CAN INSECT CARBON STABLE ISOTOPES IN TEMPERATE, C3 DOMINATED ENVIRONMENTS IDENTIFY LONG-TERM VEGETATION CHANGES?

Insects from arid and tropical Australia have been shown to be reliable recorders of local vegetation change when analyzed via stable carbon isotope (δ13C) values because they consume plants or other insects that consume the plants. Because plants using different photosynthetic pathways – C3 or C4 – have strikingly different δ13C values (averaging -27 ‰ vs. -12 ‰, respectively), the plant type can be identified and tracked up the food chain. However, it is not yet known whether insects from a temperate, largely C3 environment will be sufficiently sensitive recorders of relatively small amounts of C4 vegetation on the landscape. To assess whether stable isotope values in insects from temperate, southwest Virginia reflect the proportion of C4 plants (warm-season grasses) on the landscape, we measured the isotopic values of insects collected using aquatic and terrestrial traps from a variety of vegetation types at three separate field sites between May and October 2024. Vegetation types targeted included C3 dominated landscapes, such as forests or croplands with wheat and soy, or C4 dominated, including corn fields and native and non-native grasses. We hypothesized that insect δ13C values will reflect the relative abundance of C4 plants in the landscape. Preliminary results show compositional differences between riparian and terrestrial traps, with riparian traps dominated by Diptera families while terrestrial traps had a greater abundance of Trichoptera and Coleoptera. Forthcoming carbon isotope results will allow us to determine whether there are detectable differences in resource use by arthropods collected from riparian and terrestrial traps in predominantly C3 or C4 environments. Ultimately, we will use the results of this study to interpret carbon isotope values measured in insectivorous bat guano accumulations and estimate the proportion of native grassland habitats (C4) on Virginia's pre-European Valley and Ridge landscapes.