Effect of adult feeding on the δ¹⁵N signatures of different tissues for Popillia japonica

Abstract

As insects feed, stable isotopes from their diets are assimilated into their tissues. Stable isotopes provide a method to determine what insects are feeding on, and in some cases, where the insects have been feeding. One method for evaluating what adult holometabolous insects fed on as larvae, is to sample tissues from the integument of the adult. The integument should have isotopic signatures that reflect the diet of the larvae, and these inert tissues should be less influenced by adult feeding compared to other tissues. The objective of our research was to examine how isotopic signatures of nitrogen in various tissues of adult Popillia japonica (Newman) (Coleoptera: Scarabaeidae) beetles would be influenced by adults feeding on 15 N-enriched plant material. We collected adults from emergences cages at unenriched grasses and fed them for 0, 2, 5, or 10 days with grape plants fertilized with isotopically enriched or unenriched potassium nitrate. The d 15 N signature of elytra and soft tissues both increased over time when fed enriched plant material. The d 15 N signatures of soft tissues were higher than that of the elytra over time. Adult beetles fed highly enriched plant material displayed increased d 15 N signatures over time of tissues generally considered metabolically inactive, which suggests that adult feeding does contribute to the isotopic signatures of elytra tissue. The difference between the d 15 N signatures of soft tissues and elytra demonstrates a lower turnover rate in the elytra and highlights the fact that diet does contribute to the signature of elytra.