Ovipositor morphology, population dynamics, and biogeography of gall-inducing sawflies (Hymenoptera: Tenthredinidae)

In this series of studies, I was interested in how a common life history constraint among a phylogenetically related group of insects could have differential effects on their expressed ecology and resultant distribution and abundance across the landscape. The gall-inducing sawflies in the Family Tenthredinidae all share the common constraint of depositing eggs into plant tissue. They use a saw-like ovipositor, hence their namesake, to perform this function. In Chapter 1, I investigate the fine structure of the ovipositor of the stem galling sawfly, Euura lasiolepis, to identify the presence of chemoreceptors. The constraint that this ovipositor has in determining the expressed ecology is complicated by the resource utilized by the female. Within the phylogenetically related group of gall-inducing sawflies, there are multiple gall forms based on the niche utilized by each species. They include stem gallers, bud gallers, petiole gallers, mid-rib gallers, leaf-lamina gallers, and leaf-edge gallers. In Chapter 2, I compare the population dynamics of two different gall forms, the stem galler and the leaf-lamina gallers. I chose to compare these two gall forms because of the different resources they use on a plant. Their differential exploitation of resources within a single host plant can cause marked differences in their resultant population dynamics. I tested the resource-heterogeneity hypothesis and found that when resources are very similar they allow females to lay eggs across a wider range of possibilities than for females whose target resource is more heterogeneous. Consequently, larval performance is consistently higher in the species that targets a more homogeneous resource, i.e., the leaf-lamina gallers. Therefore, the stem gallers showed a strong preference-performance linkage and the leaf-lamina gallers did not. Variation in target resources of specialized insects can define how sensitive a species will be to a dynamic environment, thus dictating their abundance and distribution. In Chapter 3, I describe how community similarity, richness, and abundance change with distance and habitat isolation in this phylogenetically related group of insects across the range of their single host plant, Salix lasiolepis. I found a highly nested subset structure in this sawfly assemblage that was not explained by distance, but by the connectedness of their habitat. Connected habitat types have a significantly different community composition, and higher richness and abundance than isolated habitats. The nested subset pattern seen in these sawflies was most likely extinction-dominated due to the low dispersal distance of species in this taxon and the highly fragmented distribution of the host plant. The predicted species extinction order was as follows: mid-rib galler (Euura sp.), petiole galler (Euura sp.), stem-galler (Euura lasiolepis), leaf-lamina galler (Pontania sp.), leaf-edge galler (Phyllocolpa sp.) which also mimics the evolutionary order of this group with Phyllocolpa being the most primitive. Conservation planning based on our data would suggest that connected habitats or long-coherent drainages are essential for sawfly persistence because extinction risk in the isolated habitats or fragmented drainages was extreme.