Lepidopteran root-feeders as agents of change in California coastal landscapes

Lepidopteran larvae are conspicuous and important herbivores in a wide variety of terrestrial ecosystems. Stem- and taproot-feeding taxa can have particularly large impacts by severing vascular cambia, in effect girdling forbs, shrubs, and trees. Root-boring caterpillars (Hepialus californicus: Hepialidae) in California coastal prairies feed upon bush lupine Lupinus arboreus, a dominant nitrogen-fixing shrub. High caterpillar densities can kill entire lupine stands, releasing limited space, light and available nitrogen to native and invasive plant colonists and facilitating change in community structure. In field and laboratory experiments, soil-dwelling entomopathogenic nematodes (Heterorhabditis marelatus) can control Hepialus and provide protection to lupines via a trophic cascade. However, the strength of these interactions varies temporally and spatially, and caterpillar outbreaks and lupine devastation can occur even within a background of robust nematode populations. We hypothesize that caterpillars resist nematode infection both by spinning silk and by remaining sealed within root tissues. The latter resistance erodes as bushes accumulate root damage and feeding galleries become increasingly accessible to nematodes and other organisms. Nematodes select microhabitats primarily on the basis of moisture content and secondarily in pursuit of insect hosts. Desiccation risk during dry Mediterranean summers drives nematodes into moisture refugia, both within lupine rhizospheres and into deep (>80 cm) soils lacking host insects. Spatial patchiness and limited locomotion and may prevent active nematode recruitment to roots with high host densities. Thus, limited top predator mobility, variation in host behavior and local abundance, and spatial and temporal heterogeneity in abiotic and biotic factors promulgate the observed heterogeneous strength in trophic cascades.