Thursday, August 5, 2010: 10:50 AM
321, David L Lawrence Convention Center
Background/Question/Methods Entomopathogenic nematodes (EPN) are globally distributed microparasites that attack root-feeding or soil-dwelling insects. Increasingly EPN are used for biological control of invasive or agricultural pest species, yet populations often fail to persist in managed habitats and the keys to persistence in endemic populations are poorly known. In natural populations in a California coastal prairie, persistent populations of Heterorhabditis marelatus have been studied continuously for 15 years. An ecologically important host insect in this habitat, the larvae of the ghost moth Hepialus californicus (Lepidoptera: Hepialidae), attacks, girdles and kills the dominant woody shrub, the yellow bush lupine (Fabaceae: Lupinus arboreus). Field experiments show that Heterorhabditis marelatus parasitism of this moth species can indirectly benefit Lupinus arboreus through a top-down trophic cascade. Dynamic models suggest that Hepialus californicus, in turn, is critical for the local persistence of this nematode species, yet the host insect is seasonally unavailable for 4-6 months each year. Thus we hypothesized that alternative hosts supplement EPN populations throughout the year. We designed monthly field sampling over more than one year (2006-2007), testing the influence of aboveground vegetation and soil depth on the community structure and diversity of subterranean invertebrates and EPN.
Results/Conclusions Averaged over the season and soil depths, lupine rhizospheres were significantly cooler and moister, and arthropod density and richness and H. marelatus prevalence were higher relative to grassland rhizospheres. Arthropod density and richness increased with the seasonal rains (October-March) and remained elevated under lupine but declined in annual grassland soils through the following dry summer. As anticipated, EPN prevalence was higher under lupines than grasslands, but surprisingly remained high through the dry summer. However, elevated nematode incidence occurred primarily at depths greater than 30 cm in the soil column, reflecting either differential extirpation in hot and dry surface layers or active migration to deeper layers to avoid desiccation. Lupine canopies facilitate EPN populations with higher soil moisture and by harboring greater diversity and abundance of arthropod larvae in the soil. These features facilitate localized individual movement, host-finding behavior, and survival of infective juveniles, and promote recycling of additional EPN generations through Hepialus as well as alternative arthropod hosts. However, limited locomotion in the soil matrix impairs EPN aggregative recruitment to localized host irruptions and reinforces inherent patchiness of endemic EPN populations. Metapopulation dynamics, which can override limitations set by local and short-term abiotic conditions, may ultimately determine long-term EPN persistence in this natural system.
Results/Conclusions Averaged over the season and soil depths, lupine rhizospheres were significantly cooler and moister, and arthropod density and richness and H. marelatus prevalence were higher relative to grassland rhizospheres. Arthropod density and richness increased with the seasonal rains (October-March) and remained elevated under lupine but declined in annual grassland soils through the following dry summer. As anticipated, EPN prevalence was higher under lupines than grasslands, but surprisingly remained high through the dry summer. However, elevated nematode incidence occurred primarily at depths greater than 30 cm in the soil column, reflecting either differential extirpation in hot and dry surface layers or active migration to deeper layers to avoid desiccation. Lupine canopies facilitate EPN populations with higher soil moisture and by harboring greater diversity and abundance of arthropod larvae in the soil. These features facilitate localized individual movement, host-finding behavior, and survival of infective juveniles, and promote recycling of additional EPN generations through Hepialus as well as alternative arthropod hosts. However, limited locomotion in the soil matrix impairs EPN aggregative recruitment to localized host irruptions and reinforces inherent patchiness of endemic EPN populations. Metapopulation dynamics, which can override limitations set by local and short-term abiotic conditions, may ultimately determine long-term EPN persistence in this natural system.