Contrasting latitudinal herbivory patterns in Oenothera biennis (Onagraceae)
The observation that herbivory and plant defenses increase towards the equator has long been viewed as a well-supported pattern called the Latitudinal Herbivory Defense Hypothesis (LHDH). Recent studies call this hypothesis into question, which suggests alternative hypotheses might explain biogeographic patterns of herbivory and plant defense in nature. Here we test the LHDH by sampling herbivory by multiple generalist and specialist insect herbivores over the entire latitudinal native range of a single plant species, Oenothera biennis. Since latitude is a proxy for climatic variation, we also consider individual environmental variables as well as population density to gain insight into possible mechanisms for latitudinal patterns. We sampled 79 populations on a 16 degree North-South gradient from Ontario and Maine, to Alabama and Florida. We quantified leaf herbivory caused by a diverse community of generalist insects, damage by a specialist weevil on stems (Tyloderma foveolatum), and damage by three lepidopteran flower and fruit specialists. We also estimated herbivore load as the total dry mass of specialist herbivores observed per plant.
We show that latitudinal patterns in herbivory exist, but they vary dramatically among herbivore species. While generalist leaf herbivory showed no pattern, T. foveolatum damage increased with decreasing latitudes consistent with the LHDH. By contrast, three specialist flower and fruit moth species all exhibited decreased herbivory with decreasing latitude. Among environmental variables, mean temperature provided the best predictor of herbivory patterns, which explained more variation than latitude. Overall, we show that in O. biennis, every pattern of herbivory across a latitudinal gradient is possible, and this variation depends on the specific tissues and herbivores studied. These results give testable predictions for ongoing experiments that seek to tease apart the roles of the environment versus genetic divergence in both the plant and insect populations that might explain the patterns we observed across the native range of O.biennis. They also show that measuring different types of herbivores on different tissues and the actual environmental conditions rather than just latitude is critical to understanding more of the specific biology associated with latitudinal species interactions.