Background/Question/Methods Many insect herbivores engineer their host plants, building leafties, webs, galls, tunnels, tents, and mines that are simultaneously or subsequently colonized by other arthropod species. The overall objective of this study was to determine the degree to which plant architecture and leaf quality traits influence the abundance of leaf-tying caterpillars and their associates in leaf ties on eight species of sympatric oaks. During the summer of 2007 at Cuivre River State Park, near Troy, Missouri, we conducted eight censuses of the leaf tie dynamics and occupants on a minimum 15 individuals of each of eight oak species (Quercus alba, Q. macrocarpa, Q. muehlenbergii, and Q. stellata in the white oak subgenus, and Q. imbricaria, Q. marilandica, Q. rubra, and Q. velutina in the red oak subgenus). In addition, we collected leaf samples for leaf quality estimates, and measured leaf phenology and plant architecture (number of touching leaves) to determine the contribution of these plant traits to within and among species variation in leaftie dynamics and arthropod richness. Results/Conclusions One measure of the engineering impact of leaf-tying caterpillars is their influence on the number of non-tying species that colonize leaf ties. Our results show that species richness of non-tie making arthropods found in ties is influenced by host plant species and oak subgenus. Red oak species had higher species richness early in the season but lower later in the season compared to white oak species. The effect of individual species was also significant, with the number of ties on Quercus alba significantly greater than on the other seven oak species. Both plant architecture and leafing phenology contributed to the engineering effect on species richness. Trees with more touching leaves, which increases the probability of tie formation, had higher species richness per tie. Later flushing individuals had lower species richness of non-tie makers in leaf ties. This latter result is surprising given that leafties are first constructed a minimum two months after leaf flush. We will report on the contribution of leaf quality traits to host plant species and subgenus effects.