Notes on the Ecology and Control of Pine Beetles in Great Britain
Authors: Hanson H. S. 
Published: July 2009

1. The object of the investigation is the study of possible methods of control of injurious insects in pine areas in Britain, with special reference to bark-beetles and toMyelophilus piniperda, L., in particular.2. Reference is made to the economic status of the pine beetles in Britain as compared with other countries, and it is pointed out that in this country control measures must be of a more intensive character than in countries where large tracts of natural forest occur.3. An analysis of the various types of damage caused byMyelophilusis made, and it is shown that although the actual amount of damage caused in any particular year may at the time appear comparatively slight, the effects of successive outbreaks are cumulative, and frequently result in a great reduction in the value of the crop.4. The occurrence of bark-beetle outbreaks and the conditions under which they occur are briefly described.5. The factors influencing the pine beetle population are discussed, and it is pointed out that the effects of climatic and physical conditions on the development of the host-plant under forest conditions are of fundamental importance in determining the density of the bark-beetle population in standing pine woods. It is stated that apart from abnormal increase brought about by exceptional breeding facilities as a result of fire, windfall, and felling operations, the conditions favouring the increase of bark-beetles and wood-boring insects in pine forests are considered to be mainly due to exceptionally favourable conditions for vigorous growth during the early stages of the crop's development, resulting in large numbers of trees being suppressed and killed in the pole stage, by rapidly increased shade as a result of the abnormal development of predominant trees. This is inevitable when Scots pine plantations are formed on soils too rich and situations too favourable. On the other hand, exceptionally favourable conditions for bark-beetle development occur when conditions are unfavourable for the healthy development of the crop owing to bad drainage conditions and water-logging of the soil.6. The effect of overcrowding among the brood is considered to be one of the most important factors in the limitation of the bark-beetle population. Examples of how this factor operates are given and it is shown that the practical application of this factor can be brought about by the limitation of the supply of suitable breeding material.7. The relative value of various parasites and predators as factors in the control of bark-beetles is discussed.It is considered that pathogenic organisms are of little importance in the control of bark-beetles, as although these organisms may cause an epidemic when the pest has reached its maximum abundance, the object of our control measures must always be to prevent the bark-beetle population from approaching that stage of density.Evidence is cited from the literature as to the effects of Nematode worms in reducing the egg-laying capacity of bark-beetles. It is suggested that the difference in the results obtained by various writers may have been influenced by the state of the insects at the time the observations were made, since the state of development of the reproductive organs of bark-beetles varies considerably during different stages, and although they sometimes appear to be shrivelled, they are capable of recuperation after a period of feeding. In any case it is impracticable to utilise Nematode worms in control measures, and it seems probable that their usefulness as parasites of bark-beetles will be neutralised by their effects upon the Coleopterous predators.A list of the Hymenopterous parasites of pine beetles found during the progress of the investigation is given, together with notes on their relative abundance, and their importance as controlling factors is emphasized.A list of the Coleopterous predators found associated with the bark-beetles is given with notes on the most important species. It is pointed out that some species of Coleopterous predators are probably more injurious than beneficial owing to the destruction of parasite larvae ;Thanasimus formicariusand Elaterid larvae are examples. On the whole, the Coleopterous predators are considered to be of even greater importance than Hymenopterous parasites as agents in the control of bark-beetles.The larvae of several species of Diptera are very numerous in the bark-beetle brood-galleries. A list is given of the species which have been identified. Although listed as predators of bark-beetles, these Diptera are regarded as being of greater importance as destroyers of parasite larvae and are therefore being studied with a view to their possible elimination.The economic status of birds in relation to bark-beetle control is briefly discussed, and it is pointed out that although under certain conditions birds do undoubtedly destroy large numbers of adult bark-beetles, it is probable that this useful work is counterbalanced by the destruction of parasite larvae in winter. On the whole, birds are not regarded as being of importance in the control of pine beetles in Britain.The effects of competition between different species of bark-beetles is discussed, with special reference toCrypturgus cinereus, Hbst., and its influence in the control ofMyelophilus minor.8. A detailed description is given of some large-scale thinning experiments which have been in progress in the New Forest since 1933.The scheme on which the experiments are based is the result of observations carried out by the writer over a period of years during which he noted that parasites and predators of the pine beetles are most numerous in young pine stands in the pole stage, previous to the first thinning operations. The conviction was formed that under those conditions a state of biological control existed which might, under favourable conditions, be continued throughout the rotation of the crop. One of the necessary conditions for the maintenance of a permanent resident population of parasites and predators is the presence of a permanent host population. This, in turn, necessitates a continuous supply of suitable breeding material. The density of the host population is largely governed by the quantity of suitable breeding material available, and the requirements of an increasing bark-beetle population demand a corresponding increase in the amount of suitable breeding material from year to year; so long as this increase in suitable breeding material continues, the host population is maintained at a higher level than that of the parasites and predators. When, however, the supply of suitable breeding material ceases to meet the requirements of the increasing bark-beetle population, a point is reached where overcrowding of the brood takes place. The intersection and intermingling of the larval tunnels facilitates the work of destruction by the predator larvae and the balance is, for a time, turned in their favour. The increased density of the host population also favours the increase of the parasites at the expense of the host. Limitation of the supply of suitable breeding material also results in a high rate of mortality among the bark-beetle larvae. The combined effects of the increased parasite and predator population, accompanied by overcrowding in a limited supply of food material, will result in a sudden reduction in the bark-beetle population, which is followed by a corresponding reduction in the parasite and predator populations. It will be seen, therefore, that under natural conditions there is a considerable fluctuation in the rise and fall of the numerical ratio of the host population and that of the parasite and predator population. The time occupied by each cycle may be extended over a considerable period of years, the peak of each host infestation giving rise to a periodic outbreak, corresponding with the period of maximum density of the crop. The automatic opening up of the canopy results in a temporary reduction in the amount of suitable breeding material. Under silvicultural conditions, however, the sudden increase of available breeding material produced when a thinning takes place and the material is allowed to remain on the area provides abnormally favourable conditions for the sudden increase of the bark-beetle population, and an outbreak is precipitated. The occurrence of these outbreaks has led to the popular belief that the remedy lies in what is called “ clean forestry.” In theory, the elimination of all suitable breeding material would solve the bark-beetle problem, but in practice the method is often impracticable when large areas of forest have to be dealt with, as is clearly demonstrated by the frequency with which bark-beetle outbreaks occur as a result of thinning operations.The object of the experiment was, therefore, to devise a method by which the state of biological control can be continued throughout the rotation, by the provision of the amount of breeding material necessary to maintain a permanent population of hosts which would be kept at a level of economic control by the parasites and predators, and at the same time to bring into practical application the factor of overcrowding as an automatic governor to check any undue increase of the bark-beetle population. The results of the experiments are given in detail, and it is shown that the rate of growth of the crop is a decisive factor.A scheme is outlined for the study of the use of billets and trap-stems.Reference is made to operations which tend to upset the balance of insect populations.Control measures in felling, burnt, and windfall areas are also discussed.