Stem borer (Diptera: Agromyzidae) in Clematis

Record Details

Record no.:	0159
Feeding guild:	Stem borer
Taxonomy:	Diptera: Agromyzidae: cf. Phytomyza sp.
Stages observed:	trace, larva, puparium, adult
Hosts in Clematis:	C. virginiana (virgin's bower)

The pale yellow larva of this borer forms an elongate tunnel in the pith of the stem. Rather than concentrating its feeding in a localized area within the stem (as does another internally feeding agromyzid on Clematis, in record 0158), it travels for a long distance along the main axis of the stem until it completes its development, its tunnel usually spanning multiple internodes.

The tunnel has ragged walls typical of agromyzid borers, and my photos show small liquidy splotches of dark green frass deposited periodically along the length of a tunnel. In one case I also observed the remnants of a larva's cephaloskeleton left behind in its tunnel after a molt. The rear portion of the cephaloskeleton appeared as if divided in two robust arms, suggesting the species belonged to the subfamily Phytomyzinae, which I was later able to confirm with successful rearing. My photos also show that the posterior spiracles of one larva each featured orangish-brown bulbs arranged in a ring and slightly elevated above the surrounding tissue, with the pale center of the ring apparently lacking any bulbs or discernable darkening.

When it is finished feeding, usually somewhere along the length of an internode, the larva establishes a short, curving tunnel outward to just under the bark of the stem. It cuts an exit slit or window in the bark that remains closed for the time being. The larva then pupates here at the terminus of the curved tunnel, the puparium taking on the arching shape of the tunnel in a manner unusual among the stem agromyzids examined in the current study.

Also unusual for a stem agromyzid, the anterior end of the puparium is modified into a peculiar, flat, circular "lid" that remains appressed to the underside of the bark until the adult emerges, at which point the "lid" (along with the exit window in the bark) pops open. The slender, teardrop-shaped anterior spiracles of the puparium arise from the main body of the puparium, projecting from the rim where the "lid" attaches. Aside from the unusual curved shape and modified front end, the puparium possesses unremarkable posterior spiracles that appear to be rather weakly sclerotized, without development of strongly darkened plates or significant horns, although the plates are darker in color than the surrounding integument. Overall the puparium is a whitish color.

I reared a single male adult in spring 2023.

Featured Images

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tunnel is long and narrow with ragged walls and moves through the white pith layer in the core of the stem — Image #: 159-21 Stem of host, dissected, showing larva's tunnel.

slender yellow larva rests in long winding tunnel in green stem interior — Image #: 159-01 Another dissected stem, showing tunnel occupied by larva. (Photo date: July 13, 2022)

larva is about 7 to 8 times as long as wide, pale in color with a black head skeleton and orangeish-brown posterior spiracles — Image #: 159-02 Larva in tunnel in stem. (Photo date: July 13, 2022)

rounded front end of larva showing short teardrop-shaped anterior spiracles projecting from the dorsal surface a short distance back from the very front of the animal — Image #: 159-03 Lateral view, anterior end of larva. Note the shape of the anterior spiracles. Also note the way the posteriormost portion of the cephalopharyngeal skeleton appears divided in two (not three), indicating the larva belongs to subfamily Phytomyzinae.

the anterior spiracles are orangish-brown in color and broader at the tip than the base — Image #: 159-04 Dorsolateral view, anterior end of larva.

the posterior spiracles are also orangish-brown and the bulbs are arranged in a ring — Image #: 159-05 Posterior spiracles of larva.

the mouthhooks can be seen here as a short black hook with a smaller tooth just behind it — Image #: 159-06 Detail, anterior end of larva in lateral view. The image, though mostly out of focus, is intended to give a sense for the appearance of the larva's mouthhooks. (Photo date: July 13, 2022)

the tunnel moves through the pith of the stem core and is partly packed with irregular whitish globules of plant tissue (apparently callus tissue) on either side of the tiny black forked head skeleton — Image #: 159-28 Remnants of a larva's cephalopharyngeal skeleton (black wishbone-shaped object), apparently resulting from a molt, left behind in the larva's tunnel in the stem. Note that the tunnel on both sides of the remains, and especially to the right, appears to be filling in with callus tissue. (Photo date: July 7, 2023)

same as previous photo (0159-28) except a closer view, showing the yellowish material (evidently exoskeleton remnants) accompanying the head skeleton — Image #: 159-29 A closer view of the remnants of a larva's cephalopharyngeal skeleton, apparently resulting from a molt, left behind in the larva's tunnel in the stem. Note that the tunnel on both sides of the remains appears to be filling in with callus tissue.

head skeleton is entirely black and consists of a sickle-shaped mouthhook just in front of a saddle-shaped sclerite, and behind these a single mostly straight bar that stretches back and eventually forks into two slightly curved bars, one bar about half again as long as the other — Image #: 159-30 Remnants of larva's cephalopharyngeal skeleton in tunnel in stem.

the frass is a few sparse dark green clumps smeared on the wall of the tunnel — Image #: 159-07 Larval frass in tunnel in stem. (Photo date: July 13, 2022)

the ragged-walled tunnel leads straight through the pith to a point where it curves outward toward the outer wall of the stem and here the pale whitish, cylindrical puparium is embedded — Image #: 159-08 Endpoint of a larva's tunnel, with a spent puparium partly visible inside. (Photo date: May 3, 2021)

only the posterior half of the puparium is visible in this dissected view of the tunnel, with the anterior half still out of view in the intact portion of the tunnel; the puparium features arching boundary lines between the abdominal segments and paired stublike posterior spiracles — Image #: 159-09 A closer view, endpoint of a larva's tunnel, with a spent puparium partly visible inside. The anterior end of the puparium is out of view as it curves toward the far surface of the stem.

tunnel walls are whitish and rough rather than smooth, with scraps of pith leftover from larva's feeding dangling from the roof and sides of the tunnel — Image #: 159-10 Segment of completed tunnel.

the anterior half of the puparium is is slightly narrower and curves upward at about a 45-degree angle to the posterior half — Image #: 159-11 Spent puparium extracted from stem in early spring. Note unusual curved shape. The age of the stem was unrecorded, but it was likely a second- or third-year stem, with the adult having emerged from the puparium in a prior year when the stem was younger. (Photo date: May 3, 2021)

the posterior spiracles are orangish-brown knobs each consisting of a small round plate slightly elevated above the surrounding tissue, the two plates separated by about two and a half times their width; there are no clearly discernible spiracular horns — Image #: 159-12 Posterior spiracles of spent puparium.

a strip of bark from the stem, viewed from the exterior, showing a small round hole flanked by flakes of bark — Image #: 159-13 Hole in bark through which adult emerged.

the puparium is curved and whitish and sits embedded in a rough-walled tunnel in the pith — Image #: 159-14 A second example of a spent puparium in a stem. The stem has been split open lengthwise for this view and the bark of the stem has been mostly removed. (Photo date: May 11, 2021)

the anterior end of the puparium is flattened into a disc — Image #: 159-15 Spent puparium in stem.

once again the arching boundary lines between the body segments are visible — Image #: 159-16 Spent puparium in stem.

spiracles are orangish-brown knobs with the bulbs in a ring; the anal scar sits slightly in front of them on the concave (ventral) surface of the puparium — Image #: 159-17 Posterior spiracles of spent puparium.

the flattened disc shape of the anterior end of the puparium appears as a circle in this front view — Image #: 159-18 More or less frontal view of anterior end of spent puparium in dissected stem.

the seam between the edge of the disc and the rest of the puparium has partly split open, allowing the adult to emerge from this now-empty puparium — Image #: 159-19 Detail, lidlike anterior end of spent puparium.

the spiracles are teardrop-shaped with the tips rounded and broader than the bases — Image #: 159-20 Lidlike anterior end of spent puparium; the anterior spiracles, projecting from the main body of the puparium on the rim where the lid was attached, are indicated by the red arrows. (Photo date: May 11, 2021)

there is a semicircular slit in the bark through which the adult fly emerged — Image #: 159-22 Exterior view of affected stem at location where an adult recently emerged in springtime.

flat piece of bark with rear end of puparium dangling underneath and front end of puparium out of view beneath a semicircular flap in the bark exterior — Image #: 159-23 Outer layers of tissue of a dissected stem, at the location of an adult fly's recent emergence, with the spent puparium still lodged in the tissue. The posterior part of the puparium is visible but out of focus below the brownish outer layers of stem tissue, and an exit window in the stem and the lidlike anterior end of the puparium have popped or hinged open to allow the adult to emerge.

the bark flap is hinged open at about a 45-degree angle and the puparium — Image #: 159-24 Another view, outer layers of tissue of a dissected stem, at the location of an adult fly's recent emergence, with the spent puparium still lodged in the tissue. The posterior part of the puparium is visible but out of focus below the brownish outer layers of stem tissue, and an exit window in the stem and the lidlike anterior end of the puparium have popped or hinged open to allow the adult to emerge.

the bark flap and the puparium — Image #: 159-25 The outermost layer of stem tissue and the lidlike anterior end of the puparium have both popped or hinged open to allow the adult fly to emerge. Barely discernable are the anterior spiracles of the puparium, out of focus and in shadow below the double "lid.".

fly with red compound eye, gray thorax featuring long curved setae on its dorsal surface, and a slender abdomen banded yellow and gray beneath long flat transparent wings — Image #: 159-26 Lateral view of adult fly reared from the puparium in the previous images (0159-22 through 0159-25). (coll. as puparium 01/05/23, adult em. 03/27/23)

the wing bases and halteres are also yellow — Image #: 159-27 Dorsal view, reared adult.

Page created: October 16, 2023. Last update: February 9, 2026