May 30, 2024

Diastrophus Diaries 01: Species of the Western Nearctic and the Mystery of Diastrophus fusiformans

Hello iNaturalists and Gall Enthusiasts!

If you don't know me, My name is Charles Davis and I am an Entomology PhD Candidate at Penn State studying the taxonomy and evolution of gall wasps in the family Cynipidae. One of the main chapters of my dissertation is a revision of the genus Diastrophus within the Nearctic (North America). Diastrophus is a group of gall wasps that makes galls on plants of the family Rosaceae, mainly cinquefoils of the tribe Potentilleae and brambles of the genus Rubus. These gall wasps occur across the Holarctic (Northern Hemisphere) and there is even a species in the Neotropics (South America), but the bulk of the diversity is found within North America. There are 19 described species, but no one has actively worked on this group in over 20 years and there have been no new species in North America in over 100 years!

The main goal of my dissertation is to bring stability to this group of insects by assessing the validity of the species using an integrative dataset including morphological, biological and genetic data. As I evaluate the identities of these species, I also get the opportunity to describe new species of Diastrophus when appropriate.

One area of North America which has been under estimated in it's diversity for these insects is the West. As it stands now, there are only 4 species of Diastrophus that occur east of the Mississippi: Diastrophus austrior, Diastrophus kincaidii, Diastrophus turgidus, and Diastrophus fusiformans. Diastrophus kincaidii and D. austrior both make multichambered stem galls on Rubus parviflorus, and it is likely that they are the same species. Diastrophus turgidus creates multichambered stem galls on Rubus idaeus strigosus. Diastrophus fusiformans induces multichambered galls on the petioles and stems of an herbaceous cinquefoil in the genus Potentilla.

Whiles I have been able to either collect or source samples and specimens of the Rubus associated Diastrophus in the West, I have not been as fortunate with collecting D. fusiformans. There are currently only a few species described from Potentilla, and across literature it is generally assumed that these gallers are not as diverse as their Rubus associated congeners.

The only western cinquefoil galler is D. fusiformans, but at its description the host plant's species was not specified only that it belonged to the genus Potentilla. Across museum records, Diastrophus reared from Potentilla gracilis is considered to be D. fusiformans, but many of these specimens do not match the original specimens used to described the species. Potentilla gracilis is a very likely suspect for the host plant as it is one of the most common cinquefoils in the western United States, but the species is extremely polymorphic as there are several varieties across its geographic range. Furthermore, when D. fusiformans was described most cinquefoils were lumped into Potentilla, and there has been a decent amount of shifting since then with genera such as Drymocallis, Horkelia, and Horkelliela being resurrected/ established. There could be just as likely that D. fusiformans originates from galls one of these plants genera as it is likely to come from galls from Potentilla (sensu stricto).

Through my field work in the Idaho area and from contributions from several enthusiasts, we have found multiple species of galls from cinquefoils in Drymocallis, Horkelia and Potentilla. Of the wasps I have reared from these cinquefoils so far, the most likely culprit to the mystery of D. fusiformans are galls of one of the varieties of P. gracilis. I have received galls from the petioles of P. gracilis gracilis, and I have personally collected petiole galls from P. gracilis, but so far none of the adults I have reared from these galls have matched the wasps in the type series. I have also not seen galls on P. gracilis that make perceptible swellings of the stem like the galls of the type series. However, I have collected P. gracilis stems that house cryptic, imperceptible galls induced by an undescribed Diastrophus species! Additionally, the original wasp specimens used to describe D. fusiformans are from Colorado, while the majority of the fresh material I have examined originates from the Pacific North West.

I hypothesize D. fusiformans is on a variety of P. gracilis that I have yet to collect, or it is present only on a variety that is found within CO. I am hoping to do more field work out west soon to collect more galls across these amazing plants, but it is unlikely that I will be able to get complete coverage of P. gracilis's enormous range. If you want to help me solve the mystery of this elusive gall wasp species, please comment below, reach out to me via iNaturalist Dms, or email me at and!

Massive thanks to these iNaturalists for contributing samples:

Posted on May 30, 2024 01:44 AM by charles161 charles161 | 3 observations | 9 comments | Leave a comment