Sampling consistency matters-Stable isotopes (δ15N and δ13C) exhibit wide variation among individuals within bee species native to North America (Hymenoptera: Apidae).

Abstract

Stable isotope analysis is scarcely utilized among bee researchers. One reason for this scarcity is ambiguity when determining whether to utilize the entire insect/arthropod or certain body parts for isotope analysis. Here, we analyzed δ15N and δ13C isotopic compositions of the abdomens, heads, and legs of 3 bee species native to North America [Centris pallida Fox, Melissodes bimaculatus (Lepeletier), and Bombus griseocollis (De Geer)]. We hypothesized that the abdomen δ15N and δ13C values would differ significantly from those of the heads and legs because the abdomen contains metabolically active organs, while the heads and legs, with less active tissues, would show no differences in isotopic compositions. Overall, we found varying degrees of statistical differences among the different body parts for isotope values. However, variations equivalent in magnitude to trophic level differences were not observed, resulting in the ability to use a single body part rather than a whole insect body for stable isotope analysis. To strengthen this finding, correlation analyses revealed paralleled patterns in responses by species. Finally, we suggest that when designing experiments using stable isotope analysis, it is important to focus on being consistent with selected body parts to reduce inter-individual variability. We also recommend using a relatively large sample size (n ≥ 20) due to the wide variance observed within individuals, especially if the goal is to characterize population-level ecological differences.