Forest pest control can be classified into natural control and artificial control. Natural control includes climatic condition, nutrition, host resistance and biotic factors, whereas artificial control includes methods such as silvicultural control, biological control, microbial control and chemical control.
Forest ecosystem consists of a variety of components, all of which are critical to the forests long term sustainable health. All parts of a tree roots, stems, foliage, shoots and terminal leaders are vulnerable to attack by pests. Pest damage can range from slight damage that has no effect on the value of the harvested products to severe damage that stunts or kills the trees or reduces their market value. Tree pests include insects and mites, diseases, weeds, vertebrates and nematodes.
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Among these insects, fungi and other microorganisms play an important role in forest stability, succession and productivity by contributing to the degradation of dead and dying trees, nutrient recycling of decaying organic matter, creating opening for regeneration, providing a food source for other organisms, pollination and predation or parasitism of potentially harmful pests. Thus it is important for forest managers to recognize that forest management practices can have an impact on the other components of the ecosystem that could be detrimental to long term forest stability.
Interest in protecting forests from insect, disease, weed and vertebrate pests has increased in recent years. This has come about largely because of, increased awareness of the destructive capacities of pests, heavy toll they take on supplies of commercial and recreational timber, environmental concerns, effects on threatened and endangered species and availability of new specific pesticides.
Destructive insects are an integral part of forest ecosystems, that they have significant economic and social impacts as well as ecological effects on forest productivity and values. The adverse impacts should be prevented or kept at tolerable levels by ecologically sound means, which are compatible with forest management practices. Insects causing damage to forests are more explicitly defined in terms of its economic effects and disruptive influence on forest pest management schedules and operations.
Augmentation of natural controls and the manipulation of forest conditions are necessary to make them less favorable for outbreaks and to ameliorate losses on a long term, continuing basis. The silvicultural methods and management practices are considered to have some influence on the probabilities of outbreaks, reduction in damage, or reclamation of damaged stands with respect to specific kinds of pests.
Forest pest management, as now conceived, is firmly rooted in quantitative methodology related to both the measurement and analysis of forest growth and development and the dynamics of forest insect populations. Forest ecosystems are diverse and complex.
Modern concepts, methods of mathematical modeling and systems analysis are drawn upon at an early stage to gain a better understanding of their basic functional processes and relationships.
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The goal of forest management should be to maintain all components of the system in a balance that enhances the likelihood of long term sustainability. Managing tree pest effectively should be based on the thorough consideration of ecological and economic factors. An understanding of pest identification, its biology and type of damage are some of the factors that determine which control strategies and methods, if any, should be used.
Ultimately, pest management decisions represent a compromise between the value of the product, the extent of the pest damage, the relative effectiveness and cost of the control measures, and the impact on the environment. Thus, forest pest management has a sound conceptual, methodological, and technological foundation for implementation as an integral part of forest resource management planning and operations. For pest management to be an integral part of forest resource management, a “look-ahead” policy must displace the “wait and see” policy. Pest monitoring and treatment strategies must be fully incorporated into the overall planning and decision process.
The conventional control measures continue to be the major tool for most pest situations as well as basis of integrated pest management systems. It is therefore, essential that a forest manager and the upcoming forest entomologists become fully conversant with various pest control procedures and options to employ them either singly or in combinations as a component of forest pest management.
1. Natural Forest Pest Control:
I. Climatic Condition:
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Climatic condition is expressed in terms of average physical conditions occurring throughout the year in a locality experienced over a long period of time. It also determines the length of the life cycle, successive stages and number of generation of insect species in a year.
The more important climatic factors are meteorological factors, temperature, rainfall, atmospheric humidity, wind, sunshine, moon light, topographic features, mountains, plains and zoogeographical distribution. Each factor affects the insect activities in one way or the other, but in nature they seldom operate independently.
i. Temperature:
It is the single most important physical factor which regulates the insect activity in the environment. Each species of insect has a definite range of temperature, within which the insect is active and its metabolism or growth is apparent. Any deviation from this point on either side retards metabolism and which in turn influences its activities.
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Near the upper and the lower limits of effective temperature zone lies the dormant zones in which the metabolism is greatly retarded and the insect becomes completely inactive and its movement ceases. The fatal temperature at which practically all the species of insects are killed instantly is 60°C.
ii. Atmospheric Humidity (Moisture):
Like temperature, atmospheric humidity (moisture) is an important factor of environment, which profoundly influences the development, behaviour and distribution of insects. Each species has an optimum favourable zone and any deviation from this zone leads to a state of dormancy and beyond the effective range, the insect dies. In other words, “with increase in temperature, there is a corresponding decrease in relative humidity”.
Under normal conditions, both excess and deficient moisture conditions do not result in immediate death of the insect, though its activities are definitely disturbed. However, certain wood boring forest insects are profoundly influenced by variations in the moisture contents within the microclimatic conditions of its host plant. This is best explained by studying the ecological succession of wood borer fauna of a felled wood.
While the coleopterous borers of families Buprestidae and most Cerambycidae require higher moisture conditions, the powder-post beetles families Bostrychidae and Lyctidae and Hymenopterans wood borers (Xylocopa spp., family: Xylocopidae) require dry and seasoned wood and cannot survive in moist wood.
iii. Heavy Precipitation:
Heavy precipitation (snowfall, hailstorm, rainfall or frost, etc.) may also sometimes cause direct mortality of different stages of forest insects. While the effect of snowfall is like that of low temperature, hailstorm and heavy rainfall act mechanically. Lepidopterous larvae, beetles and their larvae, which are open feeders, feeding exposed on the foliage, are beaten and washed off by sudden heavy hailstorm and rain or killed by frost.
They are either drowned or injured, thus preventing their ascendance to the crown of the trees. Rainfall also increases atmospheric humidity which influences the emergence period of insects, such as soil insects and wood borers. For example, winged form of termites and ants, wood borer adults of many species of Cerambycidae and Curculionidae, start emerging with the onset of monsoon rains.
iv. Wind:
Wind is yet another physical factor and its discernible effects are both mechanical and eco-physiological in nature. Strong wind shakes the crown of trees violently, blowing off and dislocating the insects, especially the more delicate forms (larvae) from their food plant and subsequently prevent them from gaining access to the food source and shelter.
II. Nutrition:
Nutrition (food) is the second important natural control factor, which, unlike physical factors of environment, is amenable and can be controlled with certain degree of accuracy. It constitutes the primary driving force for seeking not only the right type of food for its existence and development but also to see that it is available in sufficient quantities.
Thus food most frequently act as a controlling factor on insects and influence the structure and activities of the insect communities (for that matter all animal communities). Thus, if food conditions are controlled in a way that even if the weather conditions are favourable, it checks the pest epidemics. Nutritional control may be considered under the following aspects like, food abundance, competition and quality of food.
Food abundance plays an important role in all the living organisms which are capable of multiplying to the extent of food available in an area. Forest monocultures offer ideal conditions for population buildup of insect species feeding on it. The competition may be intra-specific or inter-specific, the outbreak of secondary species under certain conditions competes with the major species for feeding space.
In intra-specific competition, all the individuals of the same species compete for all factors in the habitat i.e., space and food. In case of outbreaks, food may become the limiting factor for their survival. The quality of food has a definite effect on the abundance of forest insects and their development. The principal factors affecting the quality of food are moisture content, alternate food plants and host resistance.
III. Host Resistance:
All plant species possess certain physiological and genetic traits which impart some degree of pest resistance by making environmental inherent conditions of less favourable for the pest species. The resistance may be due to inherent conditions of unpalatability, such as hardness of leaf lamina, shoot, sap density and presence of certain chemical compounds, such as essential oils, acids, gums, resins, latex, etc. There are three mechanism of host resistance to pest recognized viz. non-preference, antibiosis and tolerance.
IV. Biotic Factors:
Biotic control comprises all such factors which tend to control the population of an organism (pest) through the activities of other organisms. The biotic factors, which play an important role in limiting the pest populations, are competition, parasitoids, predators and pathogens.
2. Artificial Control or Applied Control:
It is defined as the introduction of external elements, such as introduction of biocontrol agents, chemical control, etc. into the forest environment and manipulation in sylvan ecosystem (silvicultural control), which though temporarily, is under siege of a pest. Most common control procedures envisaged are monitoring, mechanical control, cultural control, legislative control, chemical (insecticidal) control and integrated pest management.
The regulation of insect pest population by modifying forest stands or methods of growing and harvesting trees to suppress insect pests is called silvicultural control. It is a type of artificial control as it is achieved by augmenting natural controlling factors of the environment by deliberate human intervention.
The silviculture control requires detailed knowledge about the prevailing ecological conditions of a forest that are favourable to the pests and then manipulate these conditions to the disadvantage of insect pests. Before taking- up any silvicultural pest management measures, it is necessary to have an idea of forest type and their relationship with insects.
Inter-Relationship of Forest Types and Insects:
It has long been known to foresters that a forest growing under natural conditions is less vulnerable to insect attack in contrast to those that grows in disturbed ecological conditions. It shows that a forest is much safer in an equilibrium state than a forest in broken biotic conditions from the point of view of pest attack. If a forest is grown under such conditions that are, conducive for maintaining the biotic balance throughout the rotation period, costly measures to restore the disturbed balance are not needed.
To achieve a successful silvicultural control, it is of paramount importance to have an accurate knowledge of the environmental factors involved. A forest represents complex biotic communities which may vary in a virgin or natural and in an artificial forest, particularly with reference to insect attack.
A climax forest, undisturbed by anthromorphic intervention is a virgin forest where a biotic balance is always maintained. Generally, no pest epidemic occurs in such type of forest. However, in pure coniferous temperate forest, outbreaks of defoliator and bark beetles are known to occur. A virgin forest in the true sense does not really occur in India.
The biotic complex of these forests remains in somewhat equilibrium state and that insects epidemics are rare. Epidemic occurs only under severe stressed conditions like prolonged drought or other natural calamities like fire or frost. Most of natural forests of India where biotic and abiotic pressures are minimum exhibit no insect problem but whenever these pressures increase, the insect problems crop up.
For example, epidemics of deodar defoliator Ectropis deodarae epidemic in outer Himalaya, bark beetle Scolytus major epidemic in deodar forest, chirpine defoliator Cryptothelia crameri epidemic in chir forest in outer Himalayas are instances supporting this contention. In tropical and subtropical natural forests, varied types of insect problems have arisen owing to biotic and abiotic pressures.
Kind of Losses by Insects in Forests:
Losses to insects in a forest are of varied types and range from tree mortality to reduced vigour.
Losses due to insects may be of following types:
(a) Mortality of trees particularly young ones may primarily be caused by defoliators, bark beetles and termites.
(b) Wood boring insects may damage the solid wood of a living or felled tree.
(c) Insects may cause great losses to growth increment.
(d) Natural regeneration is hampered by insect damage.
(e) Deterioration of site and degeneration of species composition may be the result of insect activity in a forest.
(f) Insect attack may cause complete failure of planting a species in an area which may be the cause for abandonment of the plantation.
It has happened in case of Acacia nilotica plantation in Madhya Pradesh which was abandoned because of severe damage by root and shoot borer Celosterna scabrator.
II. Mechanical and Physical Control:
Though non-conventional and highly specified, the mechanical and physical methods of control play an important role in pest management. These methods aim in reducing the pest population by employing certain mechanical devices or manual operations which ensure killing of insects by trapping or mechanical means.
It act as barriers and prevent insects or its life stages from gaining access to host plants (mechanical control) or by manipulating certain physical conditions of environment more particularly the temperature and humidity, in a way that they prove destructive to the pest species (physical control) and thus preventing or minimizing the insect attack.
These measures also involve the use of radiant energy in various ways, which could destroy the insect pests. In broader sense, the physical control measures comprise one or more of the following measures, viz., heat treatment, refrigeration, radiant treatment, trapping, sound percussion devices and self-destruction or auto-cidal treatment.
Many insect pests of forest tree species are amenable to control by mechanical and physical methods, however, these methods rarely provide commercial control. These methods hardly differ from cultural methods except that they involve the use of special equipment or operation and are employed in situations where insects are to be eliminated from an area without any deleterious effect on the environment.
Hand picking of noxious insects or their life stages is the simple perhaps the most ancient effective method of insect control practiced by man and which continues to be one of the most profitable methods for some insect pests (e.g., white-grubs and crickets in seed beds; beetles and larvae of some pests in forest nurseries).
Use of hand nets or dragnets for catching small and noxious insects is an extension of hand-picking procedures (e.g., beetle defoliators, grasshoppers, plant hoppers, etc.). Similarly, wire probes or other flexible materials are also employed for poking tunnels of some wood borer species (e.g. Batocera rufomaculata, Tonica niviferana, Zeuzera spp., etc.). Jarring of branches of trees is also one of the mechanical methods employed in shaking off adult or larval population of species from the host plants.
The purpose of control should be to maintain a balance between pests and hosts at such a level that economically important injury will not occur. Economic consideration is one of the most important factors to control a pest in a forest. In case of direct pests, even moderate levels of pest abundance cannot usually be tolerated. The appropriate level of control is determined by economic value of the forest stand.
Over-all, mechanical control in forestry generally means salvage or the removal of infested trees to prevent the spread of a pest. If the cut trees can be utilized, the control operations may be cost effective. In some instances, the infested trees may be too few or too remote for a cost effective operation. Cut-and-leave or cut-and-treat are options that are frequently employed for bark beetle control.
The infested trees, along with a buffer, are simply cut down and left or treated by debarking, applying a pesticide, or burning. It is assumed that any surviving beetles will disperse and not create additional infestations. On high value trees, mechanical control could include simple hand picking, erecting insect or pathogen barriers or sticky traps. Non-toxic control strategies such as insect lure method (use of attractant), insect bait station, cellulose baits, pheromone traps, sealed baits, electronic bait etc., are also found useful in control of certain forest insect pests.
Biological control is also referred as bioremediation and is defined as ingenious and resourceful employment of certain promising biotic elements (predators, parasitoids and pathogens) for suppressing pest species. Biological control refers to using natural insect enemies to control pest populations. Enemies can include vertebrate predators such as birds or mice, arthropod parasitoids such as the tachinid fly, arthropod predators such as spiders and mites.
The phenomenon involves certain degree of deliberate human interference, which, unlike biotic natural control, is manipulated by man for regulating pest populations in an ecosystem. Simply, biological control is the reliance of natural enemies to keep a pest in check. Three principles or approaches are involved in biological control viz.- importation, conservation and augmentation.
Normally importation is utilized against exotic pests where natural enemies from the pest’s home land are introduced into the infested region. It is assumed that once the natural enemy is established it will be self-maintained. Conservation includes the identification and remediation of negative influences that suppress natural enemies already in the system, and the enhancement of habitat for natural enemies.
For example, providing alternative food sources for natural enemies, such as pollen and nectar, by encouraging selected flowering plants. Using more selective pesticides that have a minimum impact on natural enemies is another approach.
Augmentation is the release of enemy populations, which were artificially reared, for pest suppression. It is used when natural enemies are missing, too scarce, or slow to arrive. There are two strategies for augmentation releases. The first is “inoculative” where the released organisms and the progeny of the released organisms accomplish control of the pest.
The second is “inundative”, where the released organisms only accomplish control of the pest. Forestry has a great potential for utilizing biological control strategies because of the perennial nature of the system and the potential for alternative food sources. The fact that pesticides are generally not cost effective is another consideration.
Microbial control is another important component of biological control. Apart from parasitoids and predators, certain entomogenous microbial pathogens, comprising chiefly the bacteria, fungi and viruses are the well-known biocontrol agents.
These microorganisms, as the name suggests, constitute a specialized group of biocontrol agents, having extremely smaller size and eco physiological characteristics, which has imparted these organisms a distinct identity, quite different from insect parasitoids and predators. Nevertheless, the principles involved for profitable utilization (inoculum, augmentation and conservation) more or less remain the same.
i. Bacterial Pathogens:
Among the bacterial pathogens, particularly the sporulating (sporeforming) bacterial diseases constitute an important group of microorganisms as they are highly resistant to drying and high temperatures. Bacillus thuringiensis, commercially named as Thuricide, is the best known bacterial pesticide with worldwide recognition. Though broad spectrum, this biocontrol agent is selective and ecologically safe in most situations.
In India, it has been successfully tested against over two dozen insect pests of forestry importance, achieving high mortality rate ranging from 30 per cent (Lymantria mathura, Dioryctria abietella) to 90 per cent (in Ascotis selenaria, Eutectona machaeralis, Clostera fulgurita, Plecoptera reflexa and Selepa celtis).
ii. Entomogenous Fungi:
The role of entomogenous fungi as effective biocontrol agent has been recognized throughout world. In India, a few fungal pathogens have been recorded infesting and killing larvae and pupae of some important forest insect pests. Beauveria bassiana, a well-known insect pathogen has been found infecting and killing larvae of Atteva fabriciella, Eligma narcissus, Hypsipyla robusta, Philosamia cynthia and Scarabaeid beetles, causing large scale larval mortality.
iii. Viruses:
Of the numerous viral pathogens, which are known to cause diseases in insects, only those (viruses) belonging to families Baculoviridae and Reoviridae reportedly hold promise as biocontrol agents against insect pests. In India, nuclear polyhedrosis viruses (NPV), isolated from the diseased larvae of poplar defoliator Clostera fulgurita, have been successfully tested in the field and found very effective in controlling the pest population.
More recently another NPV isolated from infected teak defoliator Hyblaea puera, has been tested in suppressing the defoliators population. Elsewhere, NPV of Helicoverpa, Baculovirus heliothis, Pieris brassicae and Pieris rapae are successful microbial pesticides.
A key part of pest management is to use a pesticide only when it is needed to prevent an unacceptable amount of damage. Use of pesticides may not be justified if the cost of control or potential harm to the environment is greater than the estimated damage or loss. Pesticides are a very important tool, when large pest population threatens high value trees. Knowledge of the pest’s life cycle, selection of an appropriate pesticide, proper timing of the application and use of the right application equipment will improve coverage and effectiveness.
A pesticide application may be needed to control a pest outbreak or to eradicate limited infestations of an invasive species. Pesticides should be selected and used in a manner that will cause the least harm to non-target organisms in forests, seed orchards and nurseries, while still achieving the desired management goal.
Pesticides are labeled for specific pests, crops and land-use situations. Use of insecticides, fungicides and herbicides is common in managed seed orchards, forest nurseries, intensive short-rotation plantations and in Christmas tree production. In general, the most commonly used forest pesticides are herbicides used for site preparation, herbaceous weed control, etc.
Insecticide applications are seldom used in general forest management because of high treatment costs and potential effects on non-target organisms. Situations justifying the widespread use of fungicides also are rare. In some cases, vertebrate animals must be controlled through trapping or hunting and similarly use of repellents and poison baits may be employed.
Insecticides for Forest Pest Control:
i. Insecticides:
Insecticide applications are rarely practical against forest insect pests. However, they may be useful in specific situations (For e.g., Chlorpyriphos 20EC against borers and termites in nursery, Methyl parathion 50EC against defoliators, Dimethoate 30EC, Imidachloprid and Thiometaxm 25G against sucking pests) such as limited infestations of an invasive species. Insecticidal soaps, horticultural oils, Bt-insecticides, and systemic are common choices because they have very low potential to harm the environment, non-target species and applicators.
ii. Insecticidal Soaps:
They are made from of salts of fatty acids. When sprayed directly on vulnerable stages of soft bodied insects, such as aphids and adelgids, they kill by damaging individual cells. However, they are not very effective against stages of the life cycle that are inactive or hidden, or against larger insects such as caterpillars and beetles. Insecticidal soap sprays must come into direct contact with the target pest and often results are best against specific life stages.
Timing and thorough spray coverage is essential for best results. If the vulnerable stage is active over a long period of time, several applications are needed to control most insects. Insecticidal soaps should not be applied directly to water or near a water source. Hard water is not effective for mixing soap sprays, so softened or distilled water is used for best results. Tender young growth of evergreens and shrubs in the spring can be sensitive to insecticidal soaps.
iii. Bt Insecticides:
Bt Insecticides (Bacillus thuringiensis) are protein toxins that are produced by a common soil bacterium. Many provide specific control of caterpillars without affecting other types of insects (beetles, sawflies, etc.). Bt insecticides disrupt the gut wall cells in the caterpillar digestive tract so a formulation of the insecticide must be sprayed on foliage that will be eaten by the caterpillars. Caterpillars stop feeding soon after eating the insecticide but usually do not die for several days. Bt insecticides work best against young caterpillars that are less than half-grown. These insecticides are relatively non-toxic to mammals and other animals.
iv. Systemic Insecticides:
Imidacloprid is the common name of a systemic insecticide that is used to control sap-feeding insects, such as the hemlock woolly adelgid. It is a nerve poison but it is much more toxic to insects than to warm-blooded animals. Imidacloprid can be diluted in water and applied as a drench poured around the base of a tree, or injected into the soil. The insecticide is taken up by the roots and moved throughout the tree.