J. M. FRANZ
J.M. FRANZ is a member of the Institut für Biologische Sohädlingsbekämpfung, Biologische Bundesanstalt fur Land-und Forstwirtsohaft, Darmstadt, Germany (Fed. Rep.)
Final part of a report commissioned by FAO. Part 1 appeared in Unasylva Volume 24 (4), Number 99
BIOLOGICAL CONTROL OF FOREST-TREE PATHOGENS
Plant diseases. The control of plant diseases can rarely be achieved without careful combination of many techniques. Breeding of resistant host trees is, no doubt, a "biological approach," but is beyond the scope of this review, although it may well be part of integrated control of insect pests (cf. below). Application of fungicides is usually too expensive in forests. There are some ways to avoid or suppress infestations by silvicultural measures - choice of site, removal of alternate hosts for pathogens, soil management and use of disease-free seed. Here, only a few words can be said about the utilization of organisms, hyper-parasites and predators to suppress parasitic pathogens.
The use of beneficial organisms against forest-tree pathogens has not been fully explored. The great difficulty in their application is that the beneficial agent has to establish contact with the plant parasite early so that it can prevent the pathogen from invading the tree. Hansbrough (1965) lists several cases of organisms attacking tree pathogens; but he points out that there are no known instances of the reduction of a forest disease outbreak to tolerable levels through the action of such organisms. Most parasites and predators of forest pathogens reduce the inoculum production, but not sufficiently to suppress or, better, to prevent the epiphytic growth. This can be illustrated by such cases as the purple mould Tuberculina maxima thriving well on the introduced white-pine blister rust fungus in the United States (Wicker and Woo, 1969), or the parasitic fungi and insects that live on the dwarf mistletoe on coniferous trees without reducing the parasite's damage (Wicker and Shaw, 1968). It is hoped to discover what antagonistic organisms in forest soil suppress the root rot (Fomes annosus) of pines in the eastern United States when the trees grow on undisturbed forest soil and possibly to transfer them to agricultural land reverting to forest production. Perhaps this could be done by inoculating nursery soil, thereby providing seedlings with a full complement of protective organisms.
There is an intricate relationship between microorganisms living on leaf surfaces (bacterial epiphytes) and pathogens. In greenhouse experiments, application of a mixture of nutrient broth culture of these bacteria on seedlings of Pseudotsuga menziesii resulted in control of the needle rust, Melampsora medusae (McBride, 1969). A review of this type of antagonistic action of epiphytic microorganisms to pathogens is provided by Leben (1965). As a whole, however, biological disease control in the forest will be more preventive than palliative or curative and must be based on sound ecological concepts of the forest as an ecosystem.
A recent study (Hulme and Shields, 1970) showed that the immediate injection of Trichoderma virida into birch logs is able to control decay fungi biologically, by competition. The artificially colonized fungus acts first and removes nonstructural carbohydrates from wood without substantially altering its mechanical properties. Thus, no other fungi can grow in the wood. The authors recommend, however, the systematic screening of other organisms for this purpose in order to find still better species which may also work under different environmental conditions.
BIOTECHNICAL CONTROL
In some cases it is possible to utilize specific innate behaviour characteristics which are responsive to certain physical or chemical stimuli ("Metarchon"). The effect is usually not a direct kill, as with convenient physical or chemical control methods. Because of the close connexion between biological properties of the responding organism and the stimulus, such control has been named "biotechnical" (Franz, 1969). Its effect consists usually in attraction, repellence or physiological disorder of pest organisms in a very specific manner.
Sex attractants are particularly powerful chemical attractants that aid in assembling both sexes. They occur in many insect orders (reviews: Jacobson, 1965, 1966). Their practical control value can be utilized by several techniques. Assessment of insect abundance was facilitated by exposing unmated females, their abdomen tips, or their extracts to attract and collect males, for instance with the nun moth (Lymantria monacha) and the gypsy moth (L. dispar). Some of the sex attractants of forest pests have been isolated meanwhile. Field experiments showed the value of synthesized sex lures (e.g., "Gyplure") for male gypsy moths in the detection and survey of outbreaks. The United States Department of Agriculture has carried out large-scale survey programmes with "Gyplure." Disposable paper traps are now used. They are fitted with a wick for the attractants and lined with a sticky substance to hold lured months. "Gyplure" did not work satisfactorily, however, in an effort to confuse males by widespread dissemination. Similar projects have been suggested with other sex attractants like that of the introduced pine sawfly (Diprion similis) (Casida, Coppel and Watanabe, 1963). A successful field experiment to confuse males by disseminating sex pheromone was done with the cabbage looper (Trichoplusia ni) (Gaston et al., 1967).
Many bark beetles are strongly attracted to pheromones which also could be partially synthesized, as were "Frontalin" and "Brevicomin" (from Dendroctonus species). They act neither as species-specifics, nor only as sex attractants, but they govern the phenomenon of mass aggregation of Scolytidae in combination with volatile host odour (Vité and Pitman, 1969). The application of powerful chemical attractants in the field needs careful consideration of ecological implications (Atkins, 1968). They offer the possibility of bringing the target species into contact with toxicants, sterilants, pathogens, or mechanical devices exposed in ways that do not contaminate the general environment or endanger other species.
General attractants that bring phytophagous or wood-boring insects to their food plants are less specific than sex attractants. Their potential for insect control is well known to practical foresters, who use trap-trees and the attractive odour of decaying bark to concentrate, supervise and eliminate wood-borers and bark beetles. Synthetic attractants may sometimes be of additional value since they are able to reinforce the attractive action of trap-trees for control of beetles. Also, the study of density and species composition of wood borers has been facilitated by synthetics and has resulted in a more accurate prognosis of outbreaks (Francke-Grosmann, 1963).
Repellents are substances or physical properties that cause the pest to move away from their source. They are frequently used in the protection of agricultural areas against game, and specific compounds against biting insects have been developed for protection of domestic animals and man (Jacobson, 1966; Painter, 1967). As volatile resins apparently cause the resistance of certain species of pine to bark beetle and to tip moth attack, future practical utilization of repellent odour may be possible also in the forest. The same can be said of feeding deterrents, which prevent insects from feeding on treated foliage without killing them. These are safer than persistent insecticides and are not hazardous to wildlife. Research is very active in this field. Organometallics, triazones, and plant extracts are the main feeding deterrents used in agriculture. First experiments in the forest (Thomas, 1969) showed the effectiveness of triphenyl-tin acetate for protection of pine seedlings against the pales weevil (Hylobius pales). The effect of these "antifeedants" is promising with open-feeding species, but limited, because new growth cannot be protected. The development of systemic feeding deterrents would therefore be highly desirable (Ascher, 1969).
The attempt has also been made to control insects whose growth period depends on an exact correspondence with plant growth by artificially prolonging plant dormancy by application of abscisic acid (Eidt and Little, 1968).
Attraction or repulsion by sound is mentioned here as another method of biotechnical control. Mosquitoes can be attracted toward sound resembling the wingbeat noise of the female. The response of moths to sounds similar to bat cries is a rapid change in the direction of flight. A few practical control experiments have been made (Belton, 1962), for example: the population of corn borers was reduced to one third in control plots where high frequency sounds were broadcast from dusk to dawn during the growing season. Although no trials with forest pests have been reported yet, the potential value of attractive or repellent sound or electromagnetic energy will warrant further study (Nelson and Seubert, 1966).
Insect hormones and hormone-mimicking compounds are of increasing importance in biotechnical control (Mordue, 1969; Williams, 1969). The moulting hormone (ecdyson) as a steroid can be topically applied only with difficulty. It is, therefore, of less immediate interest than the juvenile hormone, which, when disseminated at critical periods produces lethal disturbances in egg development and metamorphosis. Larvae of insects fail to form viable pupae or adults; eggs deposited from treated females do not hatch. Juvenile hormone analogues have been found in many plant extracts; they are understood as a specific defence mechanism which appeared during evolution. It is possible to synthesize juvenile hormones and some analogues (e.g., sesquiterpenes, farnesol, farnesenic acids, and others). These are partially active at nanogramme (10-9 g) levels. Research is actively proceeding because practical application seems possible owing to the extreme effectiveness of even traces of this hormone. For instance, heteroptera males treated with 1 to 5 micro-grammes of it transfer enough to several females during copulation to make them sterile. This venereal approach provides a new path to autocidal control; it creates selectivity by coupling the hormone transfer to sexual activity. Malacosoma californicum pluviale larvae and pupae were treated with farnesyl methyl ether or crude extracts of Thuja and Pseudotsuga trees; they showed increased mortality due to structural abnormalities, moulting difficulties and increased infection with polyhedral virus and microsporidia. Reduced egg numbers were produced and the progeny was less viable (Wellington, 1969). A disadvantage of the juvenile hormone and its analogues is that they do not protect against damage by larvae. The toxicity to vertebrates is low, but as yet too little is known about side effects in the environment. Whether or not insects will develop resistance to "hormonal pesticides" is controversial. No practical field experiments using forest pests are known, but they are to be expected.
BIOLOGICAL CONTROL OF WEEDS
Activity toward the utilization of organisms to suppress undesired plants has been centred in agriculture and pastures (cf. reviews by Holloway, 1964; Huffaker, 1959; F. Wilson, 1964). As some of the techniques employed and successes obtained refer also to forested areas, a few examples can be given here without discussing the fundamentals.
Most biological measures against weeds concern unintentionally introduced species. The problem is similar to the biological control of introduced pest insects. The obvious difference is the requirement of greater food specificity for phytophagous insects as compared with entomophagous ones. Natural enemies, usually stemming from the original floral region, have shown the ability to suppress the unintentionally introduced pest in several cases. A well known example is that of prickly pear, Opuntia, species in Australia and other continents which were brought under control by imported specific phytophagous insects, mainly Cactoblastis cactorum (Phycitidae). This has influenced forestry inasmuch as the prickly pear growth made large areas uncultivable, kept grazing cattle away and allowed tree growth to flourish, as is seen in the paper by Dodd (1940).
Growth of forest as a consequence of massive invasion of weeds on pastures has happened also in other cases. Removal of the forests and continuation of agricultural use of the land have then been the usual development when the introduction of specific plant-feeders was successful. Thus. biological weed control did not contribute to forest protection under such conditions.
The effort to suppress gorse (Ulex europaeus) in New Zealand and recently in California is one of the few examples of attempted biological weed control of (partial) forestry interest. The Peed-feeding gorse weevil (Apion ulicis) was imported from Europe and became established. It showed a high degree of specificity, but could not sufficiently reduce the tremendous seed production of the weed. Further specific feeders are being investigated in Europe (Zwölfer, 1969).
Scotch broom (Cytisus scoparius) is a rangeland and forest weed introduced from Europe to California. It is assumed that the introduction of a leaf- and stem-mining lepidopteron (Leucoptera spartifoliella) will reduce the broom, which not only competes with desirable forest trees but also increases fire hazard.
The European shrub Rhamnus cathartica is a weed in Canada because it serves as alternate host of a fungus parasite (Puccinia lolii) on oats. Two geometrids showing the required specificity are, at present, considered for future release in Canada ((Zwölfer, 1969).
Among the introduced plants in Europe, Solidago species are weeds for the forester because they compete with young plantations, whereas beekeepers esteem them for pollen production. Such conflict of interest has sometimes resulted in the intentional importation of plant-feeders of low efficiency.
The control of native weeds by imported plant-feeders may be quite successful. The great reduction of the Bermuda cedar (Juniperus bermudiana) in Bermuda after the accidental introduction of two alien scale insects, and the wide destruction of native chestnut in the United States by an introduced fungus (Endothia parasiatica) from Asia are examples which demonstrate the potential of imported phytophagous organisms. Both examples refer, unfortunately, to valuable trees. Similar cases may be found for weeds. Here is an open field for investigations which should be carried out with care, but should not be unnecessarily restricted by fear of outbreaks (F. Wilson, 1964).
The possibility of using plant pathogens in weed control is old, but only a few practical experiments are known so far (C.L.. Wilson, 1969). For the conversion of predominantly oak stands to pine,, the inoculation of trees with the oak wilt fungus (Ceratocystis fagacearum) has been found to be a practical means for oak removal in Minnesota. For the control of persimmon (Diospyrus virginiana), the trees can be cut and the stumps inoculated with a spore suspension of Cephalosporium diospyri without danger to untreated trees. Other projects to suppress weed trees or herbaceous weeds are being considered, and the immense potential of this approach warrants great efforts.
INTEGRATED CONTROL
Integrated control in its modern broad meaning was first defined rather simply as any combination of the effects of beneficial organisms with all other types of pest control (Franz, 1961b). Nowadays, a more sophisticated definition is preferred: "Integrated control is a pest population management system that, in the context of the associated environment and the population dynamics of the pest species, utilizes all suitable techniques and methods in as compatible a manner as possible and maintains the pest populations at levels below those causing economic injury" (R.F. Smith, 1969).
Integrated control and pest management should not be thought of as synonymous, according to Stark (1970), who proposes that pest management should be considered as the higher level activity (i.e., as a component of resource management), whereas integrated control should be the approach by which regulation of pest populations is achieved. In his review the practical aspects of integrated control are dealt with and special attention is given to the selection of examples from the field.
The feasibility of integrated control depends largely on the economic injury level. If it is very low and practically no damage can be tolerated, chances for integrated control are slim. In the forest, however, the crop can usually tolerate considerable and repeated (indirect) damage. This depends mainly on the high ability of trees to regenerate. Oaks that are defoliated in May grow a complete set of new foliage in the course of a few weeks. The regeneration of hardwoods is one reason why forest-killing outbreaks are usually limited to purely coniferous stands (Franz, 1948). In addition, the tolerance level is further increased, in managed forests at least, by the regular removal of damaged individual trees from the stand by thinning operations in the course of the growth period (Voûte, 1964).
If we take not survival of the forest but economic losses as the measure for damage that can be tolerated before control is warranted (economic threshold), the problem gains another dimension. As Stark (1970) correctly emphasizes, the concept of economic threshold, as developed for agriculture, is not adequate for most decisions in the management of forest pests. It was developed along strict profit-loss relationships, but does not recognize sufficiently the mixed values of noncommercial forests or the long-range costs. An extremely broad perspective is needed in assessing the risk from outbreaks on noncommercial forests.
In agriculture, the common causes that force farmers to consider integrated control are: the development of resistant pests; the toxic residue problem; and the upsurge of new and more frequent outbreaks after repeated chemical control. In forestry, all these unwanted side effects do exist, but are not yet problems as pressing as they are in agriculture (Schwerdtfeger, 1966).
Several papers have appeared recently on integrated control relevant to forest problems: Beirne, 1963a, 1967; FAO, 1966; Franz, 1966a; Knight, 1962; Koehler, 1959; Stark, 1970; Voûte, 1964. The availability of more extensive surveys will allow me to centre on practical and methodological examples. Because of. the key role of natural enemies in integrated control, most papers deal with two aspects: how to get along without insecticides, and if this is not possible, how to apply them in a way not too detrimental to biotic antagonists of pests.
Reduction of insecticide application
Forecasting potential outbreaks to avoid pest control operations has a long history in forestry. Once there was no insecticide problem because there were no insecticides. When they appeared forest entomologists like Escherich (1931) realized that the main purpose of pest control is to protect the crop and not to kill as many pests as possible. In his comprehensive books, very careful analyses of the complex biological situation surrounding outbreaks are presented. In some parts of Germany and in a few other European countries, supervision of pine forests based on the level of hibernating pests has been carried out for over a hundred years. The economic injury levels nowadays are known and critical density figures established for most forest types and main pests in Europe and North America. In preparing the forecast for potential outbreaks, not only quantitative data such as numbers of sawfly cocoons per unit area are to be considered. The qualitative aspects must also be properly taken into account, namely the effect of natural enemies must be measured and the trend of the gradation (outbreak) assessed (see Wellenstein, 1942; Janisch, 1959).
By using the technique of prognosis, which carefully weighs all available data of the complex and dynamic situation, many planned control operations have been cancelled or reduced when natural limitation factors were recognized early enough to be sufficiently effective. It is hoped that this type of preventive forecasting will be adopted in all countries having managed forests in order to minimize pesticide application. An example is the reduction of the size of the treatment area from 200 000 acres to approximately 30 000 (85 percent), in an infestation of the jack-pine budworm (Choristoneura pinus) in the United States, because of heavy parasitization by Apanteles fumiferanae (Benjamin, 1965).
Diagnosis of diseased specimens, rearing of parasites and hyperparasites, experience in the trends of gradations (outbreaks) and of epizootics as well as in the survival of partially defoliated trees - all these and many other factors are details in the whole picture of the situation. In other words, since foresters remained an ecologically-minded sector of plant producers, techniques are available to give reliable forecasts for the future development of incipient outbreaks. Thus, hasty or unnecessary control operations can be avoided (for other examples see Dowden, 1952).
The augmentation of natural mortality factors in the cultural and biological control of pest organisms can be achieved in forest management. Planned forest protection as part of general management is becoming widely acknowledged (Graham, 1951; Waters, 1963). To say it in a few simple words: the aim is to plant, to grow and to maintain forest types that will be, according to present experience, as pest-proof as possible within the framework of the economic task assigned to the forest. The foundation of susceptible monocultures should be avoided and special care must be taken with the use of tree species and strains outside of their natural distribution area. Selection of more resistant strains of trees will be one possibility (see Schönborn, 1966). Disruptive types of forest operations should he avoided. Sustained yield in forest production should become as important a target as productivity.
Silvicultural control is usually understood by foresters as any application of cultural measures to prevent existing pest organisms from reaching outbreak dimensions. The alteration of a susceptible forest into another, more resistant type can be achieved in the planning stage (here called forest management) as well as later on, during maturation. Techniques of thinning, of protection against storm, sometimes even of fertilization (Stark, 1965) can contribute to the general aim: to increase the environmental resistance of the forest to potential pest organisms. This type of control can, if successful, result in a low average level of pests and make other control operations unnecessary (for examples see Voûte, 1964).
In natural populations of phytophagous arthropods, there are only two types of regulating processes known that are able to act more intensively as population density increases: intra-specific competition and natural enemy action. Under favourable environmental conditions for the pest, as with all plant-feeders, intra-specific competition cannot normally be utilized. A process that reduces populations because they have exhausted the available plant substance usually is of little practical control value in plant protection. Therefore, the only acceptable force, capable of some regulation, is natural enemy action; it has a key position in any integrated control programme. Actually, it would be difficult to imagine a situation in which integration of different control methods is planned without the direct or indirect role of natural enemies being carefully considered.
Acknowledging the central position of the effect of natural enemies, because they represent the only acceptable stabilizing and regulating element in an environment threatened by pest outbreaks, we immediately understand why all the methods of integration have two tendencies in common: either to preserve and augment natural enemy action, or to alter the environment so as to make it unsuitable for the pest. We understand also why the integration of natural enemy action with chemical control methods is so much more difficult than with any other control method - indeed, formerly they were considered to be totally incompatible. Recent developments have shown, however, that an optimum control pattern can be achieved in which the advantages of all available means, both natural and artificial, are integrated.
Several examples are known in which cultural or physical control measures have been specifically modified in order not only to spare beneficial agents, but also to rely on their help. Such old and direct methods as burning bark or sun-curing can be altered so that predominantly the pests and not their enemies will be destroyed. An example is the solar-heat treatment of felled ponderosa pine against the western pine beetle (Dendroctonns brevicornis) (Person, 1940). Most cleric predators (Thanasimus lecontei) escape to the sheltered underside where they feed on the bark beetles that are not heat-killed. For this purpose, the bark must not be peeled from the exposed logs. Burning of the bark of trapping trees at times when the fewest parasites are present and leaving parts of the crown of felled trees in the forest to provide food for alternative hosts of important parasites (Nuorteva, 1959) are other examples.
To summarize these observations, many methods of pest control in the forest are known that do not rely on chemicals. Because of certain limitations of these methods and/or the very moderate scale on which they have been explored and tested, most integration problems arise through application of toxic chemicals in the forest.
How to minimize side ejects of insecticides
In the development of a minimum insecticide programme it is desirable, but not yet frequently accomplished, first to lower the general level of the pest's abundance by utilizing more or less permanently acting natural enemies, assisted, when feasible, by silvicultural practices. When and where the level of pest abundance is still above the economic threshold, temporary rapid-acting biotic agents or chemicals could be applied in order to guarantee satisfactory control. There are several ways known to make insecticides less detrimental to natural enemies.
"Selectivity is the measure of the capacity of a treatment to spare natural enemies while destroying pests " (Bartlett, 1964). Insecticides can be physiologically selective, indicating an inherent difference of susceptibility between pest and enemy to a toxic substance; but very few are. Examples relevant to forest entomology are such outdated insecticides as chlorinated nitrocarbazoles. These are also stomach poisons and proved to be effective against such forest pests as Melolontha (Scarabaeidae), Hyphantria cunea, Malacosoma neustria, Euproctis chrysorrhoea, and other Lepidoptera (Wellenstein, 1954). Unfortunately, in spite of all suggestions by ecologists and wildlife protectionists, these compounds are no longer being produced. The tendency in modern chemical production is still not favourable to the production of "tailor-made insecticides," as documented in a 1965 symposium of the Entomological Society of America. It might well be essential for the future of some problems connected to public welfare that governmental research institutions accept such a charge whenever industry denies that there is "a reasonable chance of developing financially successful products ..." (Persing, 1965).
Although they are not chemical insecticides, microbial pathogens should again be mentioned here because of their amazing specificity. Viruses and spore-forming bacilli, as far as tested in practical field experiments, infect only the target pest or sometimes, as with Bacillus thuringiensis, a group of related hosts (many Lepidoptera). The future availability of more pathogen prep orations help in cases where high selectivity is required.
The differential exposure of pest and natural enemies to the pesticide (physical or ecological selectivity) can be used to reduce the damage of broad spectrum insecticides and to achieve some degree of selectivity. It is here that modern integrated control efforts have revealed numerous new possibilities.
Several modern contact insecticides show some degree of selectivity that can be exploited. Endosulfan, for instance, is relatively harmless to bees, and systemics save the lives of non-plant-feeding arthropods, if properly applied to the tree trunk. Persistent pesticides are always more detrimental to the fauna, whereas reasonably fugitive residues allow the survival of entomophagous arthropods which enter the treated area from protected niches, as in the pupal stage in the soil, or immigrate from nearby untreated plots. Very little is known on the subject of pesticide degradation time with regard to natural enemies. These are usually physiologically more susceptible than the hosts and also more exposed to contact insecticides, due to their greater mobility.
Organic phosphorus compounds have sometimes been preferred in forest insect control because of their shorter persistence. An example is the choice of Phosphamidon instead of persistent chlorinated hydrocarbon in the control of spruce budworm (Choristoneura fumiferana) in Canada. This reduced salmon losses, which were unavoidable with DDT (Fettes, 1963). Applied in Switzerland, however, this compound caused severe losses of birds (Schifferli, 1966).
Dosage reduction can sometimes result in a degree of selectivity with broad spectrum insecticides. This method has been widely used with DDT and BHC compounds in the forest. No a priori rule for the optimum dosage can be given, because it depends on many variables. The residue level that kills enough pests to avoid damage and still saves enough natural enemies (which are usually more susceptible, as mentioned earlier) to restore an effective balance has to be found empirically. Much will depend on local conditions, trend of the outbreak and time of application.
A guiding figure may be 113 grammes of DDT (pure substance) per acre, a dosage which proved satisfactory in the huge spruce budworm operations in Canada. In Quebec, natural enemies brought about the final collapse of the outbreak after DDT treatment (Blats, 1963). Similar dosages (0.5 kg DDT/ha as aerosols) resulted in satisfactory reduction of pests and simultaneous survival of enough parasites and predators to restore the balance in campaigns against the pine looper (Bupalus piniarius) and the nun moth (Lymantria monacha) in Czechoslovakia (Martinek, 1966); 10 percent DDT sprays against the gypsy moth (Lymantria dispar) on oak in Spain were said to last not very long (perhaps due to intensive insolation) and saved the effectiveness of parasites (Romanyk, 1958). The problem of low-dosage insecticides in combination with microbial pathogens will be discussed below.
Timing is another important step toward integration of chemical control with biotic agents. Broad spectrum insecticide applications may thereby lose some of their harmful effects on natural enemies. On the basis · of a thorough knowledge of the life histories of beneficial organism, insecticides may be applied when they are in dormant, resistant, or protected stages of their life cycles, or when they are temporarily away from the pest's immediate habitat. Following are examples of the application of contact insecticides, usually DDT (sometimes in very low dosages) against the pest insects before their main parasites appeared in the field:
(a) Early larval instars - Lymantria monacha on spruce (Gäbler, 1950), Dendrolimus pini on pine (Ryvkin, 1955), (Choristoneura murinana on fir trees (Franz and Wellenstein, 1958), Archips crataegana on oak (Hochmut, 1963), Diprion Mini on pine (saving only egg parasites) (Avramenko, 1958), Neodiprion sertifer on pine (Martinek and Kudler, 1964).(b) Application of chlorinated hydrocarbons (DDT, Toxaphene) in the egg stage, shortly before eclosion of the pine sawfly Diprion pini (Urban, 1964) showed that these toxicants are able to enter the foamy cover of eggs inserted into the needle. The survival of the egg parasites, which emerge much later, is an interesting aspect.
(c) Still earlier in the life cycle and, therefore, less dangerous to larval and pupal parasites, are treatments of adults on the wing. Successful experiments are reported from Czechoslovakia against Lepidoptera (Epinotia tedella) and sawflies (Pachynematus scutellatus and Cephalcia abietina) on spruce (Martinek, 1966).
In the above examples, early spraying may have r missed some individuals not yet emerged or hatched. ; Timing in favour of biotic agents usually requires some ; sacrifice from the maximum possible efficiency of a treatment. It ensures, however, the continuation of the regulative potential of the biocoenosis. Experience has shown that this sacrifice in "efficiency" is worth its price. It needs, of course, a sound knowledge of the time coincidence of host and parasite and the will-ingress of the operator to time the application of insecticides carefully according to biological, not to technical requirements. This series of successful field experiments (which is by no means complete) demonstrates again that ecology is the basis of all rational pest control.
Any minimum insecticide programme requires the application of chemicals only when and where natural limitation factors would fail with certainty ("spot treatment"). The philosophy behind this economy in ; the use of pesticides is not to reduce the cost of the treatment. It is frequently cheaper, per unit area, to treat large areas together. The difficult task of bioloigists participating in the planning of such operations - (and they should always participate!) is to convince · technically-minded foresters that the limiting of treatment to the most heavily attacked spots will be worth the extra effort and expense. As long as the treated areas are small, and the beneficial insects eliminated from the target area can immigrate from nearby refuges, the unbalanced situation will soon be buffered.
Minimizing the area treated, one of the established methods in integrated control, has no lower limit in forestry. It begins with the size reduction of treated patches in the forest, steps down to treatment of single trees, for instance those infested by bark beetles, and reaches an extremely dimension where persistent contact insecticides are applied, e.g. the tops of young pines, to protect only the buds of the leader against the bud moth, Rhyacionia buoliana; Miller and Neiswander, 1955. Another way to keep pine-needle-feeding caterpillars away from the crown is to put insecticide rings around the trunk, using a special spray gun. Larvae of Dendrolimus pini, after hibernating in the soil, cannot climb up to their food in spring and are at the same time exposed to their enemies (Gäbler, 1950; Rudnev and Zagaikevic, 1952). Spot treatment usually requires more manpower than indiscriminate, large-scale distribution of pesticides. It will, therefore, be of particular interest in areas where abundant labour is available at low cost.
Further improvement of spot-treatment techniques is to be expected through the combination of insecticides with attractants. The classic technique of bark beetle control-the trap-tree method-is now being modernized by combination with contact chemicals or fumigants. Knight (1962) gives a neat example of integration of insecticide application against Dendroctonus engelmanni in the centre of the outbreak area and at the lower parts of the trunks, whereas the fringe zone and the tops of the trunks were covered by woodpeckers, complementing the beneficial limitation of the chemical. The development of some synthetic sex attractants and the discovery of attractants in sawflies, bark beetles and several other insect groups in addition to Lepidoptera are encouraging advances.
Insecticides can, in some instances, also be used to prepare the way for biotic agents. An example of this type of integration is the suppression of pest-insect populations by conventional insecticides prior to the release of sterilized individuals. Because the sterile population should outnumber the fertile one, the direct kill of a considerable part by insecticides (or other means) would greatly reduce the number of sterile individuals needed (Knipling, 1960).
A more delicate manipulation is the reduction of vigour through insecticide application. Experiments carried out predominantly in the U.S.S.R. indicate a certain synergistic effect of low dosages of insecticides, mainly of DDT and BHC, with some pathogens. The idea is that minute amounts of toxic substances suffice to increase the susceptibility of such pests as white grubs and caterpillars so that infection by pathogenic bacteria or fungi is made easier. The problem is, of course, to make sure that the insecticide level does not increase enough to affect such highly susceptible beneficial insects as micro-hymenoptera. Unpublished experiences with very low concentrations of DDT (down to 0.0016 percent pure compound) showed damaging influences to Trichogramma. The compatibility of many microbial pathogen preparations, and even of nematodes (DD-136), with most insecticides facilitates this approach, which needs further study and confirmation (review in Telenga, 1964). If the host (pest) population is already latently infected by pathogens (as gypsy moth larvae very frequently are by nuclear polyhedrosis) then the application of very low dosages of DDT will suffice to provoke an epizootic. Kovacevic (1965) gives several examples and recommends study and further exploitation of the method.
Outlook for the future
Local conditions are important in any ecological problem. Therefore, enough energy and money should be put into research adapted to local problems and into education of adequate staff to continue on a large scale what has already been begun in integrated control.
Integrated control means more than the invention or imitation of technical tricks to reduce the amount of pesticides used in forests and to consider the conservation and augmentation of biotic regulating agents. For the future, integrated control should be understood as the incorporation of ecologically sound principles of forest protection into the planning and management of forests.
This report attempts to fulfil a twofold purpose: to bring together the main facts of new techniques and possibilities now available in biological and integrated control of forest pests; and to emphasize future needs in research, in the provision of facilities for research and control, in education and in organization.
RESEARCH
In the past the planning of biological control projects has been frequently too casual, because the ecological implications of transferring species into new habitats and biocoenoses were underestimated. Everyone will agree that thorough studies are needed for the preparation of any project of importing beneficial organisms, in the country of origin as well as in the problem area. When we set out to find climatic analogies between the areas, it should be made certain that the meteorological factors compared are relevant to the natural enemy species concerned.
An aspect of importation that formerly was virtually unknown is the importance of day length for the onset of diapause. Transfers of biotypes genetically adapted to certain photoperiods may fail in spite of perfect similarity of other meteorological factors if one tries to colonize them under different light conditions.
For the intelligent choice of new parasites or predators, special studies on the dynamics of populations will be warranted to determine the degree of density-dependence of the test species at various population levels. Such problems as the importance of competition or other types of interference among natural enemies need investigation - though, in my belief, no theory should be an obstacle to practical field tests.
In several well-planned biological control projects, there have been inadequate follow-up studies on dispersal and effectiveness of the introduced natural enemies. Failure to achieve immediate and complete success frequently caused the project to be terminated. There is an urgent need to reopen abandoned projects when a careful appraisal of the situation indicates formerly unknown or neglected opportunities.
Turnock and Muldrew (1970) have reviewed the utilization of native or alien parasites of forest insects in North America. They list many important research aspects and emphasize that, in evaluating success or failure of any type of biological control, no "all-or-none" attitude should prevail. Partial successes may be valuable contributions to general regulation. This potential will be part of integrated control programmes and simply by being available may be particularly valuable if conditions change. It may well be that an entomophagous species formerly without impressive impact is either more resistant or, by its behaviour, better protected against pesticides. Its survival after chemical control operations may be decisive for the persistence of the effect. The flexibility of the total complex of control factors seems to be of paramount importance particularly where, as in clean, extensively cultured forests, disturbance of this complex may frequently happen. In this connexion, indigenous biotic agents deserve more attention, and the possibility of increasing their effectiveness by conservation or cultural means should be explored.
FACILITIES
Without proper facilities no efficient research can be expected. "Facilities" in this sense includes books. Some of the books most urgently needed are reviews of regional experiences in biological control. Recent examples are the Technical Communications of the Commonwealth Institute of Biological Control, No. 1 and No. 2, dealing with Australia and Canada (F. Wilson, 1960; McLeod, 1962; McGugan and Coppel, 1962). Another type of information is provided by two journals, one devoted to insect pathology (Journal of Invertebrate Pathology, Academic Press Inc., since 1959) and the other, at an international level, treating all aspects of biological control against insects (Entomophaga, Paris, since 1956). From 1956 to 1969, a special bibliography on biological control appeared annually in \. Bibliographic services will be more important in the future as the amount of published information continues to grow. The need for quick orientation in foreign literature will always be the first step in the preparation of international biological control projects. Therefore, exchange of information is one of the most prominent points in the programme of the International Organization for Biological Control (IOBC).
With a few laudable exceptions, biological control laboratories all over the world are too few and are insufficiently staffed and equipped. Considering the tremendous range of research mentioned in this brief review, the scarcity of such laboratories is, no doubt, one reason why progress in biological control is slow. Results depend, as correctly stated by DeBach (1962), on the total amount of manpower and finances put into biological control efforts. Careful analysis of available data has demonstrated the economic superiority of bio-environmental methods of pest control as compared to the continuing application of pesticides (Pimentel et al., 1966; Simmonds, 1967).
EDUCATION
Governments can best be influenced by education of the public. In most countries public opinion and public support are the prerequisites for governmental assistance. Since products are usually not for sale in biological control (with the exception of microbial pathogen preparations), propaganda is not wanted. One of the first requirements is a broader education in ecology at the university and forestry faculty level. This would lead eventually to better and more continuous support of biological control projects at the governmental level.
For more advanced education, very few laboratories and institutions are capable of accepting biologists for training as biological control experts; most training centres do not have the finances to do this on their own. Such a training programme should be an important task for international organizations.
ORGANIZATION
In biological control projects in which frequently experts from several nations cooperate, the degree of efficiency largely depends on the type of organization available. There exist two international organizations: the Commonwealth Institute of Biological Control (CIBC) and the International Organization for Biological Control (IOBC). The latter has evolved from the European/ Mediterranean "Organisation Internationale de Lutte Biologique" (OILB) under the auspices of the International Union of Biological Sciences (IUBS). The aim of IOBC is to achieve cooperation and exchange of information on biological control in all parts of the world (DeBach, 1970). This will be facilitated by setting up regional sections of the Organization and admitting individual as well as institutional membership. IOBC is closely collaborating with CIBC, which has much experience in the provision of entomophagous and weed-killing organisms (cf. its annual reports). Both organizations will be able to assist in the initiation and performance of biological control projects in the forest, directly as well as indirectly. IOBC will use its contacts with FAO and WHO to develop further the ideas of bio-environmental control and thereby contribute to the quality of our environment.
ANGUS, T.A. 1968 World Rev. Pest Contr., 7 (1): 11-26.
ANONYMOUS. 1969 Int. Pest Contr., London, 11 (5): 13.
ASCHER,, K.R.S. 1969 Congr. int. antiparasitaires, Milan, octobre 1969, Sect.. 4. 18 p. (Mimeographed)
ATKINS, M. D. 1968 Canad. Entomologist, 100: 1115-1117.
AVRAMENKO, I.D. 1958 In Biol. Metod. Bor'by Vred. Sel'skochoz. Kul'turi Lesn. Nasaz. Kišinev, Izd. Minist. Sel'skogo Choz. Moldavskoj SSR, p. 3-4.
BALCH, R.E. 1960 Canad. Entomologist, 92: 297-310.
BARTLETT, B.R. 1962 Ann. Ent. Soc. Amer., 55: 448-455.
BARTLETT, B.R. 1964 In DeBach, P. Biological control of insect pests and weeds, p. 489-511. London, Chapman and Hall.
BEIRNE B.P. 1962 Ann. Rev. Ent., 7: 387-400.
BEIRNE, B.P. 1963a Mem. Ent. Soc. Canada, (32): 7-10.
BEIRNE, B.P. 1963b Res. Inst., Canada Dept. Agric., Belleville, Ontario, Inf. Bull. No. 1. 6 p.
BEIRNE, B.P. 1967 Pest management. London, Hill. 123 p.
BELTON, P. 1962 Proc. Ent. Soc. Manitoba, 18. 9 p.
BENJAMIN, 1965 D.M. Proc. 12th Int. Congr. Ent., London, 1964: 696-697.
BIRD, F.T. J. 1961 J. Insect Path., 3: 352-380.
BIRD, F.T. & ELGEE, D.E. 1957 Canad. Entomologist, 89: 371-378.
BIRD, F.T. & BURK, J.M. 1961 Canad. Entomologist, 93: 228-238.
BLAIS, J.R. 1963 Canad. For. Ent. Pathol. Branch, Bi-monthly Progr. Rept., 19 (1): 1.
BRUNS, H. 1960 Bird Study, 7: 193-208.
BUCKNER, C.H. 1966 Ann. Rev. Ent., 11: 449-470.
BURGERS, H.D. & HUSSEY, N.W. New York, Academic Press. (In print)
CAMERON MAcBAIN, J.W. 1963 Ann. Rev. Ent., 8: 265-286.
CASIDA, J.E., COPPEL, H.C. & WATANABE, T. J. 1963 econ. Ent., 56: 18-24.
CLAUSEN, C.P. U.S. 1956 Dept Agric., Techn. Bull. No. 1139.151 p.
COMMONWEALTH INSTITUTE OF BIOLOGICAL CONTROL. 1960-69 Annual reports. Farnham Royal, Bucks., Common wealth Agricultural Bureaux. (Mimeographed)
DAVID, W.A.L. & GARDINER, B.O.C. 1960 J. Insect Path., 2: 106-114
DEBACH, P. 1958 Proc. 10th Int. Congr. Ent., Montreal, 1956, 4: 759-768.
DEBACH, P. 1962 Proc. Hawaii. ent. Soc., 18: 69-79.
DEBACH, P. 1964 Biological control of insect pests and weeds. London, Chapman and Hall. 844 p.
DEBACH, P. 1970 Document, Meeting, International Union of Biological Sciences, Amsterdam, 1969. 4 p. (Mimeographed)
DODD, A.P. 1940 The biological campaign against prickly pear. Brisbane, Commonwealth Prickly Pear Board. 177 p.
DOUTT, R.L. 1960 Pan Pacific Ent., 36: 1-14.
DOWDEN, P.B. 1952 J. econ. Ent., 45: 481-483.
DOWNES, J.A. 1959 Canad. Entomologist, 91: 661-664.
DUNN, P.H. & MECHALAS, B.J. 1963 J. Insect Path., 5: 451-459.
DUTKY, S.R. 1959 Advances appl. Microbiol., 1: 175-200.
EICHHORN, O. 1969 Z. Ang. Ent., 63: 113-131.
EIDT, D.C. & LITTLE, C.H.A. 1968 Canad. Entomologist, 100: 1278-1279.
ELMORE, J.C. & HOWLAND, A.F. 1964 J. Insect Path., 6: 430-438.
EMBREE, D.G. 1965 The population dynamics of the winter moth in Nova Scotia, 1954-1962. Mem. Ent. Soc. Canada, (46). 57 p.
ESCHERICH, K. 1931 Die Forstinsekten Mitteleuropas. III. Band. Berlin, Parey. 825 p.
FAO. 1966 Proceedings of the FAO Symposium on Integrated Pest Control, Rome, 1965. Rome, FAO. 3 v.
FETTES, J.J. 1963 Proc. Ann. Mtg West. For. Pest Comm., West. For. Conserv. Ass. Seattle,
Wash., 1962.
FRANCKE-GROSMANN, H. 1963 Ann. Rev. Ent., 8: 415-438.
FRANZ, J. 1948 Forstwiss. Zentralbl., 67: 38-48.
FRANZ, J. & WELLENSTEIN, G. 1958 Z. PflKrank., 65: 20-32.
FRANZ, J.M. Proc. 13th Int. Congr.. Ent., Moscow, 1968.
FRANZ, J.M. 1958 Entomophaga, 3: 109-196.
FRANZ, J.M. 1961a Ann. Rev. Ent., 6: 183-200.
FRANZ, J.M. 1961b Biologische Schädlingsbekämpfung. In Sorauer, P.,ed. Handbuch der Pflanzenkrankheiten. 6. Band. 3. Lief., 2. Aufl., p. 1-302. Berlin, Parey.
FRANZ, J.M. 1964 In Starr, M.P., ed. Global impacts of applied microbiology, p. 256-266. Stockholm, Almqvist &Wiksell; New York, Wiley.
FRANZ, J.M. 1966a Meded. Rijksfacult. Landbouwwetensch., Gent, 31: 512-525.
FRANZ, J.M., 1966b Proc. FAO Symp. Integr. Pest. Contr., Rome, 1965, 3: 65-76.
FRANZ, J.M. 1968 Anz. Schädlingsk., 41: 65-71.
FRANZ, J.M. 1969 Proc. 6th World For. Congr., Madrid, 1966, 2: 1915-1923.
FRANZ, J.M. & KRIEG, A. 1967 Gesunde Pfl., 19: 175-176, 178-180. 182.
FRANZ, J.M., KRIEG, A. & REISCH, J. 1967 NachrBl., Dtsch. PflSchDienst., Braunschweig, 19: 36-44.
GÄBLER, H. 1950 Z. Angew. Ent., 31: 441-454.
GÄBLER, H. 1951 Anz. Schädlingsk., 24: 35-36.
GASTON, L.K., SHOREY, H.H. & SAARIO, C.A. 1967 Nature, London, 213: 1155.
GIBB, J.A. 1960 Ibis, 102: 163-208.
GÖSSWALD, K. 1958 Proc. 10th Int. Congr. Ent., Montreal, 1956, 4: 567-571.
GRAHAM, S.A. 1951 Sci. Monthly, 72: 235-244.
HANSBROUGH, J.R. 1965 J. Washington Acad. Sci., 55: 41-44.
HANSON, H.S. 1939 Bull. ent. Res., 30: 27-65.
HASSELL, M.P. 1966 J. Anim. Ecol., 35: 65-75.
HEIMPEL, A.M. 1967 Ann. Rev. Ent., 12: 287-322.
HEIMPEL, A.M. & ANGUS, T.A. 1960 Bact. Rev., 24: 266-288.
HOCHMUT, R. 1963 Lesnicky Casopis, Praha, 9: 197-214.
HOLLOWAY, J.K. 1964 In DeBach, P. Biological control of insect pests and weeds, p. 650-670. London, Chapman and Hall.
HORBER, E. 1963 In International Atomic Energy Agency. Radiation and radioisotopes applied to insects of agricultural importance, p. 313-322. Vienna.
HORBER, E. 1969 In International Atomic Energy Agency. Sterile-male technique for eradication or control of harmful insects. Proceedings of a Panel organized by the Joint FAO/IAEA Division of Atomic Energy in Food and Agriculture, Vienna, 1968, p. 73-85. Vienna. STI/PUB/224.
HUFFAKER, C.B. 1959 Ann. Rev. Ent., 4: 251-276.
HUFFAKER, C.B. & KENNETT, C.E. 1969 Canad. Entomologist, 101: 425-447.
HUGER, A.M. 1966 J. Invertebrate Path., 8: 38-51.
HULME, M.A. & SHIELDS, J.K. 1970 Nature, London, 227: 300-301.
IGNOFFO, C.M. 1968 In Insect viruses, oaf. by K. Maramorosch, p. 129-167. Berlin, Springer.
JACOBSON, M. 1965 Insect sex attractants. Now York, Inter-science, Wiley. 154p.
JACOBSON, M. 1966 Ann. Rev. Ent., 11: 403-422.
JANISCH, E. Z 1959. Angew. Ent., 43: 371-386.
JAQUES, R.P. 1963 Canad. J. Plant Sci., 43: 301-306.
JAQUES, R.P 1967. Ibid, Part I, II. Canad. Entomologist, 99: 785-794, 820-829.
KLASSEN, W., KNIPLING, E.F. & MCGUIRE, J.U., 1970 Jr. Ann. Ent. Soc. Amer., 63: 238-255.
KNIGHT, F.B. 1962 Bull. Ent. Soc. Amer., 8: 196-199.
KNIPLING, E.F. 1960 J. econ. Ent., 53: 415-420.
KNIPLING, E.F. 1969 In International Atomic Energy Agency. Sterile-male technique for eradication or control of harmful insects. Proceedings of a Panel organized by the FAO/IAEA Division of Atomic Energy in Food and Agriculture, Vienna, 1968, p. 19-32. Vienna, STI/PUB/224.
KOEHLER, W. 1959 Trans. 1st Int. Conf. Ins. Path. Biol. Contr., Prague, 1958: 405-413.
KOVACEVIC, Z. 1965 Anz. Schädlingsk., 38: 51-53.
KRIEG, A. 1961 Grundlagen der Insektenpathologie. Viren-,Riekettsien- und Bakterien Infektionen. Darmstadt, Steinkopff. 304 p.
KRIEG, A. 1967 Mitt. Biol. Bundesanst., Berlin-Dahlem (125). 106 p.
LAAN, P.A. VAN DER, 1967 ed. Insect pathology and microbial control. Proc. Int. Colloq., Wageningen, 1966. Amsterdam,, North-Holland. 360 p.
LACHANCE, L.E. & KNIPLING, E.F. 1962 Ann. ent. Soc. Amer., 55: 515-520.
LEBEN, B.C. 1965 Ann. Rev. Phytopath., 3: 209-230.
LYON, R.L. 1965 Bull. ent. Soc. Amer., 11: 76.
LYNN, M. 1967 For. Abstr., 28 (1): 1-18.
MAGNOLER, A. J. 1968 Invertebrate Path., 11: 326-328.
MARAMOROSCH; K., 1968 ed. Insect viruses. Berlin,, Springer. 192 p.
MARTIGNONI, M.E. & SCHMID, P. 1961 J. Insect Path., 3: 62-74.
MARTIGNONI, M.E. & MILSTEAD, J.E. 1962 J. Insect Path., 4: 113-121.
MARTINEK, V. 1966 Anz. Schädlingsk., 39: 103- 106.
MARTINEK, V. & KUDLER, J. 1964 Práce Výzk. Úst. Lesn. CSSR, 29: 229-260.
MARTOURET, D.-J. 1969 Rev. Zool. Agr. Path. vég., 68: 5-12.
MCBRIDE, R.P. 1969 Canad. J. Bot., 47: 711-715.
MCEWEN, F.L., GLASS, E.H., DAVIS, A.C. & SPLITT-STOESSER, C.M. 1960 J. Insect Path., 2: 152-164.
MCGUGAN, B.M. & COPPEL, H.C. 1962 In A review of the biological control attempts against insects and weeds in Canada. Part II. Commonw. Inst. Biol. Contr., Techn. Commun. No. 2: 35 216.
MCINTYRE, T. & DUTKY, S.R. 1961 J. econ. Ent., 54: 809-810.
MCLEOD, J.H. 1962 In A review of the biological control attempts against insects and weeds in Canada. Part I. Commonw. Inst. Biol. Contr., Techn. Commun. NO. 2: 1-33.
MILLER, W.E. & NEISWANDER, R.B. 1955 Ohio agric. Exp. Sta. Res. Bull., 760: 1-31.
MORDUE, W. 1969 Proc. 5th Brit. Insecticides Fungicides Conf., Brighton, 1969, 2: 386-392.
MORRIS, O.N. 1969 J. Invertebrate Path., 13: 285-295.
MORRIS, R.F., 1963 ed. The dynamics of epidemic spruce budworm populations. Mem. Ent. Soc.. Canada, (31). 332 P.
MULDREW, J.A. 1953 Canad. J. Zool., 31: 313-332.
MULDREW, J.A. 1964 Canad. Forest Ent. Path. Branch, Bi- monthly Progr. Rept, 20 (2): 2-3.
MÜLLER-KÖGLER, E. 1965 Pilzkrankheiten bei Inekten. Anwendung zur biologischen Schädlingsbekämpfung und Grundlagen der Insektenmykologie. Berlin, Parey. 444 P.
NASH, R.F. & Fox, R.C. 1969 J. econ. Ent., 62: 660-663.
NELSON, S.O. & SEUBERT, J.L. 1966 In Symp. Scient.. Aspects Pest Control, Washington,
1966. Natl. Res. Council, Publ. 1402: 135-166.
NIKLAS, O.F. 1967 Mitt. Biol. Bundesanst., Berlin-Dahlem, (124). 40 P.
NIKLAS, O.F. 1969 NachrBl. Dtsch. PflSchDienst., Braun- schweig, 21: 71-78.
NORTH, D.T. & HOLT, G. 1968 J. econ. Ent., 61: 928-931.
NUORTEVA, M.K. 1959 Verh. 4. Int. PflSchutz-Kongr., Hamburg, 1957, 1: 1025-1027.
ORLOVSKAJA, E.V. 1962 Zesz. Problemowe Postepów Nauk Rolniczych (Warszawa), 35: 219
222.
OSSOWSKI, L.LJ. 1960 Ann. appl. Biol., 48: 299-313.
PAINTER, R.R. 1967 In Kilgore, W.W. & Doutt, R.L., eds. Pest control: biological, physical and
selected chemical methods, p. 267-285. New York, Academic Press.
PAVAN, M. 1961 Collana verde, Minist. Agric. For., Italy (7): 148-157.
PERSING, C.O. 1965 Bull. Ent. Soc. Amer., 11: 72-74.
PERSON, H.L. 1940 J. For., 38: 390-396.
PIMENTEL, D. 1961 Ann. Ent. Soc. Amer., 54: 76-86.
PIMENTEL, D. 1963 Canad. Entomologist, 95: 785-792.
PIMENTEL, D. CHANT, D., KELMAN, A., METCALF, R.L., NEWSOM, L.D. & SMITH, C. 1966 In Restoring the quality of our environment: a report of the Environmental Pollution Panel, President's Science Advisory Committee, Appendix Y 11, p. 227-291. Washington, D.C., The White House.
POINAR, G.O., 1967 Jr. Proc. Helminth. Soc. Washington, 34: 199-209.
PSCHORN-WALCHER, H. & ZWÖLFER, H. 1968 Anz. Schädlingsk., 41: 71-76.
RANDALL, A.P. 1963 Canad. Forest. Ent. Path Branch, Bi- monthly Progr. Rept, 19 (4): 1.
ROMANYK, N. 1958 Bol Serv. Plagas Forestales, Madrid, 1 (1): 2732
RUDNEV, D.E. & ZAGAIKEVIC, I.K. 1952. Lesn. Ghoz., 5 (3): 56.
RUDNEV, D.F. & TELENGA, N.A. 1958 Lesn. Choz., 11 (11): 37-40.
RÜHM, W. 1964 Anz. Schädlingsk., 37: 33-38.
RYVKIN, B.V. 1955 Lesn. Choz., 8 (6): 58-60.
SAMSINAKOVA, A. 1964 Naturwissenschaften, 51: 121-122.
SCEPETILNIKOVA, V.A. 1963 Beitr. Ent., Berlin, 13: 855-872.
SCHIFFERLI, A. 1966 Ornithol. Beob., Bern, 63: 25-40.
SCHMIEGE, D.C. 1963 J. econ. Ent., 56: 427-431.
SCHÖNBORN, A. VON. 1966 In Breeding insect-resistant forest trees. London, Pergamon Press. p. 25-27.
SCHÖNHERR, J. 1969 Entomophaga, 14: 250-260.
SCHWERDTFEGER, F. 1966 Z. Angew. Ent., 58: 252-265.
SIDOR, C. 1965 Šumarski List, Zagreb (9-10): 381-390.
SIMMONDS, F.J. 1960 In Rep. 7th. Commonw. Ent. Conf., 6-15 July, 1960, p. 20-27. London, Commonwealth Institute of Entomology.
SIMMONDS, F.J. 1963 Canad. Entomologist, 95: 561-567.
SIMMONDS, F.J. 1967 J. Roy. Soc. Arts, 115: 880-895.
SIMMONDS, F.J. 1969 Commonwealth Institute of Biological Control.. Brief resume of activities and recent successes achieved. Farnham Royal, Bucks., Commonwealth Agricultural Bureaux.
SMIRNOFF, W.A., 1961 J. Insect Path., 3: 29-46.
SMIRNOFF, W.A. 1962 J. Insect Path., 4: 192-200.
SMIRNOFF,, W.A. 1963 J. Insect Path., 5: 104-110.
SMIRNOFF, W.A. 1968 J. Invertebrate Path., 11: 321-325.
SMIRNOFF, W.A. & JUNEAU, A. 1963 Canad. Forest. Ent. Path. Branch, Bi-monthly Progr. Rept,
19 (4): 1-2.
SMITH, C.N., LABRECQUE, G.C. & BOBRKOVEC, A.B. 1964 Ann. Rev. Ent., 9: 269-284.
SMITH, R.F. 1969 Agric. Sci. Rev., 7 (1): 1-5.
STARK, R.W. Integrated control, pest management or protective population management? Proc.
3rd Ann. Northeastern Forest Ins. Work Conf., New Haven, Conn., 1970. (In print)
STARK, R.W. 1965 Ann. Rev. Ent., 10: 303-324.
STEIN, W. & FRANZ, J. 1960 Naturwissenschaften, 47: 262-263.
STEINHAUS, E.A., 1963 ed. Insect pathology: an advanced treatise. New York, Academic Press. 2v.
SWEETMAN, H.L. 1958 The principles of biological control. Dubuque, Iowa, Brown. 560 p.
TALALAEV, V. 1958 Ent. Obozr., 37: 641-652.
TANADA, Y. In Kilgore, W.W. and Doutt, R.L., 1967 eds. Pest control: biological, physical, and
selected chemical methods, p. 31-88. New York, Academic Press.
TAYLOR, K.L. C.S.I.R.O., 1967 Div. Ent., Melbourne, Techn. Paper, (8). 19 p.
TAYLOR, T.H.C. 1937 The biological control of an insect in Fiji: an account of the coconut leafmining beetle and its parasite complex. London, Imperial Institute of Entomology. 239 p.
TELENGA, N.A. 1964 Colloq. int. path. insect., lutte microbiol., Paris, 1962. Entomophaga, Mém. Hors Sér. No. 2: 531-544.
THOMAS, H.A. 1969 U.S. Forest Service Res. Note SE-118. Asheville, North Carolina, Southeastern Forest Exp. Sta.
THOMPSON, C.G. 1959 Trans. 1st Int. Conf. Ins. Path. Biol. Contr., Prague, 1958, p. 201-204.
TURNBULL, A.L. & CHANT, D.A. 1961 Canad. J. Zool., 39: 697-753.
TURNOCK, W.J. & MULDREW, J.A. The use of parasites in biological and integrated control of forest insects. Proc. 3rd Ann. Northeastern Forest Ins. Work Conf., New Haven, Conn., 1970. (In print)
URBAN, S. 1964 In Über den Einfluss von Umwelbedingungen auf die Wirkung von chemischen Pflanzenschutzmitteln. Symp. 1962. Dtseh. Akad. Landwirt-schaftswiss., Berlin, Nr 62: 197-204.
VEBER, J. 1964 Colloq. int. path. ins., lutte microbiol., Paris, 1962. Entomophaga, Mém. Hors Sér. No. 2: 403-405.
VITÉ, J.P. & PITMAN, G.B. 1969 Canad.. Entomologist, 101: 113-117.
VOÛTE, A.D. 1964 Int. Rev. for. Res., 1: 325-383.
WATERS, W.E. 1963 Proc. Soc. Amer. Foresters, Atlanta, Georgia, 1962, p. 36-40.
WATERS, W.E., 1969 ed. Forest insect population dynamics. Proceedings of the Forest Insect Population Dynamics Workshop, West Haven, Conn., 2.3-27 January 1967. U.S. Forest Serv. Res. Papers NE-125. 126 p.
WEBSTER, J.M. & BRONSKILL, J.F. 1968 J. econ. Ent., 61: 1370-1373.
WEISER, J. 1956 Z. PflKrankh., 63: 625-638.
WELLENSTEIN, G. 1942 Monogr. angew. Ent., (15): 478-534.
WELLENSTEIN, G. 1954 Verh. Dtsch. Ges. angew. Ent., 12, Frankfurt a.M., 1952: 148-156.
WELLENSTEIN, G. 1960 Z. angew. Ent., 47: 32-41.
WELLINGTON, W.G. 1969 Canad. Entomologist, 101: 1163-1172.
WICKER, E.F. & SHAW, C.G. 1968 Mycologia, 60: 372-383.
WICKER, E.F. & Woo, J.Y. 1969 Phytopathology, 59: 16.(Abstract)
WILLIAMS, C.M. 1969 Accad. Naz. Lincei, 356, Quaderno N. 128: 79-87.
WILSON, C.L. 1969 Ann. Rev. Phytopath., 7: 411-434.
WILSON, F. 1960 A review of the biological control of insects and weeds in Australia and Australian New Guinea. Commonwealth Institute of Biological Control, Ottawa, Canada. Farnham Royal, Bucks., Commonwealth Agricultural Bureaux. 102 p.
WILSON, F. 1964 Ann. Rev. Ent., 9: 225-244.
WILSON, F. 1965 In Baker, H.G. and Stebbins, G.L., eds. The genetics of colonizing species, p. 307-329. New York, Academic Press.
ZWÖLFER, H. 1967 Proc. Papers IUCN 10th Techn. Mtg, Lucerne, 1966. IUCN Publ. N.S. (9): 141-150.
ZWÖLFER, H. 1969 Proc. 9th Brit. Weed Contr. Conf., 1968, p. 1147- 1156.
ZWÖLFER, H. & PSCHORN-WALCHER, H. 1968 Anz. Schädlingsk., 41: 51-55.
