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3. Managing pests and diseases

3.1. Healthy forests - the objective
3.2. Integrated pest management - the tools

3.1. Healthy forests - the objective

The objective of managing pests and diseases in forests and forest plantations should be to keep them in a healthy, productive condition. What is a healthy forest? The concept of a healthy forest became a popular concept during the mid- to late 1980s. In economic terms, a “healthy” forest can be defined as “a forest in which pests and diseases remain at low levels and do not interfere with management objectives.” In ecological terms, a healthy forest is “a fully functional ecosystem; one in which all of its parts can interact in a mutually beneficial way.”

The healthy forest concept directs forest managers to focus on the forest rather than its pests and diseases and takes into account the natural role of insects, fungi, fire and other so called “damaging agents” and their interactions in forest dynamics. Under an overall policy of forest health protection, pests and diseases are looked upon as a symptom of an unhealthy forest rather than as the problem. This directs forest managers and forest protection specialists to address the underlying causes of the pest or disease - factors such as overstocking, over-maturity, poor site/species matching, excessive fuels and single species forests with little diversity. Striving for healthy forests also involves anticipating pests and diseases based on historical records of their occurrence and the knowledge of forest and climatic conditions that favour their abundance. This allows time to implement management practices that will make these forests inhospitable for build-up of damaging pests and disease (Ciesla 1998).

The healthy forest concept may sound like an idyllic approach to managing forest pests and diseases, especially in cases where an introduced pest is causing widespread damage in a simple plantation ecosystem in the absence of its normal complex of natural enemies. Yet even the most damaging of exotic pests and diseases respond to stand and site conditions and there is evidence that they focus their activities on “unhealthy” forests. A classic example is that of Sirex noctilio in exotic pine plantations in the Southern Hemisphere. In New Zealand, it was noted that stressed trees in unthinned plantations were the first to be attacked by this insect. Therefore, by its action, the plantations were effectively thinned (McLean 1998). Thinning is now an integral part of managing plantations threatened by S. noctilio in places where this insect has been introduced. In another example, when a cypress aphid, originally identified as Cinara cupressi, appeared in eastern and southern Africa, most severe damage occurred in plantations of Cupressus lustianica that were either overmature or had been established on low nutrient soils (Claude and Fanstin 1991, Ciesla et al. 1995, Obiri 1994).

3.2. Integrated pest management - the tools

3.2.1. The decision process
3.2.2. The action process
3.2.3. Integration of new technologies
3.2.4. Integrated pest management systems

The healthy forest concept provides the umbrella or objective for addressing damage caused by pests and diseases. Integrated pest management (IPM), on the other hand, provides the tools to accomplish this task.

The concept or philosophy of IPM as a “rational” approach to pest control was formalized during the 1960s. Crop protection specialists had become aware of the adverse side effects of dependence on chemical pesticides, including pesticide resistance, occurrence of secondary pests, environmental damage and human health hazards. This led to the realization that alternative approaches, including cultural, biological and genetic tactics, used either alone or in combination, were also needed to provide long-term, effective protection against damaging pests.

Numerous definitions of IPM appear in the literature. Smith et al. (1976) refer to IPM as “a process based on ecological principles and integrates multi-disciplinary methodologies in developing agro-ecosystem management strategies that are practical and effective and protect both public health and the environment.” Pimentel (1986) describes IPM as a “pest control method that includes judicious use of pesticide and non-chemical technologies - all of which are based on sound ecological principles.

IPM can be looked upon as consisting of two basic elements; a decision process and an action process. The decision process establishes the basis for any subsequent actions to be undertaken, including no action. The action program may consist of one or more ecologically, economically and socially acceptable tactics designed to reduce pest populations to non-damaging levels (Ciesla 1982).

3.2.1. The decision process

The decision process is often the most time consuming and complex aspect of IPM. It requires careful consideration of the pest, its host, resource management objectives and the ecological, economic and social consequences of the various available tactics. Population levels of pests are estimated and anticipated resource losses are projected, as are the costs of treatment and its anticipated benefits. If treatment costs exceed losses, a rational decision may be to not treat and accept the losses. Other questions to address include: will natural controls take over within a short enough time so that artificial controls will be unnecessary; or will the effects of proposed treatments be so adverse that they would outweigh the benefits of treatment?

Monitoring of forest pests and diseases and their resultant damage is a critical input to the IPM decision process. Pest monitoring is becoming a sophisticated process that makes use of many technologies. Pheromones and other chemical regulators are often used to monitor insect population levels. Remote sensing technologies such as aerial sketch-mapping, aerial photography and airborne video are used to map and assess forest damage. Geographic information systems (GIS) can be used to relate the location of affected areas to key resource values, terrain features, land ownerships and environmentally sensitive areas. Mathematical models can predict resultant damage caused by certain levels of pest numbers and their consequences. In some cases, pest, growth and yield and economic models are linked to make projections of pest and disease impacts. Data visualization techniques can display the expected results of alternative action tactics.

3.2.2. The action process Prevention Suppression

IPM action programmes consist of two overall strategies: prevention or direct control (suppression). Specific pest management tactics exist under each of these strategies. Prevention

Prevention consists of tactics designed to either reduce the probability of the occurrence of a pest or disease or to create environmental conditions inhospitable for its buildup into damaging numbers. Regulatory, cultural or genetic tactics are examples of prevention strategies.

Regulatory tactics are designed to prevent introductions of exotic pests and diseases and to prevent their spread once established. Examples include inspection of wood products and wooden containers at ports of entry to intercept pest species, conduct of pest risk analyses when new trade agreements are made and establishment of quarantine zones when a pest species is first discovered in a new location.

Cultural tactics are designed to create conditions inhospitable for the development of damaging numbers of pests and diseases. These include matching tree species selected for planting to suitable growing sites, controlling stocking through intermediate harvests to maintain tree vigor and timely harvesting of plantations when they reach maturity. A drastic but sometimes necessary cultural approach is to simply eliminate a tree species from a plantation programme because of its high susceptibility to certain pests and diseases. An example is Kenya’s policy to discontinue planting of Cupressus macrocarpa due to the stem canker fungus, Monochaetia unicornis, and P. radiata because of its susceptibility to Sphaeropsis sapinea (Odera and Arap Sang 1975, 1980).

Genetic tactics make use of varieties of host plants that are either more tolerant to damage or less palatable to the pest. Identification and testing of varieties of Leucaena leucocephala and hybrids with other species of Leucaena for resistance or tolerance to the psyllid, Heteropsylla cubana, was a major line of investigation following this insect’s introduction into the Asia-Pacific region (Banpot Napompeth 1994). In the southeastern United States, screening and development of pine seedlings resistant to fusiform rust is an integral part of the management of this disease (Manion 1991). Tree breeding programmes in Brazil address resistance to the canker Cryphonectria cubensis on eucalypts and breeding of P. radiata in New Zealand includes a rating for resistance to the needle fungus Dothistroma pini (McLean 1998). Suppression

Tactics directed against the pest or disease are referred to as direct control or suppression tactics. Examples include various types of biological, mechanical or chemical methods.

Biological control involves the use of natural enemies of a pest or disease to help keep its numbers in check. Classic biological control, a technique widely used in agriculture, involves the importation of natural enemies to help control an exotic pest. In Colombia, the geometrid Oxydia trychiata, an indigenous species attacking pines and cypress, was successfully controlled by the introduction and release of an egg parasitoid Telenomus alsophilae (Bustillo and Drooz 1977). In another successful example of classic biological control, releases of a parasitoid, Pauesia bicolor, resulted in the collapse of an aphid, Cinara cronartii, a North American species accidentally introduced into pine plantations in South Africa (Kfir et al. 1985, Mills 1990). In Chile, state-of-the-art mass rearing facilities have been established by forest industry for production of the European pine shoot moth parasitoid, Orgilis obscurator. One such laboratory, Controladora de Plagas Forestales, was established in 1992. In 1996, this facility produced 1.2 million parasitized larvae and 22,600 adult female parasitoids for field release (Controladora de Plagas Forestales 1997).

A key concern about biological control is the possibility that the introduced natural enemy might also attack innocuous or beneficial insects in the ecosystem. Therefore it is necessary to thoroughly evaluate candidate species prior to release to ensure their relative host specificity. Another concern is the hazard of accidentally introducing hyperparasites, natural enemies of the biological control agents, which might eventually affect the agent’s efficacy. For example, colonies of the leaucana psyllid predator Olla v-nigrum, released on the Indian Ocean island of Reunion, were subsequently discovered infested by three species of hyperparsites (Quilici et al. 1995). These could severely affect the ability of this predator to function as an effective biocontrol agent.

Augmentative biological control is a process designed to increase the efficiency of natural enemies already in place. Mass release of Trichogramma dendrolimi, an indigenous egg parasitoid of the defoliator Dendrolimus punctatus in pine plantations in China is an example of this tactic (Yan and Liu 1992). Another biological control tactic is the use of biological insecticides such as the bacterial agent, Bacillus thuringiensis (Bt), nuclear polyhedrosis viruses or fungal preparations to control insects. Bt is widely used for control of lepidopterous defoliators in both natural and plantation forests. In China and Vietnam, a fungus, Beauvaria bassiana, is used for control of the pine caterpillar, Dendolimus punctatus (Anon 1993).

Mechanical tactics include removal and destruction or rapid removal of infested or infected trees with the objective of destroying the pest. Examples include cutting and burning of trees infested by bark beetles or rapid salvage of infested trees and destruction of infested bark at the sawmill. When the pine woolly aphid, Pineus borneri, was first discovered in Kenya, the initial response was to destroy the infested pines. This insect is easily carried on air currents, however, and it was soon realized that the infestation had spread far beyond the designated area where trees were being destroyed (Owour 1991).

Use of chemical pesticides applied either from the ground or by low flying aircraft is still considered to be an integral part of IPM. However these materials are used more sparingly, are applied at reduced intervals and with greater precision. In IPM, chemical pesticides are often considered to be the tactic of last resort.

3.2.3. Integration of new technologies

An important part of IPM is that no matter how advanced, sophisticated or effective an IPM system for a specific pest or pest complex may be, there is always room for the introduction of new technologies. These may include more accurate pest monitoring, prediction of new pest management tactics, and more efficacious treatments with fewer undesirable side effects.

3.2.4. Integrated pest management systems

IPM systems consist of a combination of decision-making and pest management tools directed against a pest or pest complex and are in various stages of development. An example of an evolving IPM system is the approach being taken to manage the European wood wasp, Sirex noctilio, in pine plantations in southern Brazil. Early detection of this insect is accomplished by baiting suppressed trees in plantations with an herbicide, a procedure that attracts attacking wasps. Infestations are treated either through inoculation of a parasitic nematode, Daladenus siricidola, which renders the female wood wasps incapable of producing eggs, or thinning plantations to reduce stocking and maintain tree vigor (Iede and Ciesla 1993). Damage assessment technologies such as aerial sketch-mapping and aerial photos (Disperati et al. 1998, Ciesla et al. 1999) are currently being evaluated. Landsat satellite imagery is being tested to determine its capacity to map the location of small, non-industrial private pine plantations that might serve as reservoirs for populations of this damaging insect. Introduction of additional parasitic insects is another potential tactic for managing this insect (Iede et al. 1998).

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