title: Population Dynamics of an Insect Herbivore over 32 Years are Driven by Precipitation and Host-Plant Effects: Testing Model Predictions.
abstract: The interaction between the arroyo willow, Salix lasiolepis Bentham, and its specialist herbivore, the arroyo willow stem-galling sawfly, Euura lasiolepis Smith (Hymenoptera: Tenthredinidae), was studied for 32 yr in Flagstaff, AZ, emphasizing a mechanistic understanding of insect population dynamics. Long-term weather records were evaluated to provide a climatic context for this study. Previously, predictive models of sawfly dynamics were developed from estimates of sawfly gall density made between 1981 and 2002; one model each for drier and wetter sites. Predictor variables in these models included winter precipitation and the Palmer Drought Severity Index, which impact the willow growth, with strong bottom-up effects on sawflies. We now evaluate original model predictions of sawfly population dynamics using new data (from 2003-2012). Additionally, willow resources were evaluated in 1986 and in 2012, using as criteria clone area, shoot density, and shoot length. The dry site model accounted for 40% of gall population density variation between 2003 and 2012 (69% over the 32 yr), providing strong support for the bottom-up, mechanistic hypothesis that water supply to willow hosts impacts sawfly populations. The current drying trend stressed willow clones: in drier sites, willow resources declined and gall density decreased by 98%. The wet site model accounted for 23% of variation in gall population density between 2003 and 2012 (48% over 30 yr), consistent with less water limitation. Nonetheless, gall populations were reduced by 72%.
authors: Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011-5640. peter.price@nau.edu.
authors: Peter W
authors: PW
authors: Price
authors: Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, MI 48109.
authors: Mark D
authors: MD
authors: Hunter
mesh_terms: D000818:Animals; D001130:Arizona; D055864:Droughts; D060434:Herbivory; D006790:Host-Parasite Interactions; D006927:Hymenoptera / Q000502:physiology*; D008962:Models, Theoretical; D018547:Plant Stems / Q000469:parasitology; D010941:Plant Tumors / Q000469:parasitology; D011157:Population Dynamics; D011891:Rain; D032108:Salix / Q000469:parasitology*
keywords: bottom-up dynamics; drought stress; host plant and herbivore interaction; long-term study; predictive model tested