Host plant and immature stages of Setabara histrionica (MacGillivray) (Hymenoptera, Tenthredinidae)

The North American sawfly Setabara histrionica (Tenthredinidae: Heterarthrinae) is previously known only from adult collections but has been hypothesized to feed upon trees in the genus Prunus. We discovered a population of leaf-mining sawflies in Washington on Prunus emarginata and identified it as S. histrionica using combined morphological and molecular analysis. We observed preference in oviposition site selection on the host plant, with most eggs deposited on the margin of the basal third of the leaf, and on leaves growing within 1 meter of the ground. We include a description of the egg, larval stages, mine and phenology of S. histrionica, and an update to Smith's (1971) key to Heterarthrinae larvae.