We studied the nesting biology and developmental schedules of the cavity-nesting solitary wasp Isodontia elegans in Oregon, USA. Trap-nests contained 1–15 brood cells each, separated by partitions, which adult females made with grass fragments. Before leaving nests to hunt, females made loose, temporary closures of longer fragments of dried grass and, upon returning, briefly deposited prey just inside the nest entrance before transferring them to a cell. Cells were provisioned with katydids (Meconema thalassinum, an invasive species in North America) and tree crickets (Oecanthus fultoni), the latter predominating as summers progressed. The inner cells of nests, where most daughters developed, were longer and separated from adjacent cells by partitions thicker than those in outer cells, where most sons developed. These observations, along with the fact that daughters are larger than sons (likely because they receive a greater provision mass from mothers), indicate that investment in daughters is considerably greater than that in sons, by multiple measures. Sexual size dimorphism was evident in both cocoon mass (prepupae + cocoon case) and adult mass. From 2013 to 2019, nesting began earlier in the summer when mean maximum daily temperatures in June were higher and when the number of days in June with maximum temperatures <25 °C were fewer. Once nesting began, females typically completed 1 cell per day, the egg being laid on one of the first 3 prey placed in the cell. In the field, eggs hatched in 2–6 d, and larvae completed feeding and began cocoon spinning 5–11 d later. After removal from winter storage, offspring completing development at 29 °C emerged as adults in 22–40 d, with males emerging about 2 d earlier on average. The duration of the postdiapause period leading to adult emergence, however, was not correlated with eventual adult mass in either sex. We discuss interpopulation differences in prey species and nest materials used by I. elegans.