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 ckd5444@psu.edu and ckd.ento.eco.evo@gmail.com!

Massive thanks to these iNaturalists for contributing samples:
@lumenal
@thurmanjohnson
@friesen5000
@annieliveoak
@chris_nelson
@graysquirrel
@wisel
@ajwright

Posted on May 30, 2024 01:44 AM by charles161 charles161

Observations

Photos / Sounds

Observer

charles161

Date

August 28, 2023 02:03 PM MDT

Description

Potentillae gracilis var flabelliformis

Photos / Sounds

Observer

charles161

Date

August 29, 2023 01:15 PM MDT

Photos / Sounds

Observer

charles161

Date

August 31, 2023 12:58 PM MDT

Description

Potentilla gracilis vr fastigiata

Comments

I will keep looking!
As far as our proptery is concerned, the only relevant host I know of is Drymocallis glandulosa. Well, and of course Rubus species.

Posted by cosmopterix about 2 months ago

Interesting project! What's the best habitat to check for these galls?

Posted by adeans about 2 months ago

@adeans The best habitats to look for these galls really depends on the variety of Potentilla gracilis, as I believe some varieties prefer some habitat over others, but generally you should be able to find them in prairies, sierras, and meadows. I have found them in forestd areas as well, but typically in areas without super dense tree coverage.

Posted by charles161 about 2 months ago

@cosmopterix I have found galls on D. glandulosa! they occur on the stem, petiole and the inflorescence. One organ of the plant I have yet to check is the roots. There is a record of galls from Sequoia National forest on cinquefoil roots.

Posted by charles161 about 2 months ago

Appreciate the update! Interesting stuff.

Posted by wisel about 2 months ago

I will keep my eyes open for them! We don't have any native "true" potentillas around me, but plenty of drymocallis, horkelia, etc, so I'll be checking those.

Posted by graysquirrel about 2 months ago

Thanks for the update! I’ll keep looking for galls on Potentilla and try to ID hosts to the level of variety. It’s good to know that galls occur on Drymocallis glandulosa as I see that plant often but haven’t been checking it closely.

Posted by lumenal about 2 months ago

@lumenal so far every species of Drymocallis I have encountered during my travels in Idaho has had a Diastrophus gall on them. Still working on the taxonomy but they are proabably at least 5 species across Drymocallis.

Posted by charles161 about 2 months ago

@lumenal also the stems of P. gracilis have cryptic galls within them as well. I will tag you in my photos of them.

Posted by charles161 about 2 months ago

Very cool to read this, I will continue to keep an eye out for unusual galls to bring to your attention :)

Posted by thurmanjohnson 26 days ago

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