Occupancy surveys and uncovering range-wide patterns of gall wasp diversity in oaks

Because of the spectacular diversity of herbivorous insects, there is a long history of studying the diversity of insects that live on plants, particularly in forests. Oak gall wasps (Hymenoptera: Cynipidae), which form distinctive structure on oaks for larval development, offer a compelling system to evaluate insect species richness in oak woodlands. Galls exhibit high species diversity, but they vary widely in detectability: some are large or abundant (e.g., rough bullet gall, Disholcaspis quercusmamma), while others are rare or easily overlooked (e.g., Druon galls, which form small fuzzy balls on the leaf midvein). In this study, we use two approaches for estimating gall wasp richness on white oaks in the eastern US: traditional rarefaction modeling and Bayesian multi-species occupancy modeling. Rarefaction curves standardize sampling effort but assume equal detectability among samples, whereas occupancy models explicitly account for detection probability and observer bias but can be computationally intensive. Together, our analyses highlight how hierarchical occupancy models can complement classic methods for quantifying insect diversity on host plants.