S.T. Murphy
A review of the impact of pests on forestry and agroforestry in Africa, including current control actions and a proposal for a coordinated regional approach.
Sean T. Murphy is Leader of the Tropical Forestry Research Programme, International Institute of Biological Control, Ascot, United Kingdom.
Note: This article is an adaptation of a voluntary paper submitted to the Eleventh World Forestry Congress. 13-22 October 1997, Antalya, Turkey.
Cypress severely damaged by the cypress aphid in a rural community in Kenya
In sub-Saharan Africa, trees and forests are recognized by national governments and international development agencies as a key to the sustainable development and well-being of rural communities and the maintenance of the environment. Natural woodlands and forests, such as the miombo woodlands, the forests of the Eastern Arc Mountains of East Africa and the West African moist deciduous forests, are recognized as natural assets that are important to the preservation of biodiversity, and as a source of products (Miller and Adam, 1992). Industrial tree plantations, community/farm woodlots and tree-crop systems provide resources for sawntimber, pulp, fuel, fodder, shelterbelts and windbreaks. These planted stands contribute to the protection of the environment because they alleviate pressure on natural forest for wood products and because they play a crucial role in the prevention of soil erosion (Evans, 1992).
In response to the increasing recognition of the importance of trees, considerable progress is now being made in African national forest policy and its implementation. Over the last few decades there has been a particular emphasis on rural development forestry (community, social and farm) and agroforestry to meet specific land use objectives such as the reforestation of denuded land as well as the needs of local smallholder farmers (Burley and Wood, 1991; Wood in FAO, 1991). As a result of national policies, increasing amounts of land have been devoted to large- and small-scale tree plantations throughout the continent.
Unfortunately, many factors hinder some of the major objectives of forestry and agroforestry programmes. One particular factor that has become increasingly important over recent decades is the constraint and threat posed by pests: insects, nematodes, vertebrates, weeds and pathogens. For example, the Centre for International Forestry Research (CIFOR, 1993) states that "... inherent weaknesses in forest protection management will severely compromise or even negate the value of research in other areas if tree growth and forest development are unduly affected by pests and diseases in an unquantifiable manner".
In this article we review the impact of pests on forestry and agroforestry programmes in Africa together with current actions to control some of these pests. We then discuss future priorities for tree pest management and propose a plan for a coordinated, regional approach to tree protection in Africa in support of national forestry and agroforestry programmes.
In 1986, an alien sap-sucking aphid, the cypress aphid (Cinara cupressi), was reported from Cupressus plantations in Malawi. Feeding by the aphid caused dieback of branches and, frequently, the death of trees. By 1991 it had spread to eight countries in eastern and southern Africa and was estimated to have killed trees for a value of US$ 41 million, while continuing to cause a loss in annual growth increment of US$ 413.5 million per annum (Murphy, 1993).
The story of the cypress aphid exemplifies one of the problems affecting African trees today - the accidental introduction of exotic insect pests and diseases, which can affect both exotic and indigenous tree species. Native, African pest species rarely produce such spectacular results, but share with alien pests a capacity to reduce tree growth and fitness considerably through feeding and, consequently, a loss in annual growth increment. Finally, besides pests that directly affect tree health, invasive weed species can damage forests by competing with existing stands and preventing forest regeneration. In Africa, this problem is particularly serious today in programmes of conservation of natural forests for their biodiversity.
Pest problems in general forestry and agroforestry
Further to the cypress aphid problem, there are many other examples where a pest has severely affected an industrial or rural development forestry programme (Table). Many important trees have been affected, including species of Pinus, Eucalyptus, Milicia and Khaya. In agroforestry, the enormous effort and investment on the part of international and national research institutions to select agroforestry tree species and varieties that perform highly under African conditions can also be compromised by the introduction of a single unanticipated alien pest or the emergence of a native species as a pest.
For example, in the 1980s the leucaena psyllid (Heteropsylla cubana), which originates in Central America, spread across the Pacific and Asia, causing severe damage to Leucaena species. In 1992, the psyllid arrived in mainland Africa and is currently found in Burundi, Ethiopia, Kenya, Malawi, Mozambique, the United Republic of Tanzania, Uganda, Zambia and Zimbabwe, where it is causing severe damage to trees in smallholder plots and national research field trials. It is expected to spread to all Leucaena-growing areas of sub-Saharan Africa within the next few years.
Examples of where the use of a tree in a forestry or agroforestry programme has been severely constrained or threatened by a pest
|
Tree |
Region of country |
Pest |
Stage of tree planting programme most affected |
Reference |
|
Milicia spp. |
Cameroon, Ghana, Malawi, Nigeria, Tanzania, Togo |
Psyllids (Phytolyma spp.) |
Saplings/mature trees |
Akanbi (1990) |
|
Cupressus lusitanica |
Eastern and southern Africa |
Cypress aphid (Cinara cupressi) |
Saplings/mature trees |
Mills (1990) |
|
Cupressus macrocarpa |
Eastern Africa |
Canker (Rhynchosphaeria cupressi) |
Saplings/mature trees |
Gibson&Jones(1977) |
|
Eucalyptus spp. |
Eastern and southern Africa |
Termites (Macrotermes sp., Odontotermes sp., Microtermes sp.) |
Seedlings in nurseries/newly established plantations |
CIFOR (1993) |
|
Khaya spp. |
Ghana, Nigeria Cameroon, Côte d'Ivoire |
Mahogany shoot borer (Hypsipyla robusta) |
Saplings/mature trees |
Akanbi(1990) |
|
Pinus radiata |
Kenya, Tanzania |
Needle blight (Dothistroma pini) |
|
Gibson (1979) |
Likewise, in the semi-arid lowland regions of eastern and southern Africa, the establishment of Sesbania sesban has either failed or substantial crop losses have been incurred because of the activities of root-knot nematodes. These nematodes feed on the root systems of S. sesban, causing the death or reduced growth of trees. Thus the investments that have been made in Leucaena sp. and S. sesban in Africa as solutions for smallholder farmer needs are now threatened.
Severe pest problems in tree-planting programmes have gradually become more frequent during the course of this century. The most important factors that have contributed to these pest problems are:
· the rapid increase in the area of tree plantations which has occurred over the past 30 years;· the many plantations consisting of monocultures have enabled some pests to spread very rapidly;
· some plantations have been established on poor sites which has resulted in stress and reduced vigour and has made the trees more susceptible to pest attacks;
· most planting programmes involve exotic tree species and these trees run a high risk of attack by alien pests.
· In recent decades, national forest research institutes and forest departments worldwide have become more aware of the damage that pests can cause over time, even though the symptoms of this damage may not be readily apparent. Measuring the damage has been difficult but recent studies have shown that some pests can cause substantial losses in annual growth increment.
Pest problems in the conservation of indigenous forests
Alien pests such as the cypress aphid may have an impact on native tree species, but invasive weeds pose the greatest threat to native African forests today. Perennial weeds, for example Lantana camara and Chromolaena odorata, grow rapidly in clearings and can interfere with the regeneration of native forests in conservation areas. Particularly at risk are Africa's continental and oceanic islands, with their highly endemic floras. On the African continent, the unique Eastern Usambara mountain range of Tanzania is under threat from alien vines and shrubs, including Clidemia hirta and Lantana camara, as well as from the introduced tree Maesopsis eminii (Hamilton, 1989). In Madagascar and the Mascarene Islands as well as the African islands of the Atlantic, invasive weeds have brought some indigenous tree species to the verge of extinction.
In view of some countries' urgent need for control of invasive alien pests, several pest management programmes have been started in recent years in different parts of the continent with technical support from CAB INTERNATIONAL (CABI), the Food and Agriculture Organization of the United Nations (FAO) and other international institutions. Most of these programmes have addressed not only specific pest problems, but also the need for institutional strengthening in tree pest management. Thus, in total, these programmes have provided a momentum and a foundation for a sustainable initiative in tree pest management for sub-Saharan Africa. Three representative efforts are now described. As these examples relate to exotic pests, biological methods of control feature strongly.
An enlargement of the Leucaena psyllid
Cupressus and Juniperus
The cypress aphid problem was outlined earlier. At the end of 1990, the aphid was causing damage to exotic and native Cupressus and Juniperus trees in at least seven countries in eastern and southern Africa. In view of this, in 1991 a regional classical biological control programme was initiated with technical support from CABI. As a result, several countries have now established their own monitoring schemes and research programmes for the cypress aphid. Extensive surveys for exotic insect parasitoids have been conducted in Cupressus-growing regions of the northern temperate zone and, to date, two candidate agents have been identified. One of these agents is currently under trial release in Kenya, Malawi and Uganda. Some countries are complementing the biological control work with studies on host plant resistance.
Leucaena
The leucaena psyllid was also mentioned earlier. In Tanzania, the International Centre for Research in Agroforestry (ICRAF), in collaboration with a national programme, has initiated selection trials on Leucaena spp. and hybrids for resistance to leucaena psyllid attacks. They are also investigating the effects of harvesting regimes on Leucaena spp. production and psyllid infestation levels. Further to this activity, a classical biological control programme is being undertaken in Tanzania and Kenya. In Tanzania, this programme is being integrated with the host plant resistance trials. Two complementary candidate insect parasitoids have already been identified for the leucaena psyllid from earlier survey work conducted for a programme in Asia, and these have been released in Kenya and Tanzania.
Azadirachta indica
Azadirachta indica, neem, is extensively grown throughout the Sahel region of West Africa and provides rural communities with vital supplies of building timber and fuelwood and it is equally important as a shade and shelterbelt tree. In 1990 widespread concern arose in West Africa from the "sudden" appearance of a disease that was subsequently labelled "neem decline". Investigations in the Niger and Nigeria by technical experts from FAO and the United States Agency for International Development (USAID) have been unable to identify any primary pest organism associated with this condition, a finding that would be an important diagnostic advance in calming fears about the spread of the disease. Furthermore, the establishment of monitoring plots by national programmes has already indicated that the condition may fluctuate annually in severity. One aim of the programmes is to distribute seed representing a more diverse genetic variation with a view to increasing the adaptability of the neem tree to local conditions.
Besides neem decline, the Oriental yellow scale (Aonidiella orientalis), which causes severe defoliation and sometimes death of neem, has spread throughout the region in the last ten years. All countries where the tree is planted are now badly affected. Damage by the scale is particularly bad where trees have already been affected by neem decline. In Nigeria, a programme of classical biological control and host plant resistance studies is now under way. A survey for exotic insect parasitoids is being undertaken by CABI in Pakistan, and potential biological control agents have now been identified. In Nigeria, surveys have begun to monitor populations of the scale and preparations are being made for parasitoid releases.
As a result of the initiatives discussed above and others, there is a new impetus behind tree protection in agroforestry systems and natural forests as well as in industrial and other plantations. To consolidate this development into a coherent set of needs and actions, CABI, the Kenya Forestry Research Institute (KEFRI) and FAO convened an International Consultative Meeting of Forestry Directors and Policy-Makers in Kenya in 1995 (Allard et al. in FAO, 1995). The countries that attended this meeting included Ethiopia, Kenya, South Africa, the Sudan, Tanzania and Uganda. At the meeting, the participating countries identified specific key problem areas (e.g. priority pests, information flow, policies for healthy forests) where national programmes need strengthening.
In general, the national programmes recommended that the problems should be addressed through a programme of institutional strengthening, training and regional cooperation, i.e. by networking pest management activities among countries.
The recommendations of the meeting are an important element of a vision for future pest management in tree protection in Africa. This vision, however, needs to be based on sustainable pest management strategies and the integration of these into planting programmes at an appropriate stage, although as far as possible these strategies should be preventive rather than reactive (CIFOR, 1993. The main focus of these strategies must be on overall forest health, whether it be the case of forest monocultures, polycultures or natural forests.
Overall, given that there is a continuum of land use systems involving trees in Africa, from pure monocultures through to mixtures of crops (including agricultural crops), pest management strategies need to have an integrated approach, i.e. utilize integrated pest management (IPM), which includes sustainable, low technology, environmentally sound and cost-effective components appropriate to particular land use systems and local conditions. Important components that should be considered in an integrated approach include pest monitoring schemes and biological control, which includes the conservation, augmentation or introduction of natural enemies, host plant resistance and silvicultural practices. Particular emphasis needs to be given to IPM strategies that prevent or reduce the risk of pest problems occurring. In this context, particular attention should be paid to practices such as correct site matching and the conservation of natural enemies. These factors must be considered before planting programmes begin. Further to this, the success of any IPM strategy depends on its careful implementation; thus IPM programmes should contain a significant training component for forest monitors and local farmers.
Akanbi, M. O. 1990. Rationalizing the control of major insect pests in the Africa subregion. Unpublished report presented at XIX IUFRO World Congress, 5-11 August 1990, Montreal, Canada. 12 pp.
Ball, J. B. 1992. Forest plantations and the wise management of tropical forests. In F.R. Miller & K.L. Adam, eds. Wise management of tropical forests 1992, p. 97-109. Proceedings of the Oxford Conference on Tropical Forests. Oxford, UK, Oxford Forestry Institute.
Burley, J. & Wood, P. J. 1991. Agroforestry. In Developing world agriculture, p. 188193. London, Grosvenor Press International.
CIFOR. 1993. Management of pests and diseases in tropical forestry. Centre for International Research Strategic Planning Thematic Paper No. 14. Canberra, ACIAR. 8 pp.
Evans, J. 1992 Plantation forestry in the tropics. Oxford, UK, Clarendon Press. 432 PP
FAO. 1991. Forestry in Africa: an overview. By P.J. Wood. In FAO, ed. Exotic Pests of Conifers: A Crisis in African Forestry, p. 1922. Proceedings of a Workshop, Kenya Forestry Research Institute, 3-6 June 1991, Maguga, Kenya. Rome.
FAO. 1995. Formulation of an African Forest Pest Management Network. By G. B. Allard, S. T. Murphy, W. M. Ciesla & S. M. Mbagathi. In Proc. International Consultative Meeting of Forestry Directors and Policy-Makers, 24-28 April 1995, Maguga, Kenya. Rome. 82 pp.
Gibson, I. A. S. 1979. Diseases of forest trees widely planted as exotics in the tropics and Southern Hemisphere. Part II. The genus Pinus. London, Commonwealth Mycological Institute. 135 pp.
Gibson, l. A. S. & Jones, T. 1977. Monoculture as the origin of major forest pests and diseases, especially in the tropics and Southern Hemisphere. In J.M. Cherrett & G.R. Sagar, eds. Origin of pests, parasite, disease and weed problems. Oxford, UK, Blackwell Scientific Publications. 413 pp.
Hamilton, A. C. 1989. Conservation of the East Usambara Forest. In A.C. Hamilton & R. Bensted Smith, eds. Forest conservation in the East Usambara Mountains, Tanzania, p. 126. Gland, Switzerland, IUCN.
Miller, F. R. & Adam, K. L., eds. 1992. Wise management of tropical forests 1992. Proceedings of the Oxford Conference on Tropical Forests. Oxford, UK, Oxford Forestry Institute.
Mills, N. J. 1990. Biological control of forest aphid pests in Africa. Bull. Entomol. Res., 80: 31-36.
Murphy, S. T. 1993. Status and impact of invasive conifer aphid, pests in southern and eastern Africa. Paper presented at the IUFRO Symposium, Impact of Diseases and Insect Pests in Tropical Forests, 23-26 November 1993, Kerala, India, Kerala Forest Research Institute.
Wagner, M. R., Atuahene, S. K. N. & Cobbinah, J. R. 1991. Forest entomology in West tropical Africa: forest insects of Ghana. Dordrecht, the Netherlands, Kluwer Academic Publishers. 210 pp.
