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.

Wasp Wednesday: More on Isodontia

Last week I showed a short video introducing you to a puzzling phenomenon in suburban areas of eastern North America. A quick review: Some homeowners in the U.S. and Canada, upon sliding open their windows, have been startled to discover the tracks filled with dry grass, dead insects, and live “worms.” Even the odd wasp has flown out. What is going on here?

Certain solitary wasps called “grass-carriers,” genus Isodontia, seem to have found window tracks make ideal nesting sites. Each female wasp makes her own nest, usually a series of cells along the length of the tunnel (though one species forms a communal brood chamber much like a bird nest).

After selecting a site, she goes in search of tree crickets or small katydids which she paralyzes with her sting and carts back to the nest. Once a cell is filled with victims, she lays an egg, and makes a partition of dry grass. Then she begins the provisioning process again, until the tunnel is filled with cells. She finishes the nest by plugging the entrance with dry grass, such that it looks as if someone has shoved a tiny broom into the hole, handle-first. Her job complete, she leaves permanently.

The larva that hatches from each egg grows by consuming the fresh prey insects. These are the “worms” that people find in the window tracks. After finishing its meals, the larva then enters the pupal stage, and an adult wasp emerges later (the following summer in northern climates).

Grass-carriers are not pests, and not aggressive towards people or pets. You will not get stung unless you physically grab a female. Males have no stinger at all.

My initial theory was that the wasps much prefer using natural cavities in dead wood, but few such natural resources exist in suburban settings. My revised theory is that the wasps may find window tracks to be a superior nesting site. Perhaps the space is “roomier” than conventional natural cavities. Maybe the parasitic flies and wasps that plague Isodontia have not yet caught on to the new nesting strategy, therefore giving Isodontia an advantage that yields more success in terms of offspring reared to maturity.

Please tolerate these wasps if you can, they are fascinating to watch. Otherwise, simply flushing the wasp and cleaning the track should discourage her from trying again.

Meanwhile, look for the adult wasps on wildflowers, especially sweetclover, sumac, and grape. They will also frequent aphid colonies, lapping up the “honeydew” secreted by the sap-sucking pests. Notice that these wasps usually have the wings splayed at rest, in contrast to other sphecid wasps that habitually fold their wings flat over their abdomen while sipping nectar.

The species shown in the images accompanying this article is Isodontia mexicana, very common over most of the eastern U.S. There are also five other species occurring in North America.

Cicada shells

Almost every summer I receive several questions about strange bugs sitting motionless on fence posts, tree trunks, and other upright objects. I thought I would do an entry here to explain the mystery.

The descriptions of the creature that people give to me vary from a “cross between a crayfish and a beetle,” to “gnome-like,” and all mention the large claws on the “front end.” No one has ever seen anything like it, and it is no wonder. Normally, the creature they are seeing lives fairly deep underground.

I have taken to calling these bugs “former insects,” since the objects being seen are not entire insects but the cast exoskeletons (“skins”) of cicadas, family Cicadidae. Scientists call these shed skins “exuviae,” all that remains behind when the insect molts from the nymph stage into an adult. Dragonflies, damselflies, stoneflies, and mayflies also leave behind these ghostly but tangible shadows of their former selves.

The average person rarely sees an animated version of a cicada nymph because the mature nymphs emerge from the soil at night, climbing the nearest vertical surface, and then splitting the exoskeleton down the middle of the back to allow the escape of the wet, soft adult under the cover of darkness. The fresh, pale adults are extremely vulnerable, but their principle predators are diurnal, so they avoid instant death by coming out at night. Cicadas are large insects, so they can be quite conspicuous under the best of cryptic circumstances.

An adult "annual" cicada, Neotibicen sp.

Most people in eastern North America see the shells of “annual” or “dog day” cicadas in the genus Tibicen. They then hear the loud “songs” of the adult male cicadas, not often seeing the insect that makes such a racket. Despite the name, annual cicadas still take a long time to grow up. They live a subterranean existence as nymphs, sucking the sap from tree roots for at least five to seven years. The generations are staggered, though, so some adults emerge every summer. This is in contrast to the synchronous broods (populations) of the periodical cicads or “17-year locusts” that emerge en masse in the late spring or early summer every 13 or 17 years, depending on the latitude of the population.

I like to get questions about things like cicada exuviae. It shows that people are observant and curious, two qualities I really admire in my own species