Inter-annual variation in above- and belowground herbivory on a native,annual legume

The relative importance of subterranean versus aboveground insect damage to plants is not well understood. In particular, the simultaneous effects of above- and belowground herbivory, and the importance of highly variable abiotic factors such as rainfall, have received little attention in diverse natural ecosystems. We investigated the influence of both above- and belowground herbivory on Lupinus nanus (Fabaceae), an annual plant native to coastal California. A number of insect species damage L. nanus aboveground, and a weevil larva consumes nodules belowground. To manipulate herbivory in the field, we employed a combination of insecticides and simulated herbivory during two different years. In 1997, simulated belowground damage reduced L. nanus survival, and insecticide application to roots increased seed production and seed mass. By contrast, in 1998, only aboveground folivory significantly reduced L. nanus reproduction, and, in combination, above- and belowground insecticides did not affect flower or seed number relative to controls. A growth chamber experiment conducted in the absence of herbivory revealed that the aboveground insecticide marginally reduced flower production and the belowground insecticide marginally increased flower production compared to controls; these non-target effects made our field experiments for aboveground herbivory conservative. Finally, ambient levels of herbivory differed among years (1997, 1998, and 2000), which varied greatly in rainfall due to the effects of El Nino. The results suggest that the impacts of herbivores are temporally variable and that abiotic factors, particularly those related to large-scale changes in weather patterns, may be more important than insect herbivory to L. nanus in some years. This revised version was published online in August 2006 with corrections to the Cover Date.