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.

Polistes metricus

Paper wasps tend to be quite variable in color and pattern, even within one species, so identifying them is not easy, even for experts. Case in point is differentiating the common Northern Paper Wasp, Polistes fuscatus, from the very similar Polistes metricus.

Dark specimens of the Northern Paper Wasp are nearly identical to Polistes metricus, a consistently dark species that I have found to be less abundant than P. fuscatus in areas where their geographic ranges overlap. One fairly reliable, if subtle, clue is the shape of the abdomen. Note that the abdomen of P. metricus is highly convex on the underside, creating a nearly acute angle with the underside of the petiole (stalk-like segment connecting abdomen with thorax). This is usually much less pronounced in P. fuscatus.

Another difference is in the face of the female wasps. Females of P. metricus have an almost completely red face, the black markings confined to the ocellar triangle. Ok, so what is an “ocellar triangle?” Most wasps have a trio of tiny, “simple” eyes at the top of the head, between the large compound eyes. These simple eyes are usually arranged in a triangular pattern. Females of the Northern Paper Wasp have the black marking extended from the ocelli to the base of the antennae. Males of both species have square, yellow faces.

Female P. fuscatus. Note black face; gently curved venter of abdomen
Female P. metricus. Note convex venter of abdomen.
Female P. metricus. Note all-red face.

Polistes metricus ranges from extreme southwestern Ontario and Maine south to Florida, and west to southern Ohio, Nebraska, Kansas, Oklahoma, and eastern Texas. The Northern Paper Wasp has a much larger geographic distribution.

I was fortunate enough to find three embryonic nests of P. metricus on a recent trip to Missouri. Two nests were under the exterior of a recessed door frame in Excelsior Springs. The other was under the roof of a sign and kiosk at Little Dixie Lake Conservation Area west of Kingdom City. These are typical nesting sites, though they can also be built among shrubbery and other more exposed locations.

Both the Northern Paper Wasp and P. metricus prey mostly on caterpillars, chewing up the larvae and feeding them to their own grubs back at the nest. Look for the adult wasps on flowers as they fuel themselves on nectar. They can also be seen around aphid colonies, lapping up the “honeydew” secreted by the aphids as a waste product.

Source: Buck, M., Marshall, S.A. and Cheung D.K.B. 2008. Identification Atlas of the Vespidae (Hymenoptera, Aculeata) of the northeastern Nearctic region. Canadian Journal of Arthropod Identification No. 5: 492 pp. (PDF version).