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Biological control of forest insects

European Station, Commonwealth Institute of Biological Control, Delémont, Switzerland

Recent Work and Future Aspects

AT the present time there is a considerable risk that a short general paper on biological control with particular reference to forest insects may merely be a repetition of previous work, since the concept of, and the approach to, this field of insect control have already been discussed twice during the past year by the very competent forest biologists Dr. M. L. Prebble and Dr. R. E. Balch of the Canadian Department of Forestry (1, 2).1 And it is only a year since two other authoritative publications, by Dr. F. J. Simmonds, Director of the Commonwealth Institute of Biological Control (3), and by Prof. G. C. Varley, of Oxford University (4), have outlined the past work and future trends of biological control in general, and of the control of agricultural pests in particular. Moreover, the present author has firsthand experience of only some of the problems concerning the biological control of European forest insects introduced into North America, problems which have already been dealt with in the two Canadian publications mentioned above.

1 The italicized figures in parentheses refer to literature cited at the end of the article.

There is, however, a certain difference in approach in that these two publications attempt primarily to show what is happening in Canada with regard to the natural enemies imported from Europe during the course of biological control work. This paper, on the other hand, deals primarily with the aspect of the investigations involved in the selection and provision of useful biological control agents for importation to other areas. Such studies involve a careful ecological investigation of the pest in question in its native habitat, in order to gain some idea of the biotic factors operating against the pest in its normal environment. Those biotic agents which would appear to be most suitable for its control in the area of introduction are then selected, collected and bred for shipment. The scope of the present paper will therefore be mainly restricted to the problem of the improvement of the present methods of introduction of biological control agents.

On the subject of possible improvements in biological control a difference in present opinions is immediately apparent. Dr. Balch (2) states, for example, "that ecological studies in the countries of origin [of the pest] are necessary but unlikely to be conclusive." He emphasizes that in the biological fight against an introduced forest insect, "all available primary parasites, predators, and diseases should be imported, but priority should he given to those found in ecologically similar regions where the pest is indigenous and under adequate natural control. "

If one takes into account the close similarity existing between the forest types of boreal Eurasia and North America, the latter part of Dr. Balch's contention must undoubtedly be strongly supported. Some general methods of assessing the ecological correspondence between areas of collection of biological control agents and those where they are to be released have already been outlined elsewhere by the author (5).

However, it is doubtful whether this represents the only facet in which preliminary investigations may assist in biological control problems relating to the species and numbers of natural enemies selected, the selection of areas in which they are collected, the order of their introduction, etc. On the contrary, it is thought that it is this marked ecological similarity between the main forest types of the Old and the New World - a similarity due to the long period of common evolutionary history prior to the late Tertiary Period - which may provide the possibility, by means of detailed comparative studies, of a closer approach to more selective methods of biological control, rather than the general procedure of "trial and error" which has prevailed in the past. As Varley (4) has pointed out, this problem is all the more important since biological control measures once successfully achieved are often irrevocable.

One of the problems in which this question has now become acute is that of the European pine sawfly, Neodiprion sertifer, in North America. This Eurasian forest insect was accidentally established in eastern North America some 40 years ago. There it damages not only Scots pine plantations utilized commercially for Christmas trees but also attacks several native American pines (6). A number of parasite species have been successfully introduced from Europe against this pest - partly in connection with another biological control program concerning the related European spruce sawfly (Gilpinia hercyniae) (2). These parasites have been released in southern Ontario and in the adjacent United States. However, only one of these parasite species became abundant while three others, though established, remained quite unimportant, and seven species apparently failed to become established. On the other hand, a surprisingly large number of native American parasites adapted themselves spontaneously to the new host, exerting an average parasitism of some 20 percent (6).

It seems reasonable to suppose that some of these results could have been predicted had a comparative analysis of the pest and its complex of natural enemies, both in Europe and in North America, preceded the rather disappointing introductions of parasites from Europe which entailed the collection and shipment of nearly 30 million pine sawfly cocoons. Neodiprion sertifer is the only member of this genus in Eurasia. All other species of Neodiprion, some 25 to 30, have a purely Nearctic, i.e., American, distribution. This suggests the possibility that N. sertifer itself might not be of truly Palaearctic origin, but rather an old Nearctic immigrant which reached eastern Asia before or during the Ice Age and afterwards spread as far as western Europe compare also Ross (7). This species has only a single generation annually and overwinters in the egg stage as opposed to the other pine sawflies of Europe which are generally two-brooded (i.e., bivoltine) and pass the winter in the cocoon stage. As a result, European parasites apparently had certain difficulties in adapting themselves to the life cycle of such an "outsider" host. Indeed, only three species of Ichneumonids (Lophyroplectus luteator, Exenterus abruptorius and Lamachus eques) have succeeded in synchronizing their life histories with that of N. sertifer in Europe, and even some individuals of these parasites still show signs of being possible remnants of a former two-brooded strain. Thus, any importation of two-brooded European egg or larval parasites into Canada - with the exception of the three specific ones, just mentioned - is eventually doomed to fail because of lack of synchronization with the host. This failure will continue unless a two-brooded species of American pine sawfly occurs in the same habitat as N. sertifer, in which case the second parasite generation would be able to persist on this other host.

It seems possible that more promising parasites might be obtainable in North America itself. Assuming that N. sertifer is in fact a foreign element in Eurasia, although it has been there for a considerable time, its recent unintentional introduction into North America would appear to be of the nature of a return to its original area of development. If this is so, then the large number of native parasite species which have attacked the European pine sawfly in North America is less surprising. This complex of American parasites, however, is still rather unbalanced in that it consists mainly of cocoon parasites, with only one important larval parasite (Exenterus canadensis) and no egg parasites, which are often very effective in Europe against bivoltine species of Diprionidae. However, since about half the American species of pine sawflies are, like N. sertifer, univoltine with the same method of overwintering, the possibilities of completing this present unbalanced parasite complex of the pest in question might probably be more easily realized by an extensive search for additional parasites of the similar ecologically related species in the western States.

An example similar to this is afforded by the spruce budworm, Choristoneura fumiferana, the most important forest insect pest of Canada. Repeated attempts have been made to increase the efficiency of its natural enemies in Canada by the introduction of additional parasites of the closely related fir budworm (Ch. murinana) from central Europe. However, as far as can be judged at present, no success has been achieved. Now, Dr. H. Zwölfer of the European Station of the Commonwealth Institute of Biological Control has made a careful comparative analysis (as yet unpublished) of the parasite complexes of both these budworm. species and has shown that the complex and sequence of parasites is more complete in the spruce budworm than in the fir budworm and that this is apparently due to the historical development of these species in their respective areas. Some groups of parasites, e.g., egg parasites and Tachinids, are virtually absent from the European form, while they sometimes play a significant role in their attack on the American form. These findings give rise to the conclusion that the introduction of European parasites into the rather "saturated" complex of spruce budworm. parasites might be more difficult than vice versa. As Dr. Balch points out, such an assertion can never be absolutely conclusive, but it would appear that by means of this type of comparative study we might reach a reasonable degree of accuracy in predicting the consequences of our efforts at parasite introductions.

There is another group of forest insects, the holarctic, circumpolar ones, which, if studied comparatively on either side of the Atlantic, might provide some basis for a deeper understanding of the evolutionary aspects of host-parasite associations. This field of comparative studies of either circumpolar or closely related forest insects is as yet virtually unexplored. It would seem that, in addition to the "lack of knowledge of population dynamics" mentioned by Dr. Balch (2), it is this lack of knowledge concerning the principles of formation and composition of parasite and predator complexes which accounts for "the present little scientific basis for the selection of species of natural enemies for introduction against foreign pests."

An instructive example is afforded by the larch sawfly, Pristiphora erichsonii. It is claimed by some authors that the larch sawfly was introduced from Europe into North America some 100 years ago; others, however, are of the opinion that the species is indigenous to both areas and is thus of holarctic origin. A sawfly with the ecological habits of P. erichsonii has usually a rich parasite complex consisting of one or more species of ectoparasites of the Tryphoninae group, several endoparasites of the Mesoleiinae group, and one or more species of parasitic Diptera (Tachinids), all attacking the larval stages. In addition it is usually also attacked by a number of cocoon parasites acting in the soil, and its eggs may also be parasitized. Such a typical complex has been found during studies on Pridiphora carried out in central Europe, where to date one ectoparasite, several endoparasites, including three Tachinids, and a number of cocoon parasites have been reared. In Canada, however, the parasite complex attacking this sawfly is much poorer in species and those which have been recorded regularly are either introduced species such as Mesoleius tenthredinis, or are of doubtful geographic status. This is obviously contrary to any theory of a circumpolar origin for P. erichsonii, and our present efforts to complete this poor Canadian parasite complex by further parasite introductions from Europe would therefore seem to be fully justified.

FIGURE 1. - Curled May shoot of larch due. to shoot oviposition by the larch sawfly (Pristiphora erichsonii). Note the notch-like frass of the first instar larvae on the first needle fascicle.

FIGURE 2. - Full-grown larva of the larch sawfly bearing an egg of ectoparasitic Ichneumonid, Polyblastus tener.

In addition to what has been said above concerning the value of comparative ecological-parasitological studies, mention should be made of another comparatively unexplored field of basic research in biological control, namely the study of insect species which are apparently already under effective natural control in at least part of their distribution area. Since it was first started, biological control has naturally been largely concerned with insect species which cause economic damage to different crops (including forest trees). Many of the species concerned are also pests in their country of origin as well as in the areas of introduction. Among forest insects introduced into the New World well-known examples are the gypsy moth (Lymantria dispar) and the brown-tail moth (Euproctis chrysorrhoea), both of which are also from time to time serious pests in European oak forests. The same applies to the winter moth (Oporophtera brumata), which has recently become established in Nova Scotia, and to the pine shoot moth (Rhyacionia buoliana), which is a serious threat to American as well as to European pine plantations.

In the case of the pine shoot moth there are in addition some half a dozen other species of the same genus endemic in Europe, and these also feed on the buds of pine trees, but they usually remain at a low population level and appear to be under adequate natural control. It would seem that a study comparing these other minor Rhyacionia, species with the epidemic species, R. buoliana, might provide some insight into the reasons why the former are easily kept under natural control, whereas the latter often develops to peat proportions. A more thorough understanding of the possibilities of improving the control-system. of R. buoliana by means of biological measures might perhaps result from such a study.

Another model of this kind is to be found in the case of the two larch sawflies, Pristiphora erichsonii and Anoplonyx destructor. Both are in fact rare species in the natural larch forests of the Alps. However, outside the natural range of Larix europae, both species from time to time cause serious damage to man-made larch plantations (e.g., in England), and P. erichsonii even does so in the tamarack (Larix laricina) stands of Canada, as has already been mentioned. There is therefore an apparently adequate control system in the Alps, but this system becomes unstable and ineffective in areas where the sawflies and their host plant are not indigenous. Comparative population studies on P. erichsonii in the Alpine larch forests on the one hand, and in Bavarian plantations on the other hand, have already shown one important mortality factor which acts differently in the two regions, namely the forest ants (Formica spp.). Experiments in the Alps on protected and unprotected larch branches infested with P. erichsonii showed on average a larval mortality of 70 to 80 percent on the unprotected branches, but of only 30 to 40 percent on the twigs protected by "tangle-foot" against these forest ants. In Bavaria similar experiments showed no significant differences in the larval mortality of the sawfly, where it was around 40 percent on both types of branches; forest ants are here completely absent. This absence of forest ants in the Bavarian larch plantations investigated thus seems to be at least one of the possible reasons for the greater abundance of larch sawflies in these nonindigenous larch forests. Unfortunately the importation of forest ants into the Canadian tamarack stands appears to be very unpromising since the swampy character of these forests would probably present an insurmountable obstacle to their permanent establishment in useful numbers.

A similar example is afforded by the green tortrix, T. viridana, a common oak pest in central Europe and elsewhere. It has recently been studied in the Vosges mountains of France, where its incidence is rather low (8), and also in northern Germany (9), where it frequently reaches outbreak proportions. Zwölfer (personal information) attributes this difference to the lower incidence of larval parasites in northern Germany compared with that in the Vosges mountains. The latter area is characterized by very mixed forest stands and this is apparently a more suitable habitat for a greater variety of natural enemies than the more uniform oak stands of the German plain.

Since they deal mainly with pest species, biological control and economic entomology in general are handicapped by the very fact that their subjects of research are populations showing only varying degrees of imperfect natural control. However, pest species form only a small group of the general fauna. For example, amongst the 14 species of sawflies feeding on Norway spruce (Picea abies) in Europe, only one is considered to be a common forest pest and this only in nonindigenous spruce forests, three others have occasionally been recorded as of minor importance, but the majority are quite indifferent, as Thalenhorst has shown (10). It would appear that the current theories of natural control and particularly the models of Nicholson (11) and of Lack (12) are not very conclusive when applied to scarce species such as these. There is never any shortage of food for them, nor do the factors keeping them under more or less stable natural control seem to be necessarily density-dependent in their action. Yet such species comprise the great majority of insects (and other animals too), and this would seem to indicate very forcibly that a study of these indifferent species is essential for a better understanding and improved use of the methods of biological control, an opinion which had already been emphasized by the late Prof. Eidmann in 1949 (13).

It has been postulated by Thienemann (14) in his second biocoenotic principle that in a plant and/or animal community very rich in species, the incidence of the different species is usually low, whereas in a community poor in species, the single members often occur in large numbers, such a community or biocoenose being less stable than the former. This principle might also be justifiably applied to forest communities in general and to their insect fauna in particular. Tropical rain forests, for instance, enormously rich in plant and animal species, are very stabilized eco-systems, and insect outbreaks in virgin stands are virtually unknown. Exactly the opposite is seen in forest types such as those of pine or spruce which are usually very uniform in growth and of poor floristic and faunistic composition. Here insect outbreaks occur even in natural forests as, for example, those of the spruce budworm in Canada or the larch bud moth (Zeiraphera griseana) in the Alps, and such outbreaks are plentiful in manmade stands of trees such as the spruce monocultures of central Europe (15, 16).

With regard to European conditions, there is a widely held opinion that the comparatively poor composition both of the European forests and their fauna is due to much more pronounced effects which the glacial periods exerted on plant and animal life in Europe as compared with eastern Asia or North America. Among the parasitic Tachinids, for example, many European species also occur in the Far East, but there are also very many additional related species which are common there, but lacking in Europe. If it is true that the absence of so many species in Europe is due mainly to these historical reasons, it is feasible that a vast field of biological control possibilities could be developed in attempting to enrich and stabilize the impoverished European biocoenoses by means of natural enemies imported from eastern Asia. There is a great number of important European forest pests which are also indigenous in the Far East, e.g., Lymantria dispar, Malacosoma neustria, Dicranura vinula, Stilpnotia salicis, Operophtera brumata, many species of beetles, the pine sawfly Neodiprion sertifer, etc. It would certainly be worth-while to initiate comparative studies of their parasite complexes, and later possibly to consider introductions to fill such empty niches in Europe as might be found to exist.

The rich fauna of natural enemies which is expected to occur in eastern Asia is also, of course, of equal interest for the biological control of forest and other insects in Canada and the United States. For this reason the Commonwealth Institute of Biological Control is planning to establish a field station in northern Japan in 1961. This will allow further progress toward a more comprehensive understanding of the ecology of forest insects of the Northern Hemisphere and of the physical and biotic factors governing them over different parts of their range. It will also improve the position with regard to future selective introductions of natural enemies into North America. Doubtless, Simmonds (3) is right when he says that far from the cream having been skimmed off the general field of biological control work, we have hardly made a beginning.


1. PREBBLE, M. L. 1960. Biological control in forest entomology. Bull. Entom. Soc. America, 6:6-8.

2. BALCH, R. E. 1960. The approach to biological control in forest entomology. Canad. Entom., 92:297-310.

3. SIMMONDS, F. J. 1959. Biological control, past, present and future. Journ. Econ. Entom., 52:1099-1102.

4. VARLEY, G. C. 1959. The biological control of agricultural pests. Journ. Roy. Soc. Arm, 107:475-490.

5. PSCHORN-WALCHER, H. 1958. Climatic and biocoenotic aspects for the collection of predators of Adelges piceae Ratz. (Hemiptera: Adelgidae) in Europe. Proceed. 10th Int. Congr. Entom., 1956, Montreal, 4:801-805.

6. GRIFFITHS, K. J. 1959. Observations on the European pine sawfly, Neodiprion sertifer (Geoffr.) and its parasites in southern Ontario. Canad. Entom., 91:501-512.

7. Ross, H. H. 1955. The taxonomy and evolution of the sawfly genus Neodiprion. Forest Sc., 1:196:209.

8. ZWÖLFER, H. & KRAUS, M. 1957. Biocoenotic studies on the parasites of two fir and two oak Tortricids. Entomophaga, 2:173-196.

9. SCHÜTTE, F. 1957. Untersuchungen über die Populationsdynamik des Eichenwicklers (Tartix viridana L.). Zeitschr. f. angew. Entom., 40:1-36, 285-331.

10. THALENHORST, W. 1957. Vergleichende Untersuchungen über den Massenwechsel. der Fichten-Nematinen. Verh. d. Deutsch. Ges. f. angew. Entom., 14:95-109.

11. NICHOLSON, A. J. 1933. The balance of animal populations. Journ. Anim. Ecol., 2:132-178.

12. LACK, D. 1954. The natural regulation of animal numbers. Oxford, Clarendon Press. 343 p.

13. EIDMANN, H. 1949. Das Problem der Indifferenz: ein Beitrag zur Oekologie der Insekten. Die Naturwissenschaften, 36:268-273.

14. THIENEMANN, A. 1920. Die Grundlagen der Biozönotik und Monards faunistische Prinzipien. Festschrift f. Zschokke, No. 4, Basel.

15. SCHWERDTFEGER, F. 1954. Forstinsekten im Ur - und Nutzwald. Algem. Forstzeitechrift, 9:277-282.

16. FRANZ, J. 1948. Über die Zonenbildung der Insektenkalamitäten in Urwäldern. Forstwiss. Centralblatt, 67:38-48.

International wood research society

The need for an International Wood Research Society has been recognized for many years. Plans to organize such a society originated with delegates to the Conference on Wood Technology. The recommendations of the Conference materialized in the founding of the Society on 27 May 1960, at a special meeting in Paris attended by 15 wood scientists from nine countries. A constitution and bye-laws prepared by a special international committee were approved, and plans for organization of the society were initiated.

The new organization is a nonprofit-making, independent international society open to individuals, private enterprises, and representatives of university and government agencies working in the fields of research, education, and industrial aspects of forest products utilization. The society is endorsed by FAO, which is to assist in the organizational stages by providing Secretariat services at Headquarters. All who joined the society in 1960 were registered as charter members.

Importance of an international society

In an area of diminishing natural resources the forests, that know no international boundaries, comprise our only renewable material resource. Over the past years there has been an awakening interest in and recognition of the need for improving wood utilization and the need for forest products research, as a means of making our forests go farther and serve the nations better.

Improving the international exchange of information and ideas among those working in the field of forest products has been frequently discussed. This need has been emphasized further by the problem of keeping abreast of findings and developments at the numerous research centers throughout the world, and in keeping up to date on an international basis on the large and expanding volume of literature. The desirability of developing an independent medium for contacts and exchange of views among those interested in improved wood utilization has become of increased importance, as well as the holding of meetings to provide a forum for presentation of papers and discussions.


The International Wood Research Society has been organized to meet these needs, in promoting the advancement of knowledge and the more efficient utilization of wood and wood products.

The constitution provides for a plenary meeting to be held biennially at a time and place to be designated by the executive committee. Also a plenary meeting may be called periodically on authorization of the executive committee on the occasion of a meeting of the FAO Conference on Wood Technology. Special meetings may be convened on authorization of the executive committee at the time of meeting of kindred organizations. Provision is made for the development of regional groups to sponsor regional meetings.


The detailed procedures for publications are to be established by the executive committee. Publications of scientific papers are to be encouraged. Consideration is being given to a bulletin and to use of publication facilities of appropriate technical and trade journals. Information regarding organizational matters as well as scientific and technical notes and reports will be made available to all members by the executive secretary by periodic bulletins.

Invitation to membership

A cordial invitation to join the society is extended to individuals and organizations interested in the advancement of knowledge and the more efficient utilization of wood and its products. Applications for membership and other correspondence should be directed to: The Executive Secretary, International Wood Research Society, FAO, Viale delle Terme di Caracalla, Rome, Italy.

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