Chromolaena odorata – commonly called chromolaena, triffid weed or Siam weed – is one of the worst invasive plant species in the humid tropics and sub-tropics of the world. Its native range extends from Florida in the United States to northern Argentina, but it has invaded south-east Asia, parts of Oceania, and West, Central and southern Africa, where it is a major threat to biodiversity, agriculture and human welfare.
One continent, two invasions
Chromolaena occurs as both a shrub standing at least 3 metres tall in the open, and as a scrambler reaching a height of 10 metres among trees. It grows rapidly and produces massive quantities of small, light seeds – more than a million per plant – which are dispersed over long distances by wind, and also by humans, animals and water.
The plant thrives on disturbed land and readily invades crops, pastures and plantations. It tends to form dense thickets, which smother indigenous vegetation and increase the intensity of fire. In Africa there are two centres of invasion, each with a distinct form – or biotype – of chromolaena. The west African form was introduced from an invasive population of chromolaena in south-east Asia, while the morphologically different southern African form apparently originated from a northern Caribbean island, possibly Jamaica. Chromolaena was first introduced to Nigeria in the late 1930s, and spread rapidly through most of West and Central Africa.
It now occurs from eastern Guinea to the central parts of the Democratic Republic of Congo and the Central African Republic, and southwards to northern Angola. It decreases the region’s agricultural productivity by invading crops, grazing areas and young or neglected plantations of timber, cocoa, citrus, rubber and oil palm. The leaves cause acute diarrhoea of cattle when browsed, and skin rashes and irritation in some people after contact.
Chromolaena was probably introduced to southern Africa either deliberately as an ornamental plant, or accidentally in seed-contaminated packing material off-loaded at Durban harbour from the West Indies. The plant was recorded as naturalised in the Durban area in the 1940s, and by the 1970s had spread throughout the subtropical areas of KwaZulu-Natal province. It has since invaded other provinces to the south and north, as well as the neighbouring countries of Swaziland, Mozambique and possibly Zimbabwe. It is anticipated that the invasion front will in time converge with that of the West African / Asian biotype (which also occurs in Mauritius), as the intervening region is climatically suitable.
Threatening conservation and ecotourism
In South Africa, chromolaena is mainly considered a threat to conservation and ecotourism, as it has primarily invaded natural areas. It reduces the biodiversity of grassland, savannah and forest, and compromises game-viewing in nature reserves and national parks. It also impacts on commercial forestry, both by suppressing the growth of young pine and eucalypt trees through competition, and by allowing fire to penetrate deeper into plantations. The plant impinges on agriculture to a lesser extent than has been reported for other invaded regions. While it does a ffect subsistence grazing and cropping, commercial sugarcane and fruit-farming enterprises are able to afford the costs of control.
Chromolaena control requires an integrated approach, the methods used being dependent on the size of plant and the type of vegetation infested. Repeated follow-up work is necessary, as the plant is capable of vigorous growth from stem coppice, root suckers and seed.
|Pareuchaetes caterpillars |
Seedlings and young plants can be removed by handpulling, while herbicides are available for cut-stump treatment and for foliar application to seedlings and coppice growth. An annual burning regime effectively controls chromolaena invasions in grassland by killing mature plants and preventing new seedlings from establishing.
Initial attempts to achieve biological control of chromolaena in Africa failed, after the leaf-feeding moth Pareuchaetes pseudoinsulata and a seed-feeding weevil released in Nigeria and Ghana in the early 1970s failed to establish. The programme was revived by Ghana in 1989, and a decade later P. pseudoinsulata was well-established in the country’s infested areas. Feeding by the moth has significantly reduced chromolaena populations, while other herbaceous species have recovered. Additional potential biocontrol agents are also being investigated, particularly the stem-galling fly Cecidochares connexa, which has proved very successful in Indonesia.
However, in the rest of the West and Central African region there has been little progress with regard to biological control, due to a perceived conflict of interest with the agricultural sector, which views chromolaena as an important fallow crop.
P. pseudoinsulata and the related P. aurata aurata failed to establish, but a third species, P. insulata, is now being released in large numbers. The adult moth lives for about a week, duringwhich time it does not feed but mates and lays eggs, the female depositing groups of up to 80 eggs on the underside of chromolaena leaves. After hatching the cater-pillars feed on the leaves, sometimes completely defoliating plants. The resulting reduction in plant height and density allows other plant species to compete for the newly created space. Other biocontrol agents that are currently being assessed in field trials, in quarantine, or in their country of origin include a leaf-mining fly Calycomyza eupatorivora, a stem-boring weevil Lixus aemulus, a stem-galling weevil Conotrachelus reticulatus, a root-boring flea beetle Longitarsus horni, a stem-tip mining moth Carmenta sp., a stem-galling fly Polymorphomyia basilica, and a shoot-tip mining fly Melanagromyza eupatoriella, as well as two leaf-attacking pathogens.
|Pareuchaetes caterpillars |
Triffid Science Fiction
Triffid weeds were walking, man-eating plants in the science-fiction book The Day of the Triffids, written by British author John Wyndham in the 1950s. The name was adopted for chromolaena because of the plant’s monstrous, alien-invading characteristics!
Chromolaena and CrocodilesChromolaena is the main invasive plant species in South Africa’s Greater St Lucia Wetland Park, a World Heritage Site in the province of KwaZulu-Natal. The infestations are not only threatening the breeding habitat of crocodiles in the park, but also causing a gender bias in their offspring! The crocodiles lay their eggs in sand on the banks of Lake St Lucia, but chromolaena is encroaching onto these sites and shading the nests. Gender in crocodiles is determined by the temperature at which the eggs are incubated, and shading by chromolaena stands has been shown to lower the temperature of nesting sites by 5-6°C. This is sufficient to induce female-biased sex ratios, or may even prevent embryonic development altogether.
Reference: Matthews S. & Brandt K. Africa Invaded: The growing danger of invasive alien species Global Invasive Species Programme 2004