Biology. D. ertterae induces inconspicuous galls on the petioles, stems and inflorescences of P. gracilis. Each gall is the width and length of the final instar. These chambers are lignified and surrounded by soft layers of pithy tissue throughout larval development. Later in the season, the soft pith surrounding the chamber hardens, but is still not as hard as the sclerenchyma of the chamber. The chambers of D. ertterae typically occur between the epidermis and the pith either independently or in a series of two to five galls in succession. Unlike D. moniquae, the chambers are not in the centre of the stem as the chamber width of D. etterae is much smaller than the diameter of a P. gracilis stem, as seen in X-ray images (Figure 8(D)).
This description is based on material reared from stems of P. gracilis var. fastigiata, P. g. var flabelliformis, P. g. var indiges, P. g. var. nr. pulcherrima and P. g. var. owyheensis. However, P. gracilis is a highly polymorphic species complex that requires further revision, with five varieties recognised in Flora of North America and some species formerly being classified as varieties now recognised as distinct species such as P. brunnescens and P. pulcherima (Luc Brouillet, Digital Flora of North America 2014). Potentilla gracilis is one of the most widespread cinquefoil species in western North America, and many of the varieties of this complex are geographically isolated from each other which could serve as a further driver for species diversification within Diastrophus.
In addition to D. ertterae, there are also conspicuous petiole galls induced on P. gracilis varieties that resemble the galls of D. fusiformans. The adult wasp morphology of these undescribed species does not match the type series of D. fusiformans, and there is significant variation between adults reared from different varieties, thus this species requires further revision.