Lupinus lepidus began colonizing the Pumice Plains of Mount St. Helens in 1981, one year post-eruption, beginning with a single plant. Population growth in this primary successional site was initially extremely rapid, yet extensive colonization of available habitat has been surprisingly slow. Our previous work demonstrated a major impact of lepidopteran and anthomiid herbivores on the demography and spread rate of colonizing lupines. Here we document strong spatial patterning of insect herbivory. We measured demographic parameters and herbivory in the initial colonizing patches ("core patches"), and newly-founded patches distant from the core ("edge patches"), for ~12000 plants in ~60 patches, from 1990-95. Stem-boring, leaf-mining, and seed-eating insects increased through time and severely decreased seed production and survivorship in low-density edge portions of the expanding population, and had less effect in the core. For example, in 1994-95 stem-boring tortricid moth incidence was 77% in edge patches vs. 24% in core patches; in 1993-95 noctuid leaf-miners infested 68% of plants in the youngest edge patches, vs. 8% at the core. Together, these moths increased edge mortality to ~90% in 1995, over a baseline of ~30% (1991-93). Damage data collected in 1996, 1998, and 1999 along transects running from core areas to edge areas show that the pattern of inverse density dependent herbivory has persisted and is also present at smaller spatial scales, i.e. from the center to the margin of core patches. We are investigating whether predators, parasites, or plant quality may explain the absence of herbivores in the high density core.

Key words: colonization, density dependence, insect herbivore, Lupinus lepidus, Mount St. Helens, primary succession