Acanthospermum hispidum (bristly starbur)

A. hispidum is a troublesome annual weed of annual and perennial crops, which has spread remarkably widely from its origins in South America, almost exclusively as a result of accidental rather than deliberate transfer. The spiny fruits are a particular hazard to livestock and assist the movement of the weed in agricultural produce. There are many countries, and regions within larger countries, to which it could yet be introduced and prove invasive, with corresponding costs to agriculture.

Acanthospermum combines the Greek akanthos, meaning 'spiny' or 'thorny', with sperma, meaning seed, a reference to the large hooked spines on the seed; hispidum is from the Latin hispidus, meaning 'hairy' or 'bristly', alluding to the generally rough, hairy covering on stems and leaves (Parsons and Cuthbertson, 1992).

A. hispidum is an annual herb, up to 90 cm high with a shallow, branched tap root. The stem is erect, and exhibits regular dichotomous branching to form a more or less flat-topped plant, covered in stiff hairs. Leaves are in opposite pairs, without petioles, obovate, up to 8 cm long and 3 cm wide, slightly lobed, irregularly toothed and hairy. Inflorescences are solitary, composite heads in the axils of upper leaves, sessile, pale yellow-green and about 6 mm in diameter. Florets are quite few and are monoecious, the 5-10 ray florets being female and a similar number of disc florets male. The fruit consists wedge-shaped achenes grouped into star-shaped clusters. The achenes (also called burrs) are about 6 mm long and covered with numerous short, stiff, hooked spines, with two much longer spines at the apex.

A. hispidum originates from South America and can be considered native wherever it occurs in South and Central America and in the Caribbean (USDA-ARS, 2005), but has spread very widely in North America, Africa, Asia and Australia and now occurs in over 60 countries (Holm et al., 1997; USDA-ARS, 2005, etc). Although mainly tropical and sub-tropical in distribution, it is also recorded from temperate Northeast Russia, near Vladivostok (Buch and Shvydkaya, 1989), and from France (Ballais, 1969).

It was introduced accidentally to Florida, USA, in the 1800s (Hall and Vandiver, 1991); first recorded in Queensland, Australia in the late 19th century (Parsons and Cuthbertson, 1992); in India in 1917 (Tadulingam and Venkatarayana, 1955); and in Kenya in 1945 (Blundell, 1992). In each case there was some lag before it became invasive, but in India this was apparent by the 1940's. In Australia it was naturalized by 1906 and then spread steadily. The origin and means of introduction are generally unknown, but in Florida and at other points along the eastern USA, it is believed to have been introduced in ship ballast (Hall and Vandiver, 1991) and in most other areas it is assumed to have come in accidentally, as a crop seed contaminant, or adhering to imported wool, skins and furs, rather than deliberately. In southern India, it was first noted around railway stations, and it seems clear that railways have been an important means of spread within countries.

Although a very large number of countries outside South America have already been infested, there is certainly scope for further spread, e.g. to Mexico and Japan, as well as more widely within countries such as USA and China where many regions with warm summer temperatures remain free so far. As the weed commonly occurs in a wide range of crops in so many countries there is a highly significant risk of accidental introduction via contaminated crop seed as well as in other agricultural produce, including animal products and bags, etc., to which the spiny fruits may be attached. Deliberate introduction is somewhat less likely, though its many traditional medicinal uses could result in a temptation to import for development of pharmaceuticals. A. hispidum is listed as a prohibited or restricted species for the states of Western Australia and Northern Territories in Australia, and for Hawaii, USA.

A. hispidum is a widely occurring crop weed (Holm et al., 1997) infesting small- and large-grained cereals, legumes, tree crops (citrus, peach, rubber, vines), vegetables and oil seeds. Most crops are likely to be infested with A. hispidum when grown within the habitat range of this weed.

Acanthospermum australe is a prostrate, glandular herb, similar to A. hispidum but softly tomentose on all parts.Acanthospermum glabratum is a trailing, annual herb, uncommon in Kenya in the 1970s but becoming quite a common weed in central Kenya by 1990 (PJ Terry, Long Ashton Research Station, University of Bristol, personal communication, 1998).

A. hispidum is a common weed of arable land, pastures, roadsides and wasteland. It grows in a wide range of climatic conditions and soil types (particularly light soils). It appears to behave similarly in both native and invaded areas. It is mainly tropical and sub-tropical in distribution, but occurs up to altitudes of 1700 m in East Africa (Terry and Michieka, 1987), 1500 m in Bhutan (Parker, 1992) and 1300 m in the Dominican Republic (Jurgens, 1977). It is also recorded from temperate Northeast Russia, near Vladivostok (Buch and Shvydkaya, 1989), and from France (Ballais, 1969), but whether it is fully naturalized in these localities seems doubtful.

Physiology and PhenologyA. hispidum is a prolific seed producer; a pure stand can contain more than one million plants/ha, producing one billion seeds, weighing almost 10 tonnes (Schwerzel, 1970). After burial for one year, 80% of seed remains viable. Freshly harvested seed has 36% germination, dropping to 15% after dry storage for one year (Holm et al., 1997 - citing the work of P Thomas in Zimbabwe during 1975-82). Most seeds of A. hispidum emerge within three years of production and all seeds die within eight years whether the field is cultivated or not (Schwerzel and Thomas, 1979; Voll et al. 2001). Garcia and Sharif (1995a,b) showed that seeds were generally dormant immediately after being collected. The achenes responded positively to light, and germination was equally promoted by red and far-red light. The germination was highest in the temperature range 20-25°C. Seeds also responded to scarification.Seedlings emerge over an extended period in the presence of adequate soil moisture and when temperatures are high enough (20-30°C). Subsequent growth is rapid, the first fruit appearing 35-40 days after emergence, seed maturation occurring 45-50 days later (Parsons and Cuthbertson, 1992). Biomass and seed production are reduced when A. hispidum grows in the shade. This probably accounts for its reduced competitiveness in mixed cropping systems compared with crop monocultures (Shetty et al., 1982).Reproductive BiologyA. hispidum is monoecious, having male flowers in the centre and female flowers on the outside of the inflorescence. Self-pollination and cross-pollination occur by wind. The spiny achenes (burrs) are readily dispersed on the wool and fur of animals and through contamination of hay and crop residues. They are also carried considerable distances by streams and floodwaters.Environmental RequirementsA. hispidum apparently requires a certain minimum warmth to germinate and thrive. In South Africa it occurs in the summer rainfall areas but not so much where there is a Mediterranean climate with winter rainfall. It would presumably be susceptible to frost, but, as an annual, it can occur in temperate areas with very low minimum temperatures, as the summer temperatures, even in e.g. Northeast Russia, can regularly exceed 20°C.A. hispidum is especially common on very light sandy soils, but is by no means restricted to this soil type, occurring as a serious weed on a wide range of soils, including clays.AssociationsA. hispidum is an alternative host for several crop pests, see the section on Economic Impact for further details.

Lloyd (1956) searched for natural enemies of A. hispidum in South America (the centre of origin of this weed) but concluded that there was little hope of finding effective biological control agents. Heliothis peltigera is of some benefit in controlling A. hispidum in Gujarat State, India but it can be a pest of economic plants (Manjunath et al., 1976).

A. hispidum is a weed problem in at least 25 crops, and is one of the three principal weeds of Zimbabwe and Brazil (Holm et al., 1997). It is a declared noxious weed in Hawaii, USA and Australia. The quality of sheep wool is reduced when contaminated by the spiny achenes, and livestock are harmed when the achenes penetrate the hooves, often resulting in infection and subsequent lameness. The plant is toxic to animals when consumed on a daily basis, but is generally avoided by livestock (Holm et al., 1991)Crop yields are decreased in the presence of A. hispidum which competes for water, nutrients and light. Walker et al. (1989) illustrated a negative linear relationship between groundnut seed yield and the period of interference from A. hispidum. Each week of interference reduced seed yields by 20, 205 and 134 kg/ha, respectively, over three consecutive years, and as little as two weeks of interference caused significant seed yield reductions. Full-season interference from 8 and 64 A. hispidum plants per 7.5 m of crop row reduced groundnut seed yields by 14% and 50%, respectively. Chivinge (1990) recorded losses in soyabean due to A. hispidum planted in pots the same day, but no loss when the weed was planted after 11 or 22 days. Congo, N'zama and Tombé (1999) studied different ratios of soyabean and A. hispidum at a range of densities and concluded that the two species share the same resources without antagonistic interactions. Holm et al. (1997) note that this is a C3 plant with relatively inefficient photosynthesis but that its competitive effect may be enhance by allelopathic influence. Crop losses are also caused by interference from A. hispidum at harvest time. This is a major cause of soyabean yield losses in Brazil (Davis et al., 1979).A. hispidum is an alternative host for a number of crop pests and diseases including: Heliothis peltigera and H. armigera [Helicoverpa armigera] (Lepidoptera: Noctuidae) (Patel et al., 1971; Manjunath et al., 1976); Calidea dregii (Hemiptera: Scutelleridae), a cotton pest in Tanzania (Reed and Kayumbo, 1965); the soyabean pests Euschistus heros and Nezara viridula (Hemiptera: Pentatomidae) (Panizzi and Rossi, 1991; Mourao and Panizzi, 2000); Tomato leaf curl virus [Tomato yellow leaf curl virus] (Mariappan and Narayanasamy, 1972; Sastry, 1984); Tobacco leaf curl virus (Mariappan and Narayanasamy, 1977; Swanson et al., 1998); Groundnut ringspot virus, the causal agent of spotted wilt in tobacco, (Nunes-e-Silves et al., 2000); Xanthomonas campestris pv. sesami (Nayak and Sharma, 1980); and Verticillium albo-atrum, a wilt-causing pathogen (da Silva and Tokeshi, 1979).

A. hispidum appears to occur relatively rarely in undisturbed habitats, and has not been reported to have an undesirable impact on natural vegetation.

The spiny fruits of this weed can cause pain and injury to man and animals, and there are some concerns about toxicity to livestock, but there are no reports of serious impact on tourism or other human activities.

A. hispidum is used in traditional medicine for its anthelmintic and antalgic properties in central Africa (Menut et al., 1995). The root is used to cure coughing and bronchitis and a boiled tea of the leaves reduces fever, promotes sweating and cures diarrhoea (Holm et al., 1997). Various tribes in west Africa use A. hispidum for the treatment of arthritis, leprosy, migraine, rheumatism and stomach complaints (Burkhill, 1985). Summerfield and Saalmüller (1998) refer to the use of A. hsipidum as a traditional treatment for infectious diseases in Benin and show that, in the porcine immune system, A. hispidum enhances the proliferation of T lymphocytes. Summerfield et al. (1997) also showed inhibitory activity of A. hispidum extracts on several herpes and pseudo-rabies viruses.

Cultural Control

Quarantine must be considered an integral part of any control programme. All animals brought onto a farm, as well as those transferred from infested to burr-free parts of the farm, must be checked and any burrs removed (Parsons and Cuthbertson, 1992).

Seeds of A. hispidum lose their viability within eight years (Schwerzel and Thomas, 1979), so that prevention of seeding will eventually exhaust the soil seedbank and eradicate the weed. Seedbank depletion can be achieved by ploughing to bury existing seeds to a depth of at least 10 cm, followed by the sowing of an appropriate perennial pasture. Any seedlings which emerge in the pasture should be removed by pulling, hoeing or with herbicides (Parsons and Cuthbertson, 1992).

Cultivation or slashing prior to seed set is beneficial but slashing after seed set will quickly spread the plant (Miller and Schultz, 1997). Mowing A. hispidum and other weeds growing through a groundnut crop was shown by Wehtje et al. (1999) to be beneficial as an alternative to a late post-emergence treatment.

Chemical Control

Herbicides that are reputed to give control of A. hispidum include: 2,4-D, diolamine, acetochlor, atrazine, bentazon, butachlor, cyanazine, diuron, fluometuron, imazethapyr, lactofen, linuron, mefluidide, metolachlor, napropamide, oxyfluorfen, paraquat, pendimathalin, simazine and terbutryn. Pendimethalin is reported to be selective against A. hispidum in groundnut (Gowda et al., 2002), though Reddy et al. (2002) found A. hispidum to be less susceptible than other broad-leaved weeds to pendimethalin, fluchloralin and metolachlor in field bean. A. hispidum was shown by Luo and Matsumoto (2002) to be somewhat more susceptible to the graminicide fluazifop-butyl than other broad-leaved weeds but Gowda et al. (2002) did not confirm this in the field. Foloni and Christoffoletti (1999) indicate promising selectivity in soyabean for the new herbicides carfentrazone and carfentrazone-ester.