The influence of different age classes on the seasonal activity and reproduction of four medium-sized carabid species inhabiting cereal fields

Reproduction and survival of four carabid species commonly occurring in cereal fields in central Sweden were investigated. The adult-overwintering Pterostichus cupreus L. bred in early summer (June), and the largest proportion of sexually active beetles were found to be in their first breeding year; only a minority were 2 (or more) years old. In contrast, the larval-overwintering P. melanarius Illiger, P. niger Schaller and Harpalus rufipes De Geer had a biennal life cycle, and mainly old beetles (2 years or older) contributed to the population's reproductive effort each year. The long prereproductive adult phase in the larval-overwintering species and the relatively short summer in central Sweden prevented many new-generation adults from breeding in their first year. Several overlapping breeding cohorts are therefore present each year, and overwintered beetles bred throughout summer and during part of the autumn. Mandible wear (combined with other characters) was used to determine the age of beetles. However, the rate of mandible wear also was found to vary, depending upon the extent to which each species burrowed in the ground. The ecological significance of (a) a long adult life and (b) the existence of several overlapping breeding cohorts for reproduction and survival of populations in cereal fields is discussed.