The Regulation of Population Density in the Bark Beetle Scolytus scolytus (F.)

Abstract

Scolytus scolytus and other bark beetles have the advantage for studies of natural populations that a range of population densities can be obtained in a single generation by considering each sample log as a separate population. Sampling has been carried out at intervals during 1 generation of S. scolytus to try to determine which of the causes of mortality of the immature stages have a regulatory effect on the beetle population. Examples of mortality factors showing density-independent, density-dependent and inversely density-dependent responses to Scolytid density were found. The ectoparasites and woodpeckers showed a response only above a threshold density. The behavioral mechanisms underlying these responses and some of the interactions between the mortality factors are discussed. Two of the mortality factors, the subcortical predators and the ectoparasites, both acting on the 3rd to 5th instar larvae, are considered to be important in regulating the population density. The response curve of the subcortical predators shows that their action is density-dependent at low Scolytid densities but becomes inversely density-dependent at high densities. The ectoparasites show a density-dependent response but only above a threshold host density. This density is slightly below that at which the response of the predators becomes inversely density-dependent. It is suggested that the ectoparasites compensate for the loss of regulatory power of the predators at high densities. It is concluded that populations are likely to be regulated by different mechanisms at different densities.