Part I. Forest insect control
Part II. Forest disease control
In one of the large Central European countries, a number of professionals dealing with the protection of plants from insect and disease hazards are organized into an "Association of... Plant Physicians." As the name implies, the members of this organization consider their profession as analogous to that of the human or veterinary physician, at least as far as scientific principles are involved (though certainly not from a financial point of view).
To these three related sciences - dealing with plants, animals and human beings - the basic principle that "prevention is better than cure" applies equally forcibly, but, unfortunately, the situation is too often far from this ideal. The chief reason why this is so has repeatedly been emphasized by this Symposium. It stems from lack of knowledge concerning the fundamental causes of outbreaks of forest pests. And, since it is not known how to prevent them, like physicians, usually only a "cure" can be resorted to. If this situation is ever to be changed, much more basic research must be done, especially in population dynamics and ecology of forest enemies. Such research is of utmost priority, as stated in the first general recommendation passed by the Symposium.
It was primarily with "control" that the speakers of Meeting No. IX were concerned.
Four methods are available, and three of them: chemical control, biological control, and silvicultural control, were extensively discussed by the Symposium. The fourth method, mechanical control, was not reviewed in detail since this method has lost much of its former importance. Apart from its other limitations, a shortage of manual labor in Europe, and the vast extension of forests and the cost of labor in North America often render this type of treatment impractical. However, in countries in which manual labor is cheap and abundantly available, mechanical control of certain insect pests might be feasible.
It has been generally agreed that each forest pest problem should be carefully assessed before deciding for or against control. However, a principal handicap in such an assessment is the frequent lack of quantitative information on direct and indirect losses from forest diseases and insects. As the Symposium has outlined in one of its recommendations such quantifications would place our efforts in research, survey and control, on a much more solid footing, and would strengthen our position with the practicing forester and the public.
The weapon most relied upon at present in the battle against forest insects is chemical control. The commercial production of a number of extremely powerful insecticides and the ease with which these can be applied over vast areas permit quick and effective control of the majority of insect defoliators. This development reminds one of the recent achievements with sulfonamides and antibiotics in human medicine.
However, the modern organic insecticides have a number of serious drawbacks in that they can cause unwanted and sometimes harmful side-effects (as may the antibiotics, in human medicine), killing not only the pest in question but also its natural enemies and many other animals. Moreover, some are poisonous to man himself. These ill-effects, although long recognized by both forest and agricultural entomologists, have only recently been widely discussed and, in fact, investigated by the American Congress. Hence, the goal is now for development of more specific, and less toxic, insecticides.
Another serious drawback is that insecticides often do not remove the basic cause of pest outbreaks. Thus, they frequently have to be applied year after year, a procedure that considerably increases overhead expenses.
It is for these reasons that alternate types of forest insect control, biological and silvicultural, have been most intensively discussed at the Symposium. It is nevertheless recognized that entomologists will depend on chemicals for decades to come.
Biological control in its classical application has been directed primarily against introduced insect pests. However, the recent developments in microbial and autocidal control offer also attractive opportunities to control native pests as do methods of conservation of native natural enemies and biotechnical control measures such as sex attractants, etc.
The importation of entomophagous insects against exotic pests is particularly attractive with forest insects because of the long-term rotation of the forest crop.
This provides a more stable environment for acclimatization of the imported natural enemies than for example ephemeral cultivations on arable land.
It has been shown from experience that a careful study of the entire host-parasite complex in the country of origin is of paramount importance before any introductions may be recommended. Many failures in the past can be attributed to the lack of such investigations into the host-specificity and interrelations of the various parasite species discovered. Currently, a much disputed argument is whether only one or a few species (that is, the most effective and best adapted) should be introduced. Otherwise all available primary parasites, aiming to establish the entire complex of natural enemies as it exists in the original situation could be introduced. Interference between several introduced parasites has been shown in some examples (for instance, the European pine shoot moth in Canada) but there are other examples on record in which a complex of various parasites has done better than a single species (for example, European spruce sawfly in Canada). Each case, therefore, needs special consideration.
A serious shortcoming of this type of biological control stems from the fact that in many cases inadequate assessments of the effectiveness of introductions have been made. Only very recently have reports been made available of effects of the biological control operations in the United States, Canada and Australia. Thus, only by careful, long-term research can results be evaluated and improved. The selective breeding of introduced strains of parasites, the search for parasites of host species closely related to the pest in question and the improvement of parasite or predator complexes by inter-areal transfer of natural enemies (for example, from the west to the east of North America) are additional means that should be more fully explored.
The use of pathogen-, such as virus, bacteria, and fungi, attaching noxious forest insects is another field of biological control that is currently much investigated in many countries. Some of these pathogens (for example, Bacillus thuringiensis and Beauveria) lend themselves to industrial production, thus fulfilling largely the plea for more specific "insecticides." The method of application is similar to that of chemical insecticides with which they can be used without interference.
Entomopathogenic micro-organisms have much similarity to the antibiotics in human medicine, but the specificity of the various strains of micro-organisms for different pests has to be carefully considered prior to application.
Usually, microbial control is not as quick and sometimes not as effective as chemical control. However, this has certain advantages since it does enhance the chances to carry over the pathogen from one pest generation to the next one, thus producing a lasting effect.
One of the newest developments in the field of biological control is the technique known as autocidal control; that is, limiting the pest species through the mass liberation of genetically inferior strains. Males made sexually sterile by gamma radiation and then released in large numbers in the outbreak area will mate with females that will then produce no viable offspring. In forestry, this method has been applied recently to a local Melolonthid population in Switzerland with promising results, and many more projects are under way in different parts of the world. The detection of chemicals that sterilize males or both sexes may considerably simplify autocidal control. These chemosterilants can be directly applied to a pest population, thereby greatly reducing the cost.
The development and release of strains of insects that possess a genetically inferior gene complex (that is, carrying many lethal factors) is another way of lowering the vigor of pest populations and of reducing them.
In general, biological control seems particularly suited to forest insect problems since the relatively stable environment of a forest guarantees freedom from such adverse effects as interference by pesticides or disturbing agricultural practices.
Since biological control agents once successfully established are self-perpetuating, and eventually will spread over international boundaries, smaller countries might co-operate with common projects.
Two international organizations for biological control exist already. These are the Commonwealth Institute of Biological Control (CIBC) for the countries of the British Commonwealth, and the Commission internationale de lutte biologique contre les ennemis des cultures (CILB) for most of the countries of western Europe and the Mediterranean.
Some aspects of biological control, as for example the enrichment of habitats with flowering plants producing pollen and nectar for adult entomophagous insects, are transient to the third method of pest control - silvicultural control - that received great attention during the Symposium.
Silvicultural control is an indirect method aimed at:
(a) prevention of insect pest outbreaks;
(b) alteration of forest conditions during the outbreak in order to combat it;
(c) prevention of injury to trees;
(d) acceleration of recovery of devasted stands.
For newly established stands precautions must be taken from the very beginning. One of the most important principles in this respect is the proper selection of suitable tree species, and of suitable races of a given species. As has been abundantly experienced in the recent history of European forests, the greatest risks are involved when tree species are introduced into new areas.
The most impressive example is Picea abies, which was originally confined to mountainous areas above 1,000 meters in Central Europe. During the past centuries, foresters in low-lying areas, hoping for greater production per unit area, started to replace the original oak-beech forests with plantations of Picea abies in pure stands. Now that the first or second generation of these conifer stands are being harvested, the disadvantage of pure stands can be fully appreciated. Not only have soil conditions rapidly deteriorated, hampering the establishment of new stands, but also the timber is of poor quality due to repeated fungal and insect attacks. This has resulted in its use for wood pulp and low quality timber. Among the insect species encountered the most dangerous are Lymantria monacha and Pristiphora abietina, which are almost unknown in natural spruce forests.
Most people attribute the mass occurrence of such species to a different "biotic balance" in pure stand forests. For example, it is thought that the natural enemies of such pest insects may be absent due to lack of flowering plants and alternate hosts. Further, the trees may show an increased susceptibility in unnatural conditions. However, more investigation into this problem is badly needed to elucidate the causal pathways leading to this so-called "disturbance of the biocenotic balance." Certainly this example should be a warning to other countries with similar reafforestation problems.
Since tree-breeding is a comparatively new science, the selection of resistant strains is still in its infancy as compared with similar endeavors against agricultural pests. More emphasis has been laid in recent years on the improvement of established plantations by chemical fertilizers, a method universally used in agriculture. Recent work in Germany has shown that while some sucking insects are promoted, the defoliator populations often become effectively reduced when, for example, nitrogenous fertilizers have been applied. This is a promising field for further research on a broader long-term basis.
One of the primary requirements in silvicultural treatment of the standing crop, as well as during its harvesting, is known in Germany as "clean hygienic management." This involves selective cutting and thinning including the removal of suppressed, physiologically weakened trees, and also "clean" methods of harvesting and logging. With regard to the latter, the modern, mechanized procedures of felling and logging may be a step backward. For instance, some methods of extensive felling seen in North America are certainly far from sanitary.
As has been outlined by the Symposium, a field so complex as silvicultural control can only be effectively dealt with through intensive co-operation between the forester, on the one hand, and the specialists: entomologists, pathologists, phytosociologists, on the other.
In conclusion, it can be said that the future in forest insect control lies in the integration of the three control methods discussed above. For emergency cases chemicals will always be depended on. However, by striving for a better understanding of the ecological relations between forest conditions and insect outbreaks it should be increasingly possible to replace chemical treatment by preventive measures or at least by more natural methods of control.
Disease reports
The control of forest tree diseases and insect pests was an ultimate consideration of the Symposium. During the conference, many comprehensive reports summarized world literature and current research on forest diseases. These papers were discussed vigorously by scientists representing the several participating countries and enabled comparisons of both common and unique problems developing under widely differing forest situations.
The presentations treated first major types of diseases; then, important tree diseases of countries or regions throughout the world. In most reports, known control measures for specific diseases were outlined and evaluated (see Chapter 5). In addition, general papers reviewed the principles, procedures, and problems of controlling forest tree diseases (see Chapter 3).
International diseases and plant quarantines
The Symposium directed its attention primarily to "internationally dangerous diseases" and possible ways and means of preventing their movement. Thus, it was emphasized repeatedly throughout the conference that "effective quarantines are the first line of defense against international spread of forest pathogens." Principles, problems, and possibilities for "restricting the movement (of pathogens) on seed, living plant material, and wood in its various forms" were discussed in papers by regulatory officers from many countries throughout the world (see Chapter 6). Various procedures for disinfecting, disinfesting, and decontaminating plant products and packing materials were listed (see preceding chapters). The Symposium recommended several ways by which plant quarantines could be strengthened (see chapter on Recommendations, 7-20, 39-43) and stressed the need for further research, especially on seedborne pathogens and their control.
Perhaps one point merits repetition in view of the Symposium's goal to initiate and to strengthen international co-operation in disease control. While much of the discussion of quarantine regulations pertained mainly to the importing country, certain responsibilities also were placed on the exporting country. No doubt the "golden rule" of plant quarantines might be, as already stated by Farstad: "Each country should be prepared to provide as much protection for the country purchasing its products as it expects from the country from which purchases are made." The Symposium suggested certain measures to exporting countries and agencies for achieving such guarantees (see Recommendations 11, 13 and 15).
Principles of forest disease control
General principles governing forest disease control were variously defined. Classical concepts of plant disease control including exclusion, eradication, protection, and immunization, so widely and effectively used with agricultural crops, are applicable in forest practice. The first two are directed against the causal agent or disease incitant; the last two toward the host plant.
Direct control measures, specifically formulated to limit or even to eradicate a particular pathogen, included temporary or emergency treatments directed against newly introduced pathogens before they become firmly established or against existing diseases that have reached, or threaten to reach, epidemic proportions under particularly favorable circumstances. Though costly, their use may be justified in certain situations where forest crop or individual tree values are unusually high. Cited examples included the use of fungicides, insecticides, nematicides, herbicides, soil fumigants, and special cultural practices in forest nurseries, Christmas tree plantations, on occasion in forest plantings, and commonly with shade and ornamental trees. Recent research has suggested possible uses of antagonistic micro-organisms, antibiotics, systemic chemotherapeutants, growth regulators, and silvicides in tree disease control.
Indirect control measures attempt to correct or to avoid fundamental maladjustments in forest conditions from which disease outbreaks receive their initial stimulus. Such measures rely on disease prevention rather than cure and are applied over a long period of time. These "silvicultural controls," though sometimes considered incidental to normal management practices, often assume a major role and may largely determine the type of silviculture employed (for examples, see Chapter 1). Their immediate costs must be balanced against the losses which they prevent and the returns they ensure (see Chapter 3 for further consideration of economic factors governing disease control programs; also chapter on Recommendations, 2, for action of the Symposium).
The Symposium recognized that as natural stands are replaced by planted or otherwise artificially regenerated forests, new disease problems will arise. Past experience has demonstrated possible dangers in planting a single species at one time over large areas, of careless choice of tree species or of seed source, of using inferior planting stock, of planting on badly deteriorated sites, and of neglect of new plantings. However, added values and opportunities for intensive management will permit the application of additional, and perhaps more effective, control measures from the time the planting is made. With planted stands, the opportunity also exists of using resistant planting stock of superior type, developed by selection or hybridization. In several cases, resistant tree varieties already are available for commercial use.
Most disease reports cited numerous examples of effective silvicultural controls for avoiding or minimizing disease losses. In fact, it appeared that most current forest disease control is indirect in nature and entails adjustments in forest management practices. Recognizing these possibilities, the Symposium strongly urged greater application of the considerable information already available, but, at the same time, continued research by forest scientists leading to improved control methods. Further, the Symposium called for increased research "to provide additional information on the identification, distribution, and biology of forest pathogens;" on infection processes, environmental effects, epidemiology, host-parasite interactions, disease mechanisms, defense reactions, and genetic variations in both host and pathogen. Such basic research must have generous financial support even though immediate application to disease situations in the field may not be readily apparent. Significant "breakthroughs" that thrust a science far ahead generally result from such fundamental studies. At the same time, the Symposium recognized the need for better quantitative evaluations of disease losses so that research programs related to control efforts could be oriented in areas of greatest need.
Responsibility and training
Since the major responsibility for the application of control measures rests with the practicing forester or forest manager, he at least must be aware of the main diseases that threaten his woodlands, and be able to recognize their signs and symptoms, to interpret their significance in terms of present and future damage, to know and to apply control measures where stand values permit or to know how, when, or where to get help from professional forest scientists. Needless to add, the forest pathologist has the responsibility not only of uncovering basic facts, but also of organizing and presenting them in a manner understandable to the forester in the field. To these ends, the Symposium considered various mechanisms by which information could be assembled, organized, summarized, and distributed to various audiences throughout the world (see chapter on Recommendations, 3, 5, 20, 26, 28, 30, and 33). Likewise, considerations of training required for effective forest scientists, forest managers, and regulatory officers provoked concrete recommendations by the Symposium (see Chapter 8, and Recommendations, 4, 16, 17, 23, 24).
Forest pest control in the United States
In summary, the guiding principles governing control of forest diseases in the United States include:
1. The pest must pose a major threat, based on pre control evaluations.2. Application of control measures must be thorough and complete, with strict adherence to technical standards.
3. Research must be dynamic and continuing so as to provide the best, up-to-date control measures.
4. Forest managers and the public must be well informed in order to appreciate the expectations or limitations of control programs.
5. People concerned must be persuaded to ensure understanding and co-operation. Co-operative aspects of pest control involve the Federal Government, states, private forest owners, and public representatives.
