Lifetime Fitness and Evolution of Reproductive Pattern in the Herbivorous Lady Beetle

Abstract

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. These data are based on mark—recapture data for >9000 adult beetles and detailed life tables over 5 yr. 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 (except for the last one) tended to have higher lifetime fitness. The major causes of these between—site differences wee seasonal variation in intensity of egg and larval mortalities due 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 delayed reproduction at site F. The relatively longer reproductive life—span of females was responsible for the prolonged reproduction at site F. Results of laboratory experiment that 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.