biomass increased dramatically over the first 4 years in the unsown plots but then declined thereafter being influenced by a negative plant-soil feedback. Negative feedbacks between plant and soil communities may develop when soil pathogens increase rapidly in the presence of their host, ultimately causing adverse plant conditions (Bever et al. ¹⁹⁹⁷).

The most prominent insect for control of common ragwort is the cinnabar moth (Tyria jacobaeae, Arctiidae). The larva of cinnabar moth is specialized on feeding on common ragwort. It uses the PAs contained for host detection, sequestering, and detoxifying them through N-oxidation (Naumann et al. ²⁰⁰²). Furthermore, PAs stimulate oviposition by females of the cinnabar moth (Macel and Vrieling ²⁰⁰³). The biology of the cinnabar moth has been reviewed by Dempster (1971, ¹⁹⁸²). The cinnabar moth is univoltine. Adult moths emerge in late spring from overwintering pupae and oviposit clusters of eggs on the underside of leaves. Large rosettes or bolting plants are usually selected for oviposition. The first instar larvae skeletonise the leaves. During the following four instars the larvae move to the top of the plant consuming the leaves and developing floral parts. The larvae can thus completely defoliate common ragwort plants forcing older larvae to move to neighboring plants. Although larvae will feed on rosettes they do prefer flowering plants. After the fifth instar larvae pupate in litter or soil. Early assessments were that the cinnabar moth provided partial control of common ragwort (Hawkes and Johnson ¹⁹⁷⁸). However, common ragwort can recover from defoliation (Cox and McEvoy ¹⁹⁸³, Islam and Crawley ¹⁹⁸³). Cinnabar moth from Europe was released as a biological control agent against common ragwort in the USA (McEvoy et al. ¹⁹⁹¹), Australia (McLaren et al. ²⁰⁰⁰) and New Zealand (Harman et al. ¹⁹⁹⁰). However, many of these releases did not lead to an establishment of cinnabar moth. These failures were attributed to disease and parasitism (Field ¹⁹⁸⁹), field predation (Cameron ¹⁹³⁵; Dempster ¹⁹⁷¹; van der Meijden et al. ¹⁹⁹¹) and the importation of biotypes from Europe, which were ill adapted to the conditions overseas withstanding considerable desiccation in summer but suffering from water logging during winter. Water logging has been shown to be detrimental to the survival of cinnabar moth pupae (Dempster ¹⁹⁷¹). The cinnabar moth’s limited power of dispersal has also been blamed for its failure to establish (Syrett ¹⁹⁸³). Several dispersal distances per generation have been recorded for the adult moths ranging from 20 m (Crawley and Gillman ¹⁹⁸⁹) to over 300 m (Harrison et al. ¹⁹⁹⁵). Mark-recapture data from Rudd and McEvoy (¹⁹⁹⁶) suggested that the larger the scale of observation the greater the movement observed.

The flea beetle Longitarsus jacobaeae (Alticinae) has been introduced as a biocontrol agent against common ragwort into the USA (Mc Evoy et al. 1991), Australia (McLaren et al. ²⁰⁰⁰) and New Zealand (Dastgheib ²⁰⁰⁵). In the case of Australia the species introduced was later identified to be Longitarsus flavicornis (Field ¹⁹⁸⁹). Also the flea beetle is an insect specialized on common ragwort since it is able to sequester PAs and store them as non-toxic N-oxides (Narberhaus et al. ²⁰⁰³, ²⁰⁰⁴).

The flea beetle is univoltine (Ireson et al. ¹⁹⁹¹). Adults emerge from pupae overwintering in the soil. They feed by rasping through the leaves leaving a characteristic pattern of shot holes (Windig ¹⁹⁹¹). The eggs are laid in the soil surrounding a plant. The major damage to the plant is caused by the larvae boring into leaf petioles and roots. Here they feed throughout the winter passing through three instars completing development in spring (Ireson et al. ¹⁹⁹¹). The larvae are mobile and will move away from dying plants. In the USA females undergo a summer aestivation, which delays oviposition for up to 5 months with an intense feeding period 2–8 weeks before egg laying (Frick ¹⁹⁷⁰).

Flea beetles released in the USA reduced the number of vegetative plants of common ragwort by 95% and flower production by 39% (James et al. ¹⁹⁹²). Flea beetles introduced into Australia were able to reduce densities of common ragwort by as much as 90% (Ireson et al. ¹⁹⁹¹, ²⁰⁰⁰).

The effect of the flea beetle is complementary to that of the cinnabar moth. The larvae of the cinnabar moth feed on the flowers and leaves in summer. The adults of the flea beetle feed on leaves, while the larvae feed on roots and leaf petioles during autumn, winter and spring. The combination of these two biocontrol agents reduced common ragwort to less than 1% of its former abundance in Oregon (McEvoy et al. ¹⁹⁹¹). A meta-analysis of the effectiveness of biological control of ragwort species, using available publications up to the year 2004, concluded that indeed the combination of the cinnabar moth with the flea beetle was most effective with an average reduction in densities of common ragwort of 99.5% (Roberts and Pullin ²⁰⁰⁷). The cinnabar moth alone reduced the number of common ragwort plants by 52.3% on average and the flea beetle alone by 96.5%.

The ragwort stem and crown boring moth Cochylis atricapitana (Tortricidae) from Europe was released as a biocontrol agent of common ragwort in Australia (McLaren et al. ²⁰⁰⁰) and New Zealand (Gourlay ^2007a). The moth can produce 2–3 generations per season with adults appearing in spring and autumn. Eggs are laid individually on the undersides of leaves. The hatching larvae begin mining into the plant tissue boring into crowns and stems of plants (McLaren ¹⁹⁹²). The moth passes through five instars. Pupation takes place in the plant or surrounding plant soil. Larval feeding severely damages the root crowns of rosette plants and can cause their death. A 40% reduction in plant height of flowering plants and a significant reduction in both size and survival of rosettes have been observed (McLaren et al. ²⁰⁰⁰; Gourlay ^2007a).

The most recent biocontrol agent used is the ragwort plume moth Platyptilia isodactyla (Pterophoridae), which has been released in 1999 into Australia (Faithful et al. ¹⁹⁹⁹) and in 2006 into New Zealand (Gourlay ^2007b). Its most common host is marsh ragwort (Emmet and Heath ¹⁹⁸⁹). The plume moth has 2–3 generations per year with adults flying in spring and autumn (Emmet and Heath ¹⁹⁸⁹). Eggs are mainly laid on the underside of leaves. Hatching larvae tunnel down into the crown of rosettes or into the stalks of flowering plants where they pass through five instars (McLaren et al. ²⁰⁰⁰). They complete development by pupation inside curled leaves or in the soil surrounding the plant (Masri ¹⁹⁹⁵). Larval feeding severely damages the crown of the plant with only two to three larvae being capable of killing a whole plant (McLaren et al. ²⁰⁰⁰). Densities of marsh ragwort have been reduced by 60–80% (Gourlay ^2007b).

Different herbicides have been tried on ragwort species with varying degrees of success. Generally, young rosettes are easier to be controlled compared to older plants. The 2,4-dichlorophenoxyacetic acid (2,4-D) works well on younger plants but performs poorly on late rosettes, budding or flowering plants (Black ¹⁹⁷⁶). Picloram showed a better performance than 2,4-D (Thompson ¹⁹⁷⁴, ¹⁹⁷⁷) and 2-methyl-4-chlorophenoxyacetic acid (MCPA) (Thompson and Saunders ¹⁹⁸⁴). On young plants glyphosate (Makepeace and Thompson ¹⁹⁸²) performed less well compared to 2,4-D and Picloram, while it performed better on flowering plants (Thompson ¹⁹⁸³). A combination of 2,4-D, MCPA and 2,4,5-trichlorophenoxyacetic acid (2,4,5,-T) controlled more than 98% of common ragwort (Forbes ¹⁹⁷⁸). A mixture of 2,4-D and MCPA achieved 90% control of common ragwort as did a mixture of dicamba and MCPA, while up to 100% control was achieved with a mixture of Fluroxypr and Aminopyralid (Neumann et al. ²⁰⁰⁹). Asulam successfully controlled marsh ragwort (Forbes ^1977b). A significant reduction of populations of common ragwort densities have further been reported using different herbicides such as Hexazinone (James and Mortimer ¹⁹⁸³), Flazasulfuron (James et al. ¹⁹⁹⁷), Clopyralid (Whitson et al. ¹⁹⁸⁶; Clay and Dixon ¹⁹⁹⁸), Tricopyr and Fluroxypr (Dixon and Clay ²⁰⁰¹) as well as Amidosulfuron, Pyridate and Tribenuron (Dixon and Clay 2004). Meta-analysis of the effectiveness of herbicide application to control ragwort species, including published data up to the year 2004, showed that 2,4-D, MCPA and glyphosate are effective against common ragwort, while Asulam is effective against marsh ragwort (Roberts and Pulin 2007).

However, chemical control in pastures can cause harmful side effects influencing pasture growth, especially white and red clover (Black ¹⁹⁷⁶). Furthermore, herbicide applications are problematic in nature conservations sites. Therefore, mature plants, which require higher rates of herbicides should be selectively treated with localized spot-sprays (Thompson ¹⁹⁷⁷, ¹⁹⁸³; Forbes ¹⁹⁸²). Another approach to minimize damage of pasture by applying chemical control is the Australian “Mitchell technique” (Watt ¹⁹⁸⁷). Mature plants are allowed to complete their life cycle in an ungrazed situation so that the plants die naturally. Subsequent establishment will then entirely be based on seedling emergence. Seedlings are controlled by herbicide application together with grazing and maintenance of a dense pasture sward.

In order to integrate chemical and biological control of common ragwort the effect of herbicides on the ragwort flea beetle has been studied. Data indicated that at field rates clopylarid, glyphosate, MCPA and mixtures of 2,4-D and picloram or dicamba had no lethal effects on adult flea beetles (Dastgheib ²⁰⁰⁵; Gourlay ^2007c). However, sub-lethal effects especially on oviposition were not excluded. No information is available on the impact of herbicides on the remaining biocontrol agents.

Ragwort species, especially the common and the marsh ragwort still remain a weed problem. Their spread within Europe is even increasing (Suter et al. ²⁰⁰⁷; Suter and Lüscher ²⁰⁰⁸). This increase is related to modifications in agricultural management. Farming has concentrated on bigger farms with less manpower leading to less intensive pasture management. Furthermore, ragwort species invade pastures through nature conservation and nature compensation areas, through ruderal areas with a high level of disturbance and through the public domain such as roadsides and railway banks (Suter et al. ²⁰⁰⁷). A successful control of ragwort species needs to be based on scientific findings to be implemented by a combined effort of farmers and public authorities. However, as pointed out by Roberts and Pullin (²⁰⁰⁷) the availability of scientific information on weed control to farmers, land manager and nature conservation practitioners is problematic. Besides scientific publications other forms of communications such as newsletters and more recently free access websites are important.

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