A One Millimeter Mystery

Sometimes a minor disaster turns into something positive, like a fallen tree limb revealing a hidden relationship between a wasp, a fly, and a saw blade. All of this in the front yard of our house in Leavenworth, Kansas, USA.

Yikes!

Upon returning from a week-long road trip in late June that took us into Arkansas, southeast Missouri, and southern Illinois, we pulled up in front of our home to discover that a massive tree limb had broken off the ancient Pin Oak, miraculously landing between our house and the neighbor’s house, with no significant damage to either structure. This makes the third such incident since we moved here in May of 2021.

Ugh, I've got a big job ahead of me.

We do not own a chainsaw, so I started cutting off the smaller branches with a couple of manual saws that we have. The odor emanating from the cuts attracted several wood-boring beetles right away. Close behind them were parasitoid wasps looking to oviposit on the eggs or larvae of their beetle hosts. While this was entertaining, and resulted in adding a new longhorned beetle to our home list of animal life, a more intriguing scenario attracted my attention.

My saws bring all the wood-boring insects to our yard.

Awhile later, I noticed several minute black specks moving over the surface where I removed the branch. They had to be insects, but I could not tell what kind. I took a few photos, and was shocked to find they were miniscule wasps. Some had greatly elongated abdomens, others did not.

At least the long-bodied wasp is a Synopeas sp. platygastrid wasp.

I submitted a couple of photos to the Hymenopterists Forum, an interest group on Facebook, to solicit an identification from true expert specialists. They did not disappoint.Bob Zuparko suggested they might be in the family Platygastridae, and that was confirmed by Kendrick Fowler. He also suggested a genus, Synopeas, and subgenus Dolichotrypes. He wasn’t sure the wasp with the “normal” abdomen was even a male of the same thing. It might be something else entirely.

Fowler went on to explain that this is a known behavior, the attraction to freshly cut oak, and that the host is presumably some sort of fly in the gall midge family Cecidomyiidae. That floored me because until then I thought all gall midges attacked foliage and/or stems. Also, how did this behavior evolve? There weren’t saws of any kind until recently, in the evolutionary sense. Beavers?

I decided to dig a little deeper and discovered that there is a genus of gall midges that oviposits in this situation: Xylodiplosis. I went back out and looked at some of the branch stumps again. Amazingly, I managed to find a few gall midges laying eggs. They were much more difficult to photograph than the wasps, and not as numerous, either. Why the wasps arrived before their hosts is a mystery to me. Oh, and there is also Ledomyia, another genus of gall midge that lives in freshly cut wood like this. I’m honestly not positive which one I documented.

Female gall midge, probably Xylodiplosis sp., ovipositing.

It turns out that Xylodiplosis gall midges have all kinds of enemies. They are attacked by nematode worms (family Ektaphelenchidae), mites (family Tarsonemidae, tribe Pseudotaesonemoidini), even another kind of gall midge (Lestodiplosis xylodiplosuga). Most of the research on these has come out of Europe, by the way, so it may not apply here in North America.

Back to the wasps. Synopeas larvae do do not begin to develop until after the host gall midge larva leaves its lair in the xylem wood to pupate in the soil. The adult wasp emerges about fourteen days after the unparasitized adult of the host gall midge, according to one source (Rock and Jackson, 1985). In their findings, the rate of parasitism was about five percent, and that included another platygastrid wasp in the genus Leptacis.

In at least one of my photos of the wasps (first photo of them in this post), I can barely make out the short spine on the scutellum (top rear of thorax) that separates Synopeas from similar genera of platygastrids. Identification of species is not possible without examination of a specimen under high magnification. There are currently forty-four known species of Synopeas found in the Nearctic (North America more or less north of Central America). I will leave you to go farther down the research rabbit hole.

Sources: Awad, Jessica N. 2020. “Building a diagnostic framework for the genus Synopeas Forster (Hymenoptera: Platygastridae: Platygastrinae) based on reared specimens from Papua New Guinea.” Master of Science thesis, University of Florida.
Crawford, J.C. and J.C. Bradley. 1911.”A New Pelecinus-like Genus and Species of Platydateridae,” Proc. Ent. Soc. Wash. 13: 124-125.
Gagne, R.J. 1985. “Descriptions of new Nearctic Cecidomyiidae (Diptera) that live in xylem vessels of fresh-cut wood, and a review of Ledomyia (s. str.),” Proc. Ent. Soc. Wash. 87(1): 116-134.
Hooper, D.J. 1995. “Ektaphelenchoides winteri n. sp. (Nematoda: Ektaphelenchidae) from wood fly larvae Xylodiplosis sp. (Diptera: Cecidomyidae),” Fundamental and Applied Nematology 18(5): 465-470.
Khaustov, Alexander A., Arne Fjellberg, and Evert E. Lindquist. 2022. “A new genus and species of Pseudotarsonemoidini (Acari: Heterosstigmata: Tarsonemidae) associated with xylophagous gall midges in Norway,” Systematic and Applied Acarology 27(6): 1020-1034.
Rock, E.A. and D. Jackson. 1985. “The biology of xylophilic Cecidomyiidae (Diptera), Proc. Ent. Soc. Wash. 87(1): 135-141.
Skuhrava, M. and K. Dengler. 2001. “Lestodiplosis xylodiplosuga sp. n., a predator of Xylodiplosis sp. (Diptera: Cecidomyiidae): morphology of developmental stages, biology and behaviour,” Acta Societatis Zoologicae Bohemicae 65(1): 57-68.