Chromolaena odorata - the invincible invader

The spread of the weed Chromolaena odorata formerly known as Eupatorium odoratum, which was only recently introduced into the continent, is assuming worrying proportions. The plant is taking over as forests in the tropical humid zones of Africa are being cleared. According to some agronomists, the introduction of this invasive plant is a real disaster story. The common names that local people have given to Chromolaena odorata are an indication of its fearful reputation. In Cameroon it is known as 'King-Kong', in Zaire as 'Cholera' and in Cote d'lvoire as 'S\82kou Tour\82'. It originated in the West Indies and tropical America, but was deliberately introduced into Africa as a cover crop for protecting plantations from erosion and weeds, as well as for regenerating the soil. Failure to keep it under control has meant that the plant has now become a major weed itself. Scientists at CIRAD/EMVT (Elevage et M\82decine V\82t\82rinaire des Pays Tropicaux) say that it is now considered to be a major pest of perennial crops, of some annual crops and also of pasture land where ecological conditions are favourable. Their description of its growth habit suggests a formidable adversary. The branches intertwine and form an impenetrable thicket 2.5 metres in height. If the plant is supported by shrubs or bushes, which it can completely smother, it can reach over 4 metres in height. Worse than couch grass Throughout the Sudano-Guinea ecological zone of West Africa, and its equivalent in Central Africa, there is great concern expressed about what some agronomists are referring to as a botanical disaster. 'It is so tough it can regrow even from the stumps left when all the top part of the plant has been destroyed. It colonizes every available sunny space and discourages even the most determined farmer. It is possible to control Chromolaena if the stumps are removed completely and the ground is kept thoroughly weeded, but it is very hard work.' says Pascal Marnotte of the Department of Annual Crops at CIRAD, Montpellier. In their efforts to control its spread, scientists have come up against a major problem. The seeds travel long distance caught in the coats of animals or in the mud that sticks to passing vehicles. Every year new areas are infested with Chromolaena, which can form a dense thicket over several hundred hectares within a few years. In Cote d'Ivoire, Ghana and the Central African Republic new plantations have little chance of survival if farmers are not constantly vigilant. Young plants can be smothered by Chromolaena as soon as they emerge from the ground, and in recent years some little-used minor roads have actually been rendered impassable by this invasive weed. 'When pastoralists in the Central African Republic attempt to settle all or part of their herds in one place, Chromolaena discourages their efforts and hampers their permanent settlement,' say EMVT scientists who are particularly concerned with the problems facing pastoralists in this, the most seriously affected country in the region. Trails are disrupted, campsites are covered over, watering holes become inaccessible and fodder crops become overgrown with the weed. 'Furthermore, the dense cover becomes a refuge for predators such as jackals and hyenas which attack calves and young animals,' add the EMVT scientists. It is no wonder that the livestock farmers curse this plant when it invades their territory. Why has Chromolaena taken such a hold in Africa? In its country of origin the plant does not have the same devastating impact because it co-exists with parasites which control its spread. Attempts have been made at biological control through the introduction into Africa and Indonesia of Pareuchaetes pseudoinsulata, a leaf eating caterpillar. 'Although its introduction into Guam has been successful, the insect has barely survived in Cote d'lvoire. It is probably being eaten by ants.' explains Dominique Mariau of CIRAD. 'It is proving very difficult to establish and, so far, populations are low; but it is too early to come to any final conclusions.' The other option is to make annual applications of chemical weed killers, but these are generally too expensive for most farmers. Alternatively, farmers can resort to traditional methods of control such as slash and burn; or they can plant fast-growing hedges of Tithonia diversifolia or Leucacna glauca, which compete with the Chromolaena and have a suppressing effect on it. Johan Huguenin, a researcher at CIRAD/ EMVT who has been working on this problem in the Central African Republic for a long time, says that it is not possible to eradicate the plant completely even by combining curative and preventative control. However there are ways of limiting its development to a level at which it is not harmful to livestock and, furthermore, it can continue to help to prevent other forms of pasture degradation.