Yet another new herb gall wasp genus? A new group of Silphium gall wasps

If you read my posts last summer about the "top galls" to look for in 2023, you might recall some discussion of blister-like galls growing on the leaves of Silphium plants. These unassuming leaf swellings are quite unusual amongst Silphium gall wasps, especially given that most species induce only inconspicuous galls hidden within the host plant's tissues, and that the cauline leaves of most Silphium species are quick to shrivel up and break away during the growing season. You can see the original observation of the gall in Salina, Kansas on Silphium integrifolium: https://www.inaturalist.org/observations/167275394

I was fortunate enough to receive some samples of these galls from Ebony Murrell (@egmurrell) last year (more on her later on), and this week I was lucky enough to finally have adults to examine! To nobody's surprise, they are yet another undescribed gall wasp species, but perhaps more surprising is just how different they are from other species, both described and undescribed. An adult female of the new species is shown below and in the associated observation (https://www.inaturalist.org/observations/213220718).

All Silphium gall wasps are currently classified in the genus Antistrophus, a member of the herb gall wasp tribe Aulacideini. While there are 10 species currently classified in the genus (and an 11th on the way in an accepted manuscript of mine), these species are best classified in multiple genera on the basis of genetic and morphological data, and the Silphium gall wasps will become two independent genera. One genus is comprised of small-bodied, reddish gall wasps that primarily induce hidden galls in the stems (currently known as the rufus species complex), while the other genus is larger, black wasps that induce conspicuous galls on the apical stems and in the flowerheads. However, the species reared from the leaf blister galls doesn't resemble either of these putative genera well! As you can see in the below photos, this wasp has a deep red body, and is (relatively speaking) on the larger side of the Silphium wasp size spectrum. There are several differentiating characters on the mesosoma (=thorax in non-wasps), including the texture of the integument and the distinction of the notauli (the submedial line-like impressions) that are especially meaningful. Also interesting are some characters on the head and metasoma (=abdomen in non-wasps), including the protruding medial area on the face, the puncture patterns on the metasoma, and the surface texture of the head. Overall, the morphology of this new species suggests that it may best be classified in a separate genus. This distinction requires validation using genetic data, but this will come later on this summer.

Going further down the rabbit hole, these galls have also been observed on at least two additional Silphium species, S. gracile (https://www.inaturalist.org/observations/12082873) and S. cf radula (https://www.inaturalist.org/observations/165426473). Of course, the identity of the gall inducer on these other species is unknown. Could each Silphium species have its own blister gall wasp species? Or could one species be associated with each of these plants? Do other Silphium species have leaf blister gall wasps? And (the big ticket question) do these galls have any detrimental impacts on plant health like some of the other rosinweed gall wasps? Only further collection and rearing will enable investigation of these questions. Based on what I'm seeing in other Silphium gall wasps, I'm guessing these galls will eventually be discovered in most Silphium species, and each host plant species will have a specialist leaf galler. If you have Silphium species in your area, please consider checking the cauline leaves for unusual blister-like swellings like those shown in the above photos!

Lastly, I need to thank the excellent Ebony Murrell for her endless support in studying the gall communities of Silphium. She has gone above and beyond, and I'd like to think that our collaboration is turning her into a gall-obsessed maniac like the rest of us. She was the first person to bring these leaf blister galls to my attention, and kindly sent me many gall samples, including the gall that yielded the adult female photographed above. Ebony and the rest of The Land Institute (https://landinstitute.org/) have been incredibly supportive of my gall wasp studies, and we're currently devising plans to work on this exciting group of insects and their fantastic host plants.

Posted on Μάιος 03, 2024 0726 ΜΜ by louisnastasi