The Development and Expression of Population Tables for the Bark Beetle Scolytus scolytus (F.)

Many of the bark beetles (Coleoptera, Scolytidae) are particularly suitable for population studies. There is only a relatively short period of the life cycle in which the adult is free, and in which distribution of the species occurs and new breeding sites are sought out. The remainder of the life cycle is spent in a small area below the bark. A maternal gallery is constructed in the phloem and the eggs are laid along its sides. The larvae make galleries through the phloem and, when mature, pupate in the bark or sapwood, eventually to emerge through holes cut in the bark. The cause of death and the stage at which it occurred can usually be determined from the traces left below the bark in the galleries. By using suitable sampling methods, it is possible not only to follow the changes in a Scolytid population with time and the causes of these changes, but to study the effects of population density on the various mortality factors. The present paper is concerned with the first of these objectives as applied to a population of Scolytus scolytus (F.). The second will be dealt with in a later paper. S. scolytus breeds only in elm. Its biology is described in detail by Beaver (1965), but follows the general pattern given above. The seasonal history is as follows. The species overwinters in the larval stage. The first generation adults emerge from the end of May throughout the whole summer to the beginning of September, as the smaller larvae develop and metamorphose. From the galleries started by the earliest emerging of these adults, a partial second generation emerges from mid-August to mid-October. These adults construct galleries and the resulting larvae overwinter together with the remaining larvae produced by the first generation adults. It is important to note that due to the prolonged emergence period and partial second generation, there is a considerable overlap in the stages of development present at one time.