Stem borer (Diptera: Drosophilidae) in Amphicarpaea

Record Details

Record no.:	0048
Feeding guild:	Stem borer
Taxonomy:	Diptera: Drosophilidae: Stegana vittata
Stages observed:	trace, larva, puparium, adult
Hosts in Amphicarpaea:	A. bracteata (American hog peanut)

fly with yellowish-brown thorax, bright red compound eyes, and dark wings held in an unusual arched posture, forming a sort of dome over the abdomen — Adult fly ex hog peanut stem.

I found a puparium and a larva of this fly in 2021 and 2022, respectively, each in a tunnel in a lower stem of the host. In the 2021 example, the plant had wilted due to the tunneling in the stem.

The larva's body was yellowish-brown and somewhat translucent, with the tracheae and other internal structures partially visible through the integument. In the front was a pair of curved mouthhooks aligned exactly side-by-side. The paired anterior spiracles of the larva each consisted of a central stalk, thickened at the base and tapering toward the apex, bearing a number of distally projecting papillae or tubules, the whole structure at least vaguely reminiscent of a Christmas tree. The posterior spiracles were knob-shaped and reddish-brown in color, much darker than the relatively pale ground color of the larva's body.

The puparium from 2021 was cylindrical, rather elongate (roughly 4.5 times as long as wide), with a modified anterior end that was flattened into a slighly concave disc, the anterior spiracles projecting at approximately a right angle to the plane of the disc. Within the containing stem, this blunted anterior end of the puparium sealed off access to the insect's tunnel (and thus to the rest of the insect's body) neatly and cryptically, a behavior termed "phragmosis" that is also exhibited by many other burrow-forming animals (Rice 1969; Wheeler 1927; Wheeler & Hölldobler 1985). An adult emerged from the puparium the following spring, and B. Sinclair (pers. comm.) identified it.

The weight of evidence points to S. vittata as the stem borer that created the tunnels, and not as a secondary inhabitant of the stems; for further discussion of this evidence, see Eiseman & van der Linden (2024). To my knowledge, no Stegana species in North America has been previously recorded as a stem borer in an herbaceous plant (but see Laštovka & Máca (1982), O'Grady (2002), and Plakidas (2023) for examples of other worldwide collecting and rearing records).

Anatomical terminology for spiracles in this report is based on the description of Drosophila melanogaster in Wipfler et al. (2013).

Featured Images

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shriveled leaves dangling from a thin vine contrast with healthy leaves of other plants behind them — Image #: 48-01 Host plant wilting due to a Stegana borer in the stem. (Photo date: August 23, 2021)

curved stem split open revealing rough-walled tunnel — Image #: 48-09 Tunneling accomplished by the borer larva in the lower stem.

dark yellow larva stretched out inside hollow stem surrounded by scrapings on the sides of the tunnel walls — Image #: 48-20 Larva (recently deceased) in its tunnel in the lower stem of the host. (Photo date: August 20, 2022)

tubelike structures (tracheae?) are visible through the larva's integument as is the dark head skeleton — Image #: 48-22 Larva in tunnel in stem.

the spiracles are shaped like a coniferous tree — Image #: 48-17 Anterior end of larva, with the red arrows indicating the larva's anterior spiracles.

the tubercle-like posterior spiracles are darker than the surrounding body tissue of the larva and protrude from the rear end — Image #: 48-18 Some views of the posterior spiracles of a dead larva.

mouthhooks, side by side, extend slightly from larva's front end; they are black and sickle-shaped — Image #: 48-19 Anterior end of larva, showing mouthhooks.

the outer walls of the stem are peeled away to show the cylindrical puparium which fits neatly inside and fills the tunnel diameter — Image #: 48-03 Puparium in tunneled-out Amphicarpaea stem (dissected). (Photo date: August 21, 2021)

the flat front end of the puparium is angled and the curved surface of the puparium's middle is covered with remnants of stem pith — Image #: 48-04 Puparium in dissected stem.

the surface is reddish and sclerotized with three horizontal bands — Image #: 48-05 Flattened, slightly concave anterior surface of puparium.

viewed edge-on the spiracle is somewhat flattened and protrudes stiffly from the hardened front end of the puparium — Image #: 48-06 Views of the flattened or slightly concave anterior end of the puparium (top two panels); an anterior spiracle projecting at a right angle from the plane of the anterior end of the puparium (just above center in the bottom left panel); detail of anterior spiracle, edge-on view (bottom right panel).

the spiracles taper toward their tips and feature numerous short tubelike protrusions studded along their length — Image #: 48-07 Anterior spiracles of puparium.

spiracles are dark, almost black, and shaped like short knobs and each projects at an oblique angle from the rear end — Image #: 48-08 Posterior spiracles of puparium.

it's a straw-colored drosophilid with black and brown stripes, red compound eyes, and darkly pigmented wings curved over the abdomen — Image #: 48-10 Reared adult, lateral. (Photo date: April 8, 2022)

the wings are held in an unusual arched posture, forming a sort of dome over the abdomen — Image #: 48-11 Reared adult, dorsal.

with cupped wings, pigmented black and dark gray — Image #: 48-12 Adult.

red compound eyes and antennae with plumose bristles — Image #: 48-13 Head of adult.

rear end tapers to a rounded point sheltered by the curved wings — Image #: 48-15 Posterior end of adult, rear view.

the adult fly is cleaning itself by rubbing its front legs over its head — Image #: 48-23 Adult, grooming.

fly rubbing its front legs together beneath its rounded head dominated by huge red eyes — Image #: 48-24 Adult, grooming.

Specimen Data for Images

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Field photo taken 08/23/21 (01); coll. 08/20/22, photos same day (dead larva in stem: 17-20, 22); coll. 08/21/21, photos of puparium and larval damage taken on 08/21/21-08/22/21 (03-06, 08-09), adult em. 04/07/22 and photos of adult and spent puparium taken on 04/07/22-04/08/22 (07, 10-13, 15, 23-24).

References

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Eiseman, C.S. and J. van der Linden. 2024. New rearing records of Drosophilidae (Diptera: Ephydroidea) from plant tissue. Proceedings of the Entomological Society of Washington 126(2): 172-182. [return to in-text citation]
Laštovka, P. and J. Máca. 1982. European and North American species of the genus Stegana (Diptera, Drosophilidae). Annotationes Zoologicae et Botanicae (Slovenské Norodné Múzeum) 149:1-38.[return to in-text citation]
O'Grady, P.M. 2002. New records for introduced Drosophilidae (Diptera) in Hawai‘i. In Records of the Hawaii Biological Survey for 2000—Part 2: Notes. Bishop Museum Occasional Papers 69:34-35.[return to in-text citation]
Plakidas, J. 2023. Stegana antigua Wheeler 1960. Diptera: Drosophilidae - Stegana antigua. Contributor post on BugGuide.net. Retrieved July 9, 2024 from https://bugguide.net/node/view/2322536.[return to in-text citation]
Rice, M.E. 1969. Possible boring structures of sipunculids. Am. Zoologist 9: 803-812.[return to in-text citation]
Wheeler, D.E. and B. Hölldobler. 1985. Cryptic phragmosis: the structural modifications. Psyche 92 (4): 337–353.[return to in-text citation]
Wheeler, W.M. 1927. Physiognomy of insects. Q. Rev. Biol. 2: 1-36.[return to in-text citation]
Wipfler, B., Schneeberg, K., Löffler, A., Hünefeld, F., Meier, R., and R.G. Beutel. 2013. The skeletomuscular system of the larva of Drosophila melanogaster (Drosophilidae, Diptera) – a contribution to the morphology of a model organism. Arthropod Structure & Development 42(1): 47-68.[return to in-text citation]

Page created: November 12, 2023. Last update: February 1, 2026