Another Micro Mystery

One way that I find insects in winter is by scraping off paper wasp nests that have been abandoned by the past season’s generation of wasps, and placing the nests in a transparent container. What usually happens is that one or more kinds of insects will eventually emerge from these nests. Allow me to share one example.

Dibrachys sp.

I started collecting recent, abandoned paper wasp nests (Polistes metricus is the common species around our home here in Leavenworth, Kansas, USA) when I saw a social media post by Sloan Tomlinson (@thatwaspguy on Instagram). He had reared a type of small parasitoid wasp as a result: the eulophid wasp Elasmus polistis. That species is a parasitoid of the brood (larvae, pupae) of paper wasps. Cool. I could add another species to my home biodiversity list.

So many tiny wasps!

I was excited to find, in January of 2022, that this method had worked, as I saw tiny wasps running around inside the container with the old paper wasp nest. Photographing the little creatures, and then cropping those images, it became apparent that what I had was not what I expected. I was left with a mystery that took me awhile to solve, even though I’m fairly good at researching.

I eventually found a journal article chronicling a study of various parasitoids of paper wasps collected from nests in Missouri, the state immediately adjacent to Kansas. In fact, Leavenworth is right across the Missouri River from Missouri. One of the creatures listed was a wasp in the family Pteromalidae. They had only eighty-seven specimens, though. I was looking at hundreds by the time they finished emerging.

Male wasps attempting to mate with a female.

The species name given was Dibrachys cavus. More recently, it has been revealed to be a “species complex,” and has a new assigned name: Dibrachys microgastri. It represents one of *three* species, any one of which could potentially be my creature. Collectively, they are parasitoids of pretty much any insect with complete metamorphosis. That makes it difficult to determine exactly which one I have. Even placing a specimen under a microscope might not be enough magnification for these two-millimeter wasps.

Dibrachys is unusual for a single genus of wasps in having such a wide range of potential hosts. At least some species, or perhaps most, are hyperparasitoids of tachinid flies and braconid or ichneumon wasps that are themselves parasitoids of moth pupae. This makes me wonder if these minions are part of this puzzle that I documented in 2022.

I find unsolved mysteries intriguing, and delight in them even if I never reach any verifiable conclusion. There will always be *something* that defies explanation in the natural world.

This little cobweb weaver spider may have been making a killing, literally.

Sources:Gibson, Gary A.P., John T. Huber, and James B. Woolley (eds). 1997. Annotated Keys to the Genera of Nearctic Chalcidoidea (Hymenoptera). Ottawa, Ontario: NRC Research Press. 794 pp.
Peters, R.S. & Baur, H. 2011. A revision of the Dibrachys cavus species complex (Hymenoptera: Chalcidoidea: Pteromalidae). Zootaxa, 2937 (1), 1-30.
Whiteman, Noah K. and Brett H.P. Landwer. 2000. Parasitoids Reared From Polistes (Hymenoptera: Vespidae: Polistinae) Nests in Missouri, With a State Record of Elasmus polistis Burk (Hymenoptera: Elasmidae). J Kansas Ent Soc 73(3): 186-188.