POPULATION BIOLOGY OF AN HERBIVOROUS LADY BEETLE

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The adaptive significance of the timing of oviposition in the thistle-feeding lady beetle Epilachna niponica was investigated at two localities (site A and Site F) in the northwestern part of Shiga Prefecture, central Japan. I followed cohorts produced over a season and measured four components of lifetime fitness : egg survival, larval survival, female adult survival from emergence to the reproductive season in the following year, and lifetime fecundity. Large variation in lifetime fitness was evident among cohorts within a population, but the two local populations showed a clear difference in patterns of cohort fitness. At site A, cohorts produced early in a season had higher lifetime fitness than later cohorts, whereas at site F, later cohorts tended to have higher lifetime fitness.The major causes of these between-site differences were seasonal variation in intensity of egg and larval mortalities fue to arthropod predation and host plant deterioration. Field observations revealed a significant difference in oviposition phenology between the two sites : early reproduction at site A and delayd reproduction at site F.The relatively longer reproductive life-span of females was responsible for the prolonged reproduction at site F.Results of a laboratory experiment which eliminated environmental variables agreed with the field observations, and suggested a genetic basis for the oviposition schedules. Correlation of reproductive pattern at each site with higher lifetime fitness of offspring suggests selection on the timing of oviposition to improve the lifetime reproductive success of females.