Optimal attack density in the larch bark beetle, Ips cembrae (Coleoptera: Scolytidae).
Abstract
The effects of attack density of Ips cembrae on the gallery length, eggs and adult offspring production and mortality were studied in 0.1 m2 bark samples from 100 larch trees (Larix dahurica [L. gmelinii]) during the summer of 1989 in a burned area of the Tu Qiang Forest District, Great Xingan Mountains, China. The results showed that there was a positive linear relationship between gallery length and number of egg niches. The number of egg niches and adult offspring produced per gallery declined exponentially with attack density, and the egg to adult mortality increased exponentially as the attack density increased in the absence of host resistance. The exponential relationships were attributed to intraspecific competition between female adult parents for breeding space and between larvae for limited food resources. In addition, there were 2 optimal attack densities, 13.9 attacks per 0.1 m2 and 8.7 attacks per 0.1 m2, at which the number of egg niches (935) and adult offspring (496), resp., were maximum. An upper threshold of attack density, 31.8 attacks per 0.1 m2, beyond which the number of offspring adults produced from a bark area became less than the input of adult parents, was also found.