Population dynamics of gyrinid beetles : II. Reproduction

Between 1974 and 1983 data concerning egg production and recruitment were obtained by sampling populations of the whirligig water beetle Gyrinus marinus Gyll. All females reproduce, but they differ in the number of eggs laid per oviposition and in the frequency of oviposition. There are two reproducing generations a year, one in spring (April-June) and one in summer (July-August). Females emerging in July and August reproduce immediately after emergence, but at a lower level than the females of the spring generation. In June the egg production of spring females also decreases. No significant differences in egg production occur between populations in different sites or in different years. Individual properties and circumstances of the females probably have a distinct influence on variation in egg production. A female probably oviposites and is fertilized at least once a week. Without, fertilization egg production and the viability of the eggs decrease after two weeks. On the average, 60 per cent of the eggs hatch.Development from egg to teneral takes about 10 weeks in spring and autumn and about six weeks in summer. The hatching and development of eggs laid in April is probably retarded until the half of May. Variation in the length of the developmental period will cause variation in recruitment and in the number of hibernating beetles. Variation in eggviability and in larval and/or pupal development are probably of equal importance to the variation in the numbers of tenerals emerging. The variation in egg production is of no importance.Recruitment (number of tenerals/female) of the generations reproducing, in spring and in summer are independent of each other. Between years the rate of recruitment differs significantly, and probably depends more on environmental conditions such as weather than egg production seems to do.