Life Tables and Mortality Factors for Saperda inornata (Coleoptera: Cerambycidae)1

The poplar gall Saperda, Saperda inornata Say, attacks the stems of suckers and the small branches of trembling aspen, Populus tremuloides Michx., in Michigan. Objectives of this research were (1) to study the mortality and survival of natural populations of S. inornata, (2) to identify mortality factors, (3) to determine their significance relative to generation mortality, and (4) to identify key factor(s) or key periods responsible for population variations. Over a 5-year period, natural populations of S. inornata were studied at 6 locations in Michigan. Annual egg niche surveys were conducted in 6 sampling areas, each with 75 plots of 30 P. tremuloides suckers per plot. Annual cohorts of beetle attacks were inspected and sampled twice yearly until final adult emergence. Specific identities of mortality factors were determined from dissections of infested twigs. S. inornata has both a 1-year and 2-year life cycle in Michigan. Survival between different sampling areas and different annual cohorts was compared by means of life tables representing pooled data on 4 complete generations. Larvae were found to have better survival on certain sites in some years, but all life tables indicated that S. inornata has greater potential for infestation than was indicated by egg-niche survey results. The proportion of adults which matured in 1 year varied with different cohorts, probably depending upon the time of oviposition and temperatures during the summer months. An average life table for S. inornata in northern Michigan is presented with causes for mortality indicated by sampling period. Most egg-niche failures occur in the 1st few months from undetermined causes. Mortality from desiccation and from parasitism by Euderus lividus (Ashmead) (Eulophidae) is confined to the 1st year, while all other known causes for mortality affect each larval period. Parasites, mainly Lixophaga sp. (Tachinidae), Odinia sp. near xanthocera (Odiniidae), Meteorus cognatus Muesebeck, Iphiaulax curygaster (Brullé), Cenocoelius sanguineiventris (Ashmead), and Bracon spp. (Braconidae), were identified. Woodpeckers predation was most damaging during the 2nd year. Key periods and key factors in the population controls are discussed.