Effects of larval versus adult density conditions on reproduction and behavior of a leaf beetle

Abstract

Population density is an important social environmental factor influencing various life history traits of individuals in many species. In how far density also affects behavioral traits, and particularly at which life stage during ontogeny the adult behavioral phenotype is shaped, has been rarely examined, especially in holometabolous insects. We tested the effects of larval versus adult density on development, reproductive success, and the adult behavioral phenotype of a non-social beetle, Phaedon cochleariae. Therefore, we reared individuals either constantly isolated, in groups, or switched them to the other rearing density condition after metamorphosis. We expected beneficial consequences on the growth and reproduction of isolated individuals because these individuals did not have to compete for food and resting sites. The development time was significantly influenced by the larval density with isolated larvae developing faster than group-reared individuals. Furthermore, the reproductive success was higher for individuals kept isolated as larvae, regardless of the adult density, pointing to a silver spoon effect (i.e., persisting fitness advantage based on favorable, less competitive early conditions). Three behavioral dimensions were found in adult beetles, boldness, activity, and exploration. Adult boldness behavior and activity were influenced by either larval or adult density conditions in dependence of the context and/or sex. Thus, the present study highlights the major influence of the experienced larval density on adult reproductive success and demonstrates the high complexity in which the behavioral phenotype of a holometabolous insect is shaped by a social environmental factor. We investigated the effects of keeping larvae and/or adults either isolated or in a group on the adult reproduction and behavior of a leaf beetle. This study was performed to understand which life stage has a stronger impact on the adult phenotype in an organism that undergoes a metamorphosis. We found that the larval stage was most decisive for the adult reproductive success, with isolated larvae laying more eggs as adults than individuals kept in groups. Furthermore, adult boldness and activity behavior were shaped by larval and adult density conditions differently between females and males. These results demonstrate that both larval and adult experience can be involved in shaping the adult behavioral phenotype of a holometabolous insect.