Trophic Levels and Anomalous δ15N of Insects on Raspberry (Rubus Idaeus L.)

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Abstract
Unexpectedly elevated δ15N values (8 to 16‰) were found in adult raspberry beetles (Byturus tomentosus), collected shortly after emergence from over-wintering sites. Elevated δ15N values (11 to 14‰) were also found in over-wintered larvae, but young adults, collected shortly after metamorphosis in autumn, and larvae collected from raspberry drupelets and receptacles in summer, did not show such elevated δ15N. δ15N values of feeding larvae (∼ 2‰) were close to those of their food plant, red raspberry (Rubus idaeus). The elevated δ15N values in overwintered larvae and adults of raspberry beetle are consistent with an hypothesis of extensive amino acid nitrogen recycling during prolonged fasting. Predatory seven-spot and two-spot ladybirds (Coccinella septempunctata and Adalia bipunctata), (δ15N-∼7‰) showed an expected trophic level increase of ∼1‰ relative to their prey, the large raspberry aphid (Amphorophora idaei). The potential is assessed for using continuous flow isotope-ratio-mass-spectrometry (CF-IRMS) at natural abundance levels for studying invertebrate development and food webs.