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Dealing with pests and diseases in tropical forests

B.K. Bakshi

B. K. Bakshi is Director of Biological Research of India's Forest Research Institute and Colleges, Dehra Dun.

Diseases and pests are the major biological determinants of forest productivity. Investment in research and control programmes to deal with these problems, says the author, constitutes only a small percentage of the overall value of forests. In the long run, the development of trees with genetic resistance, slow and costly as this may be, is the most effective solution.

With the rapid increase in world population, the amount of forest per caput is declining, and particularly so in the less industrialized or developing areas of the world. It is estimated that the land under forest in developing countries is about 2100 million hectares, or more than half of the forested land on earth. Considerable effort is needed therefore to increase the productivity of the existing forests and to afforest suitable areas. Diseases and insect pests constitute the major biological determinants of forest productivity in the natural forests and particularly in plantations, thus offsetting the effort in increasing wood production to meet the growing needs of an increasing population.

Mortality in Eucalyptus tereticornis due to "pink disease" caused by Corticium salmonicolor.

About 30 years ago extensive plantations of fast-growing species were started in many developing countries. The overall area under plantation for the developing countries at the end of 1964 was about 4.4 million hectares, with 75 percent of this area equally distributed between Latin America and Pacific Asia. These plantations are expected to produce a sustained yield of about 45 million cubic metres a year, which is equivalent to nearly half the present annual drain of industrial roundwood of 108 million cubic metres from the forests.

The natural tropical forest is for the most part of mixed composition with associated undergrowth. The crop is of uneven age, with the degree of resistance and susceptibility to a pest varying according to species and age, thus minimizing the risk of rapid spread of pests. Even in a susceptible species, there is a natural selection of resistant individuals over the decades. Indigenous pests therefore normally do not cause any catastrophic, or even appreciable, damage in a natural forest unless it is profoundly disturbed through silvicultural and management practices. This happened, for example, in "wet" sal in India where intensive fire protection measures carried out up to the 1920s resulted in a serious outbreak of a root disease which had to be brought under check through control burning, a necessary silvicultural measure in such forests. Insect pests on the other hand, such as sal heartwood borer and deodar defoliator, may build up to an epidemic level during certain years and remain at an endemic level during the intervening period. Even then, a biological balance is maintained between, the natural vegetation and the insect pests.

Sal timber riddled with tunnels of sal heartwood borers (Hoplocerambyx spinicornis), showing pupating larvae and pupa in tunnels

Mortality in sal (Shorea robusta) due to sal heartwood borer.

Plantation risks

In plantations, preferred species are raised usually as a pure crop in even-aged stands intensively managed toward increased productivity by reducing genetic variation, eliminating competing vegetation, maintaining optimum stand density and practicing other cultural operations. All these operations may change the ecosystem drastically and expose plantations to the risk of diseases and insect pests. The pathogens and insect pests in the tropical forests consist of a richer complex of species which exhibit a more prolonged period of activity as compared with those occurring in forests of temperate climatic regions. The quantity and quality of available food in plantations may lead to epidemic insect outbreaks such as, for example, teak defoliators, semul shoot borer and ailanthus defoliator. Similarly, khair raised as a pure crop on a reforested site suffers from serious mortality owing to Ganoderma lucidum root disease, which is normally endemic in the natural forest. Inadvertent entry of a pest into a country may pose a threat to indigenous species, particularly when raised in plantations. Catastrophic losses due to chestnut blight, Dutch elm disease, white pine blister-rust, gypsy moth and European pine-shoot moth in North America, Rhab-docline needle cast and Scleroderris canker in Europe are classical examples.

Await rotation

The initial success of an exotic is no guarantee for the future until it has successfully completed at least one rotation, because pathogens and pests require time to build up and local pests may take time to adapt themselves to the new host. Exotics are exposed to two hazards: they may be attacked by an indigenous parasite against which they may not possess any resistance or by an inadvertently introduced foreign pathogen. An example of the first hazard is Eucalyptus, which, planted extensively in India, is seriously affected over 40000 hectares owing to attack by the pink disease that occurs on a wide range of indigenous hosts. Celosterna scrabrator, the root and shoot borer of indigenous Acacia, has found a suitable host in Eucalyptus, on which the pest has become serious in dry areas. In east Africa, exotic cypress and juniper plantations are attacked by the local pest, Oemida gahani.

A striking example of the second hazard is the Dothistroma blight, innocuous on Pinus radiata in its indigenous home in California, but the cause of serious damage in exotic plantations in different countries. Instances of exotic insect pests causing damage to exotics include European wood wasp on softwoods in New Zealand and Australia and Australian cerambycid on Eucalyptus in South Africa, the Mediterranean region, Brazil and Argentina. Many diseases and insects have thus already caused serious damage to plantations. The threat is expected to become greater when more areas are brought under plantation. General awareness of this is either lacking or limited in most developing countries.

Strong research base

To meet this threat, it is necessary to build a strong research base for forest diseases and insect pests. For an effective approach to research in this field what is needed is specialization in forest pathology and forest entomology after attaining a good general proficiency in the basic disciplines and a knowledge and understanding of all biological sciences and related disciplines. In many cases the pathogens and the insect pest may operate either singly or in combination in a complex of climatic factors and soil conditions. A team approach consisting of all related disciplines then becomes essential.

Root-rot mortality in khair (Acacia catechu) caused by Ganoderma lucidum forming sporophores at base of tree

Sporophore of Polyporus shorea at base of sal tree.

At the outset, a forest disease and pest survey unit is needed to appraise the disease, diagnose the cause, study the biology, ecology and epidemiology, assess damage and ascertain the potential threat, and exercise continuous surveillance of all forested land for any new outbreaks. Eventually control measures will have to be developed which may be indirect through adjustment in silviculture, management and cultural practices or direct by applying lethal chemicals.


Use of sterile males, food attractants, sex lures, antifeedants, and juvenile hormones are some of the other recent methods being experimented with for the control of insect pests. However, tree improvement programmes will have to be developed in order to give trees genetic resistance to diseases and pests. This method, although slow and costly, is potentially the most effective long-range solution. Biological control, particularly for insect pests, will have to be worked out, either from the natural-enemy complex or through augmenting it by introducing parasite predators and pathogens that are destructive to the pest but harmless to the vegetation. An effective forest plant quarantine organization should be set up to provide safeguards against accidental entry of new pathogens and insects.

Sal heartwood borer, adult

Surveys form the basis for assigning priorities for intensive research into economic problems and for outlining the depth of research needed to solve the more pressing problems. Thus, when the impact of Phytophthora cinnamoni on Eucalyptus marginata in southwestern Australia was surveyed - 80000 hectares, or 5 percent, of the commercially productive forests having been devastated - there was large-scale funding of research and twenty-five scientists were assigned to the problem. The damage to radiate pine due to Dothistroma blight in New Zealand necessitated instituting a project under which a chemical control of the disease was successfully evolved. As a result, radiate pine continues to be the main species planted in New Zealand, occupying over 90 percent of new plantings. Fomes annosus root disease has attracted the largest number of pathologists in different countries working on this problem and international conferences are held under the auspices of IUFRO (International Union of Forestry Research Organizations) to discuss the latest information on the subject. Control measures have been developed for this disease and research into control methods is continuing. To combat the Amylostereum-Sirex disease cum pest problem on radiata pine, a National Sirex Fund was established for every year from 1962 to 1967/68. This fund amounted to 0.4 percent of the value of Australia's annual exotic conifer production.

Money spent on research and control programmes is fully justified and ultimately constitutes only a small percentage of the over-all value of forests.

In most developing countries, a research base for forest pathology and forest entomology is lacking. In those where such a base exists, the number of specialists in the field is far from adequate in comparison to the magnitude of the problems which may be present in a variety of plantation species raised over large areas. Due to the absence of regular surveillance, occurrences of diseases or pests are not noticed generally until considerable damage may have taken place. Advice from experts in related disciplines in agriculture may be available in the country but in most cases is not useful since the problems may be unrelated and the approaches different. Likewise, imported expertise for a limited period may not be helpful as the problem may require considerable time and effort to undertake the basic studies necessary for developing a control programme. Even if the control of a pathogen or a pest is known, its adaptation needs technical skill, manpower and funds for extension work.

Meagre resources

In the developing world, very meagre resources are devoted to research and development in general. With a rapidly growing population in most developing countries, the most immediate concern is food production, and the major emphasis is there, while related fields such as forestry do not get adequate attention.

This low capital input for forestry research is a major constraint to the growth of forestry in developing countries.

Dynamic forestry programmes are urgently needed in developing countries to satisfy increasing domestic requirements for wood and wood products and for these countries to participate more actively in world trade. Up to now research and development efforts in forestry have been directed mainly at the raw materials and market conditions of developed countries. It should be understood that for forestry to go forward in developing countries there needs to be adequately financed research and development work in developing countries geared to the raw materials and the needs of these countries.

Moth of Eterusia pulchella causing defoliation in Pinus kesya.

Many diseases and insect pests are common to most countries and may be of potential importance to others. Therefore, protection of forests against pests and diseases needs world-wide attention. The problem in any region cannot be viewed in isolation but must be seen in a global context. Cooperative efforts should be stimulated, regionally and internationally, through such agencies as FAO and groups such as IUFRO by arranging technical programmes, symposia and conferences to discuss the status and needs of research.

Particular emphasis should be put on the creation of regional institutes for forest diseases and insect pest research, particularly in developing countries. Countries with similar forest vegetation and problems should be grouped together to form the nuclei for action-oriented research for countries which badly need to do this kind of work but cannot afford the facilities.

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