In insects, mating performance and female post-mating reproductive fitness is closely related to female and male density. In nature, females and males of the willow leaf beetle, Plagiodera versicolora (Laicharting), exhibit the phenomenon of swarming damage. This study investigated the effects of female and male density on their mating performance and on female post-mating fecundity, egg hatching rates, and egg-laying patterns. Female and male density had a significant positive effect on the time from pairing to successful mating and on female fecundity, the duration of the egg-laying period, and the number of egg clutches, but it had a significant negative effect on the egg hatching rate. Female and male density had no significant effect on female post-mating longevity and the number of eggs per clutch. Our study shows that variation in female fecundity and egg-laying patterns is an adaptive strategy for P. versicolora at relatively high population densities. This strategy helps to expand the distribution of offspring and lower the competition for food resources among offspring, which, in turn, helps to maintain or increase population abundance.