2. STATUS AND CONTROL OF CHROMOLAENA IN KWAZULU-NATAL 1 Chromolaena odorata
2.1.3 Chromolaena control research .1 Mechanical control
Slashing and uprooting of chromolaena is labour intensive (85 man days ha·l) and control is short (Goodall& Erasmus 1996). The method causes high levels of disturbance, creating an ideal environment for further weed germination. Labour intensive techniques are best suited to fallow cropping systems where clearing efforts can be combined with soil preparation under shifting cultivation in resource-poor regions (Muniappan& Marutani 1991; Slaats 1995). Follow-up operations involving slashing and ring-weeding up to four times per annum are required to prevent chromolaena from interfering with crop production. In South Africa mechanical control is usually practised with the application of herbicides. Examples include slashing tall growth (a) to promote coppice that can be sprayed later, or (b) applying herbicide to the stumps (Erasmus 1988).
2.1.3.2 Cultural control
Chromolaena has become an important component in shifting agriculture in West Africa (de Foresta& Schwartz 1991; de Rouw 1991). Rainforests are cleared for crops, as were coastal forests in KZN during the Iron Age (Section 2.2.3). When crop production in rainforest clearings becomes uneconomical the farmers move into new areas and repeat the process.
Abandoned fields are soon invaded by chromolaena, preventing forest succession. A method of nutrient recycling by mulching with chromolaena has been developed in fallow cropping in West Africa (Slaats 1995). Fallow-cropping with chromolaena is preferred to clearing forest, the latter yielding a lower return on the energy invested during labour intensive site
preparation (Roderet al. 1995; Slaats 1995). Chromolaena also displacesImperata
cylindrica which is a serious weed of crops and fallow land in the tropics (Ivens 1975).
Fallow-cropping with chromolaena has not been tested in South Africa. The agricultural benefits are overshadowed by the ecological impacts. New research intends to determine its potential in economically depressed regions where crops could be cultivated in place of dense chromolaena infestations. Great care is needed to ensure the chromolaena is not promoted as a new 'agroforestry wonder-plant'. Mulches, leguminous ground covers and signal grass (Brachiaria decumbens) were also found successfully to compete with and reduce the incidence of chromolaena (Wu& Xu 1991) and are indirect methods of control. In rural areas it is hoped that clearing of dense infestations will be integrated with crop production on otherwise useless ground.
2.1.3.3 Biological control
In its country of origin chromolaena is not invasive, although it is sometimes a minor problem during plantation establishment. No control methods other than slashing are required in the Neotropics, the reverse of the situation in Africa and Asia. The only logical explanation is the cumulative effect of biotic factors, principally host-specific insects, other arthropods and diseases, which attack the plant throughout its range in the New World, but are largely absent from Asia and Africa (Cruttwell McFadyen 1991).
Biocontrol involves the use of natural enemies, especially insects and fungi, from the weeds' countries of origin, to decrease reproduction and growth to a level where other control methods can be more effectively employed. Chromolaena biocontrol research began in 1988 with the aim of introducing a suite of insects to attack different parts of the plant (leaves, stems, roots, flowers). In 1989 a defoliating mothPareuchaetes pseudoinsulata was released with unsuccessful establishment in the wild (Zachariadeset al. 1999). Another defoliating mothP. aurata aurata was released more numerously between 1990 and 1993 but it was also unable to establish in the field. Several other insect species have been imported, cultured and tested but onlyP. insulata has been authorised for immediate release, but no field data are available yet. Several species still under quarantine appear promising, namely
Melanagromyza eupatoriella (stem-tip borer), Lixus aemulus (stem borer), Calycomyza sp.
(leaf miner) andLongitarsus sp. (root borer). Research on these insects is at differing stages
but one species is due for release at the end of 2000, followed by another species in 2001. If the remaining insects prove host-specific, they should be introduced into the wild by 2004 (Zachariades pers. comm., Plant Protection Research Institute, Private Bag X6006, Hilton 3235). Once an agent is released, it takes several years for it to have an impact on the weed populations, if it does establish a population.
2.1.3.4 Chemical control
Chromolaena has conclusively proved easy to kill by herbicides. A number of herbicides were screened between 1983 and 1993 (Erasmus 1985; Erasmus& van Staden 1986a;
Erasmus& van Staden 1987; Erasmus 1988; Goodall1997) and by 1995, seven formulations comprising 16 products were registered for chromolaena control (Table 2.1). These include
11 foliar, two cut-stump and three soil applications (Vermeulen et
at.
1995). Commercial enterprises employ chemical control effectively, e.g. forestry companies, but this method is inappropriate for unskilled subsistence farmers in South Africa.Table 2.1 Herbicides registered for the control of Chromolaena odorata in South Africa (Vermeulen et
at.
1995)Active ingredient Site of Herbicide mixture No.
application (%concentration) products
glyphosate 359 g Q-I foliage H/100Qwater 7
triclopyr 480 g Q-I foliage 375 mQ/1OOQwater 1
triclopyr 480 g Q-l stumps 1Q/1OOQ diesel 1
metsulfuron methyl 600 g kg-I foliage 25 g/100Q water 2
sulfosate 720 gQ'! foliage 0,67Q/IOOQ water 1
imazapyr 100 g Q-I stumps 2Q/1 OOQ water 1
tebuthiuron 200 g kg'l soil 1g/m·2 2
tebuthiuron 752 g kg-I soil 1kg/10Qwater 1
2.1.3.5 Integrated Control
At present effective integrated control of chromolaena is only feasible by combining
mechanical, cultural and chemical control, depending on the type of ecosystem invaded and the density of infestations (Table 2.2).
Table 2.2 Integrated strategies for the control of Chromolaena odorata in different situations (Goodall et al. 1996a)
Habitat
Savanna
Density
sparse
Strategy
Spot-spray leaves with triclopyr, or slash and apply imazapyr to stumps, or uproot plants by mattock. Remove seedlings by hand in future follow-up operations, or spray with triclopyr.
Spray leaves with triclopyr if plants are< 1.2 m. Sash taller plants and apply dense imazapyr to stumps, or spray the regrowth with triclopyr. Follow up with
regular spraying until it becomes feasible to remove seedlings by hand.
Indigenous forest
sparse
Spray leaves with triclopyr ifplants are<2m. Slash taller plants and apply imazapyr to stumps, or spray the regrowth with triclopyr. Plants may also be uprooted if populations are low enough to warrant the expense. Remove seedlings by hand in future follow-up operations, or spray with triclopyr.
Spray leaves with triclopyr if infestations are< 1.2 m. Sash taller plants and dense apply imazapyr to stumps, or spray the regrowth with triclopyr. Follow up
with regular spraying until it becomes feasible to remove seedlings by hand.
Timber plantations
sparse
Spray leaves with triclopyr if plants are<2m. Slash taller plants and apply triclopyr to stumps, or spray the regrowth with triclopyr. Plants may also be uprooted ifpopulations are low enough to warrant the expense. Follow up control depends on the silvicultural policy.
Fallow land
Border infestations
Gardens
Spray leaves with triclopyr if infestations are<1.2 m. Sash taller dense infestations and apply triclopyr to stumps, or spray the regrowth with
triclopyr. Follow up control depends on the silvicultural policy.
Slash, uproot and burn infestations or incorporate debris as a form of green all densities manure. Follow-up with manual weeding (hoeing) or using registered
herbicides appropriate for the specific crops.
If accessible by vehicle chromolaena thickets growing along forest margins, dense watercourses and roadsides can be sprayed with triclopyr from LDV and
tractor-mounted sprayers(GoodaIl1997~.
all densities Remove seedlings by hand. Slash and uproot taller plants.
Municipal
sparse
Spot-spray leaves with triclopyr, or slash and apply imazapyr to stumps, or uproot plants by mattock. Remove seedlings by hand in future follow-up operations, or spray with triclopyr.
Spray leaves with triclopyr if plants are<1.2 m. Sash taller plants and apply dense imazapyr to stumps, or spray the regrowth with triclopyr. Follow up with
regular spraying until it becomes feasible to remove seedlings by hand.
2.1.3.6 Rehabilitation in coastalforest
Trials in infested coastal forest at St. Lucia compared control efficacy of mechanical control (uprooting and uprooting combined with burning) and chemical control integrated with grass planting and fertilization (Erasmus 1991). Fire reduced the intensity ofreinfestation and
improved establishment of sown grasses. Grass establishment two years later was poor in unburnt plots due to the thick mulch layer left after clearing. Setaria megaphylla grew the best where chemical control was applied (2.5 t ha-I) compared with the next bestPanicum
maximum(1.4 t ha-I). P. maximumgrew best in both manual control treatments butChloris gayana yields were also significant. Natural grasses establishing in plots that were not planted was highest in the plots treated with herbicides (0.9 t ha-I) and lowest in the plots which were burnt (0.4 t ha-I). Nitrate and phosphate application increased grass yields but over-sowing, or planting of grass, was expensive and was considered only feasible in situations where large areas of exposed soil were left after clearing chromolaena.
2.1.3.7 Developing landscape level control strategies
In an attempt to find tactical solutions to deal with the chromolaena problem on a large scale a control strategy was developed embracing both 'big picture' (Goodallet al. 1996a) and sustainable chromolaena control on a local scale (Goodall& Naude 1998). The strategy comprises two functioning levels that are interdependent. The "macro strategy" analyses the problem on a national or regional scale without being prescriptive while "micro strategies"
focus on the planning and management of chromolaena in prioritised areas on farms, nature reserves and plantations. Macro strategies incorporate (a) inventories integrating weed
surveys, mapping and modelling of chromolaena spread; (b) setting up planning, management and regulatory structures; (c) identifying regional priorities; and (d) allocating strategic resources. Micro strategies consist of management plans containing control strategies for treating key areas. The success of managing chromolaena on a regional scale depends on the collective commitment of all the affected parties and an organised management structure at the macro level that can promote job empowerment opportunities and deliver on coordinated objectives.