Influence of host plant vs. natural enemies on the spatial distribution of a pine sawfly, Neodiprion autumnalis

1. The pine sawfly, Neodiprion autumnalis, infests ponderosa pine, Pinus ponderosa, growing at low densities near the bottom of an altitudinal gradient in Arizona, U.S.A. The relative importance of host-plant quality vs, natural-enemy effects in determining the spatial distribution of this sawfly was examined over a 3-year period. 2. Field and laboratory bioassays were conducted on all life stages of N. autumnalis at two forest stand densities (high greater than or equal to 23 m(2) ha(-1) low less than or equal to 7 m(2) ha(-1)) and at two elevations (bottom slope = 2390 m, top slope = 2540 m). These experiments were used for constructing life tables of N. autumnalis that compared the effects of host-plant quality on oviposition preference and progeny performance with the effects of natural enemies at different tree densities and elevations. 3. Life-table analyses determined that mortality attributed to host-plant effects during the egg and larval stages had the largest impact on fitness between tree densities and elevations. 4. Natural enemies caused a significant reduction in progeny survival, but their effects were similar across all tree densities and elevations during egg and larval life stages. However, cocoon-stage survival did vary between tree densities and elevations due to natural-enemy effects. 5. It was concluded that the observed oviposition preference for, and higher progeny performance on, trees at low densities and bottom slope elevations were caused primarily by host-plant effects. 6. These results further the argument that heterogeneity at the resource level (i.e. bottom-up forces) determines potential outcomes of multitrophic level interactions.