Ageratum (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Ageratum L.

Protologue: Sp. pl. 2: 839 (1753); Gen. pl. ed. 5: 363 (1754).

Family: Compositae

Chromosome number: x= 10; A. conyzoides: 2n= 20, 40, A. houstonianum: 2n= 20

• Ageratum conyzoides L.

• Goatweed, billy goat weed (En).

Ageratum comprises approximately 30 species, all of which - except for A. conyzoides and A. houstonianum - are restricted to the Americas and adjacent West Indies. Goatweed is a pantropical weed which originates from South and Central America. It extends about 30° north and south of the equator.

A. conyzoides was already being cultivated as an ornamental at the end of the 17th Century, in Europe. A. houstonianum was not cultivated before about 1820, but is now a very popular garden plant, whereas A. conyzoides has fallen out of favour. Both species occur as a weed in South-East Asia, but A. conyzoides is more widespread and common.

A. conyzoides plays a role in traditional medicine in many parts of the world. It is widely used externally to treat skin diseases, wounds (both disinfection and haemostasis), ulcers and boils; internally as febrifuge and to treat diarrhoea and haemorrhages. Local uses reported include applications as an emetic (internally), and to treat eye diseases (externally), pneumonia (externally), sore throat (infusion as syrup), stomach-ache (internally), gonorrhoea and catarrh (internally). In Indonesia, an infusion of the roots is used against fever, and an infusion of the leaves to wash sore eyes, and to treat stomach-ache and wounds. In Malaysia, leaves are used externally to heal wounds, cuts, scratches and itches and to alleviate tooth-ache; a decoction of the root is taken for treating coughs and a decoction of the whole plant is taken against asthma. The juice of fresh leaves is widely used as a vulnerary in the Philippines, and leaves cooked in coconut oil are also applied to wounds. A decoction of the herb is used to treat stomach troubles. In Papua New Guinea, a solution of crushed leaves is taken to treat diarrhoea and juice from squeezed leaves is used to treat sore eyes; the leaves are used in New Britain internally against fever and dysentery and externally to heal wounds. In Thailand, the leaves are used for treating wounds, itching and eye inflammations. The whole plant is used as antipyretic, diuretic, carminative, anti-amoebic and emmenagogue. In Vietnam, A. conyzoides is reported to be effective in the treatment of allergic sinusitis and rhinitis; it is also used in hair care. Extracts may be used as insecticide.

Both A. conyzoides and A. houstonianum are sometimes grown as ground cover, e.g. in rubber plantations in Java. In China, A. conyzoides is used as a cover crop in citrus plantations, and is reputed to be effective in the biocontrol of mites as it hosts predacious mites. A. houstonianum is often cultivated as annual ornamental, in tropical as well as temperate regions.

Goatweed is only used locally medicinally and is not traded on the international market. It is only planted on a fairly large scale in China. Goatweed is rarely cultivated on a larger scale to obtain insecticides, since the active compounds can be synthesized.

Goatweed has a disagreeable odour which has been described as the smell of a billy-goat or of salty shrimps. When dried it smells of coumarin.

Both goatweed species contain an essential oil (in A. conyzoides 1.6% w/v), which can be isolated from the fresh leaves and stems. This oil is characterized by the presence of volatile chromene derivatives (up to 85%) of the precocene (e.g. 7-methoxy-2,2-dimethylchromene (precocene I), and ageratochromene (precocene II)) and encecalin type. The proportions of precocene I and II differ between the species: the oil of A. conyzoides is rich in precocene I (about 80%) and poor in precocene II (less than 1%), and that of A. houstonianum contains approximately equal amounts (23-32% of precocene I and 24-44% of precocene II). The presence of terpenes as well as of chromenes has been established; β-caryophyllene is the main constituent (10% in A. houstonianum) after the precocenes of both oils.

Analysis of the aerial parts of A. conyzoides yielded 11 chromenes in total, and also the lignan (+)-sesamin and the sesquiterpene caryophyllene-epoxide (caryophyllene oxide). Another study found the largest amounts of precocenes in the leaves, followed by the flowering heads, whereas stems and roots had only minor amounts.

The genus Ageratum is known to be rich in flavonoids, especially of the polymethoxygenated flavone type. The following have been identified in A. conyzoides: 5,6,7,5'-tetramethoxy-3'4'-methylenedioxyflavone, 5,6,7,8-tetramethoxy-3'4'-methylenedioxyflavone (= linderoflavone B), 5,6,7,8,5'-pentamethoxy-3'4'-methylenedioxyflavone (= eupalestin), 5,6,7,8,3',4',5'-heptamethoxyflavone (= 5'-methoxynobiletin), 5,6,7,8,3',4'-hexamethoxyflavone (= nobiletin), 5,6,7,3',4',5'-hexamethoxyflavone, 5,6,7,3',4'-pentamethoxyflavone (= sinensetin) and 5,6,7,3',4',5'-hexamethoxy-8-hydroxyflavone. Flavones from A. houstonianum include 5,6,7,8-tetramethoxy-3'4'-methylenedioxyflavone (= lucidin dimethylether), eupalestin, 5,6,7,8,2',4',5'-heptamethoxyflavone (= agecorynin C), 5,6,7,8,2',3',4',5'-octamethoxyflavone (= agehoustin A) and 5,6,7,2',3',4',5'-heptamethoxyflavone (= agehoustin B).

The pyrrolizidine alkaloids 9-angeloylretronecine, lycopsamine and echimidine have been isolated from A. conyzoides. Furthermore, both species contain friedelin, β-sitosterol and stigmasterol; A. houstonianum additionally contains friedelan-3β-ol and also accumulates benzofuran derivatives in the roots.

Precocenes I and II have been synthesized in a single step in 2 hours, starting from a substituted monophenol and 3-methyl-2-butenal in pyridine at 140 °C.

Tests with oral administration of a leaf extract to rats in Africa showed that A. conyzoides has analgesic properties, but less than morphine. In tests with mice, an extract of whole plants induced hemagglutination against sheep red blood cells at a concentration of 24 μg/ml. A fairly good antiphlogistic effect by comparison with hydrocortison and inhibiting activity on experimentally induced tumour development in mice have been reported from Vietnam. The flavones have been suggested as the substances responsible for the promotion of wound healing. Crude material isolated from A. conyzoides leaves exhibited antibacterial activity in vitro against Staphylococcus aureus. Ageratum oil showed antibacterial activity against 20 bacteria in a test using 22 bacteria.

The essential oil has antifungal properties; in tests it inhibited the growth of Alternaria alternata, Aspergillus spp., Colletotrichum truncatum, Fusarium oxysporum, Helminthosporium tericum, Penicillium italicum, Rhizoctonia solani and Trichoderma viride. An aqueous extract of A. conyzoides also showed nematicidal activity against Meloidogyne incognita. Moreover, it exhibited insecticidal activity against diamondback moth (Plutella xylostella) in cruciferous crops, pulse beetles (Callosobruchus chinensis and C. maculatus) in stored leguminous seed, and corn weevils and red flour beetles (Tribolium castaneum) in stored maize. At concentrations of less than 10 mg/l it significantly inhibits emergence of adult mosquitoes; this inhibition has been attributed to the antijuvenile hormone activity of the chromenes. Precocene I and precocene II are able to induce precocious metamorphosis, cause sterilization and/or force diapause in certain insects, especially Heteropterans. The methoxyflavones possibly also have toxic effects on insects. A chloroform extract of leaves showed insecticidal activity against Drosophila melanogaster and Dysdercus cingulatus which was comparable to the standard insecticide malathion and better than the activity of the natural insecticide rotenone. Extracts derived from flowers of A. houstonianum have also shown ovicidal, antifeedant and repellent properties against several insects; acetone extracts were more effective than ethyl acetate extracts, and leaf and shoot extracts were less effective than flower extracts. Precocene II showed toxicity for American dog ticks (Dermacentor variabilis). A 2 cm thick layer of powdered leaves can control potato tuber moth (Phthorimaea operculella) in stored potatoes for up to 120 days. Locusts (Locusta migratoria) fed on goatweed during the nymphal stage were sterile; this implies that goatweed might be used for biological control if planted near breeding sites or on possible migration routes. Whitefly was disrupted in its development by A. houstonianum, though female insects were attracted by the plant to lay their eggs; this too indicates good prospects for biological control.

Antiviral activity against some legume viruses has also been reported.

In tests with rats in the Philippines, crude aqueous extracts of A. conyzoides applied through stomach intubation caused severe poisoning at concentrations of 15-20% after 5 days. The extract retarded follicular development in the ovary, and caused inflammation and degeneration of liver cells. Goatweed is also suspected of hepatotoxicity in livestock. Cattle deaths from liver damage in northern Sumatra have been attributed to pyrrolizidine alkaloid poisoning.

Goatweed shows allelopathic effects on crops such as rice and wheat by inhibiting seed germination and root elongation of seedlings.

Goatweed pollen is reported to cause commonly respiratory tract allergy, even in very low concentration.

Other plant sources of insecticides include Azadirachta indica A.H.L. Juss., Derris elliptica (Wallich) Benth. and other composites such as Tagetes spp. and Tanacetum cinerariifolium (Trev.) Schultz-Bip. Flavones with supposedly anti-inflammatory activity are also found in other Compositae such as Achillea millefolium L.

• Annual erect herbs, at the base sometimes decumbent and rooting, up to 120(-150) cm tall; roots fibrous.
• Leaves opposite in lower part of plant, higher ones alternate, simple, serrate-crenate above the entire base, with sparse long hairs above and below, glandular on lower side, pinnately veined or faintly 3-veined, distinctly petiolate; stipules absent.
• Inflorescence consisting of (1-)4-18 peduncled heads arranged in cymose clusters; involucre campanulate, involucral bracts 2-3-seriate, 2-ribbed, green with a pale or reddish-violet top; head 60-100-flowered.
• Flowers usually bisexual, 5-merous; corolla tubular in varying shades of blue and lavender or white, corolla lobes short, triangular; stamens inserted on inner side of the corolla, with fused anthers forming a ring around the style, with an apical appendage; ovary inferior, 1-celled, with 1 erect ovule on the bottom of the cell, style with 2 long filiform arms with thickened hairy tops, far exserted from the corolla.
• Fruit an oblong achene, dry and indehiscent, 1-2 mm long, 5-angular, scabrous on the angles, blackish with a pale base, on the outer margin of the top with a pappus consisting of 5(-6) free, membranous, awn-tipped, off-white scales 1.5-3 mm long.
• Seedling with epigeal germination; cotyledons leafy, arbicular, glabrous; hypocotyl up to 5 mm long, epicotyl absent or extremely short.

A single plant of goatweed can produce up to 40 000 seeds. There is no marked dormancy and germination is promoted by light and inhibited by burial. Seed of A. conyzoides can germinate at comparatively low temperatures (10-20 °C), which explains its occurrence at higher altitudes, whereas maximum temperature for germination is around 30 °C. For seed of A. houstonianum, the minimum temperature for germination is reported as 20 °C and the maximum temperature 35 °C.

The life cycle can be completed in less than 2 months. Flowers and fruits may be present throughout the year. The occurrence of male sterility has been documented for both A. conyzoides and A. houstonianum. The usually male fertile A. conyzoides is self-pollinated and cross-pollinated by insects. A. houstonianum has been reported as self-incompatible. Fruits are dispersed by wind but may also cling to the fur of animals.

A. conyzoides and A. houstonianum are closely related, and the latter has even been considered as merely a variety of the former (A. conyzoides var. houstonianum (Miller) Sahu). Although both resemble each other closely and are often confused, there are some reliable morphological distinguishing characteristics, particularly in the involucral bracts and leaf-bases. There are also minor differences in number of flowers per head (on average more in A. houstonianum), and length of corolla, anthers and style (on average longer in A. houstonianum). Both grow in similar habitats and both may become weeds. A. conyzoides has a short-day ecotype and a day-neutral ecotype.

Several cultivars of A. houstonianum are popular garden plants, e.g. cultivar "Blaue Donau" in Europe.

Goatweed can be found from sea-level up to 2500 m altitude. It is a very common weed in numerous annual and perennial crops. It commonly grows in roadsides, waste places and grassy fields. In shifting cultivation in Thailand, A. conyzoides is one of the dominant species in the second year after clearing of the forest, together with Chromolaena odorata (L.) R. King & H. Robinson. A. houstonianum has a more restricted altitudinal range than A. conyzoides; in Java it occurs up to 1700 m altitude.

Propagation by stem cuttings is more efficient in A. houstonianum than by seed or root, resulting in rapid growth and reproduction. Protoplasts have been isolated from leaves, stems and callus of A. houstonianum, and callus has regenerated.

A. conyzoides is the host to several pathogens causing serious diseases in commercial crops: for Pseudomonas solanacearum attacking Irish potato in India and for the nematodes Meloidogyne incognita (despite the reported nematicidal activity against the same species) and M. javanica causing yellowing and wilting in black pepper and vegetables in the Philippines and Pratylenchus sp. attacking upland rice in the Philippines. A geminivirus called ageratum yellow vein virus, which is transmitted by the whitefly Bemisia tabaci, causes a vein-yellowing disease in A. conyzoides; it has been reported from Malaysia and Singapore. A similar virus, the tobacco krupuk virus, is reported for Indonesia. A. conyzoides is a host of cotton bollworm in Thailand and for Nysius inconspicuous in India (a pest on sesame). A. houstonianum has been reported in India as a host for green scale (Coccus viridis) which is a major pest of coffee.

Both species are important weeds in arable and plantation crops. The lepidopterous insect Parauchaetes pseudoinsulata can complete its life cycle on A. conyzoides and might be a promising agent to control this species biologically in areas where it is a serious weed.

Both goatweed species are widespread in anthropogenic habitats, suggesting a broad genetic variability. The existence of different photoperiodic ecotypes is of interest for breeding, as it provides an opportunity to obtain planting material adapted to specific conditions. Breeding programmes could focus on optimum medicinal and insecticidal activity which is reflected in high concentrations of active compounds such as precocenes. The breeding value of male sterility is rated as low.

Goatweed has several widespread and comparatively well-documented medicinal applications. It has outstanding prospects as an insecticide. The insecticidal properties offer possibilities for the control of mosquitoes that are vectors of malaria and filariasis, and for the control of insects like locusts, bugs and mites. The effect on phytophagous mites in citrus deserves further research. Goatweed is easy to cultivate, which makes it suitable for large-scale production for industrial use as well as for home gardening for local use.

Since goatweed establishes spontaneously, it is a cheap and beneficial soil cover crop. Moreover, certain forms of goatweed are attractive ornamental plants.

• Adesogan, E.K. & Okunade, A.L., 1979. A new flavone from Ageratum conyzoides. Phytochemistry 18(11): 1863-1864.
• Bioka, D. & Mabika, A., 1993. Effet analgésique d'un extrait brut d'Ageratum conyzoides chez le rat [Analgesic effect of a crude extract from Ageratum conyzoides on the rat]. In: Schilcher, H., Phillipson, J.D. & Loew, D. (Editors): First World Congress on Medicinal and Aromatic Plants for Human Welfare, Maastricht, the Netherlands, July 19-25, 1992. Pharmacology, phytotherapy, human welfare, regional aspects. Acta Horticulturae No 332: 171-176.
• Durodola, J.I., 1977. Antibacterial property of crude extracts from a herbal wound healing remedy - Ageratum conyzoides. Planta Medica 32(4): 388-390.
• Gonzalez, A.G., Aguiar, Z.E., Grillo, T.A., Luis, J.G., Rivera, A. & Calle, J., 1991. Chromenes from Ageratum conyzoides. Phytochemistry 30(4): 1137-1139.
• Gonzalez, A.G., Aguiar, Z.E., Grillo, T.A., Luis, J.G., Rivera, A. & Calle, J., 1991. Methoxyflavones from Ageratum conyzoides. Phytochemistry 30(4): 1269-1271.
• Johnson, M.F., 1971. A monograph of the genus Ageratum L. (Compositae-Eupatorieae). Annals of the Missouri Botanical Garden 58(1): 6-88.
• Kaul, M.L.H. & Neelangini, 1989. Male sterility in diploid Ageratum conyzoides L. Cytologia 54(3): 445-448.
• Ohta, T., Kuhr, R.J. & Bowers, W.S., 1977. Radiosynthesis and metabolism of the insect antijuvenile hormone precocene II. Journal of Agricultural and Food Chemistry 25(3): 478-481.
• Sharma, V.S., 1987. Comments on the identity of Ageratum conyzoides L., and A. houstonianum Mill. - two naturalized weeds in India. Feddes Repertorium 98(11-12): 557-560.
• Siebertz, R., Proksch, P. & Witte, L., 1990. Accumulation and biosynthesis of the chromenes precocene I and II in Ageratum houstonianum. Phytochemistry 29(7): 2135--2138.

• Ageratum conyzoides
• Ageratum houstonianum

• Slamet Sutanti Budi Rahayu, Rina Ratnasih Irwanto & L.J.G. van der Maesen