Latitudinal gradients in herbivory on Oenothera biennis vary according to herbivore guild and specialization


  • Corresponding Editor: L. A. Dyer.


The latitudinal herbivory-defense hypothesis (LHDH) posits that herbivory and plant defenses increase toward lower latitudes. Recent studies provide contradictory evidence and suggest alternative explanations for biogeographic patterns in plant–herbivore interactions. Here we test the LHDH by sampling herbivory from multiple generalist and specialist insect herbivores over the entire latitudinal native range of the plant species Oenothera biennis L. (Onagraceae). We sampled 79 populations on a 16° north-south gradient from Ontario and Maine to Alabama and Florida. From each population, we quantified herbivory across feeding guilds by considering leaf herbivory caused by generalist insects, damage by a specialist stem-boring beetle, and flower/seed herbivory by three Lepidoptera that specialize on Oenothera. We also related environmental and population density variables to herbivory. Our results show that latitudinal patterns vary dramatically among herbivore species. While generalist leaf herbivory showed no latitudinal pattern, stem borer damage increased with decreasing latitude. By contrast, the specialist flower/seed herbivores all caused less damage at lower latitudes. Temperature explained slightly more variation in herbivory than latitude, while precipitation and population density were less important. Overall, we show that every pattern of herbivory (positive, negative and no relationship) is possible across a latitudinal gradient, and this variation depends on the insects' degree of specialization and feeding guild.