Stem borer (Diptera: Agromyzidae) in Galium

Record Details

Record no.:	0244
Feeding guild:	Stem borer
Taxonomy:	Diptera: Agromyzidae: cf. Ophiomyia sp.
Stages observed:	trace, larva, puparium, adult
Hosts in Galium:	undetermined G. sp. 2

Tiny pale grublike larva extracted from orangish-brown tunneling under surface of stem at node — Young larva in between inner and outer layers of stem node

The externally visible sign of this borer consists of rusty reddish-brown discoloration to the stems, especially at the nodes but also in the form of shallow subsurface linear tracks along the internodes. Depth-wise, the larva's feeding mostly occurs in the space between the outer sheathlike layer and the tough inner core layer of the stem, and the outer layers of tissue are generally thin enough to allow the rusty discoloration to show through. The larva tunnels shallowly along the lengths of the internodes, with the tunnel sometimes spiraling around the stem but usually not doubling back on itself (i.e., an individual larva apparently only travels the length of a given internode once). Upon reaching a node, the larva performs some more concentrated excavation of the inner tissues, and if it is still young, after a little while it will leave the partially excavated node and enter the next internode to continue its movement and feeding along the length of the stem. After moving through multiple nodes and internodes in this way, the larva finishes its development in a node, which it more or less completely hollows out with the possible exception of some of the inner core layer. The larva then prepares an exit window in the epidermis and forms its black puparium just beneath the exit window within the hollowed-out node.

The anterior spiracles of the final-instar larva and puparium feature an archlike protrusion at the base of the spiracle stalk that projects slightly above the main body of the puparium, but not as prominently as the distal end of the stalk.

I reared adults in spring 2022. The adult and puparium both resemble Ophiomyia, and the recently described O. galiodes has been reared from Galium plant material in spring (see Lonsdale 2021), but its feeding mode was unrecorded.

This borer should not be confused with the Ophiomyia stem miner that may also be found in Galium stems in the study area (record 0242). The miner creates a blotch mine in the internode just under the epidermis, causing the affected area to turn a silvery or light brown color.

Featured Images

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Image #: 244-01 Affected stem, showing rusty discoloration at node. (Photo date: July 9, 2021)

Image #: 244-09 Affected stem, showing rusty discoloration centered at node. (Photo date: August 13, 2021)

Image #: 244-10 Stem of Galium ?triflorum adjacent to node, showing spiralling rusty-colored tunnel of larva visible through stem epidermis.

Image #: 244-11 Stem of Galium ?triflorum adjacent to node, with outermost layer of stem tissue removed, revealing spiralling rusty-colored tunnel of larva.

Image #: 244-02 Young larva (arrow) in a dissected node. (Photo date: June 28, 2021)

Image #: 244-03 Young larva in a dissected node. (Photo date: June 28, 2021)

Image #: 244-04 Ventral view of young larva from node of host.

Image #: 244-05 Ventral and ventrolateral views of young larva's anterior end.

Image #: 244-06 Posterior end of young larva.

Image #: 244-07 Lateral view of young larva from node of host.

Image #: 244-08 Another ventrolateral view of larva's anterior end.

Image #: 244-12 Node inhabited by mature larva that has finished its feeding and mostly hollowed out the interior of the node. The small hole is part of the exit window the larva has established prior to pupating in the node. (Photo date: August 13, 2021)

Image #: 244-13 Mature larva in hollowed-out node. (Photo date: August 13, 2021)

Image #: 244-14 Mature larva, removed from node.

Image #: 244-15 Anterior end of mature larva in lateral view, showing anterior spiracles (top arrow) and mouthhooks (bottom arrow).

Image #: 244-16 Anterior end of mature larva in lateral view.

Image #: 244-17 Anterior end of mature larva in ventrolateral view.

Image #: 244-31 Views of a mature larva's anterior spiracles (bottom right panel) and posterior spiracles (other three panels).

Image #: 244-18 Puparium, freshly formed. (Photo date: August 14, 2021)

Image #: 244-19 Anterior (left panel) and posterior (right panel) spiracles of freshly formed puparium.

Image #: 244-34 Anterior spiracles of freshly formed puparium. Note the archlike protrusions at the bases of the spiracle stalks; see next image (0244-33) for better views of this feature on another puparium.

Image #: 244-33 Several views of single anterior spiracles from a spent puparium, showing the archlike protrusion at the base of the spiracle stalk.

Image #: 244-20 Overwintered dead stem of the host, dissected at the node where a larva finished its feeding and pupated in the prior growing season. The tough inner cylinder of plant tissue is all that remains of the inner tissue of the node; the rest has been eaten away. Note that even the outermost layers of the tough inner cylinder were scraped away by the larva, and the remaining tissue blackened as a result. The black puparium of the culprit is out of focus, above center. (Photo date: February 11, 2022)

Image #: 244-21 Puparium in hollowed-out node of overwintered dead stem of the host. (Photo date: April 4, 2022)

Image #: 244-22 Another view of a puparium in a hollowed-out node of an overwintered dead stem, with this view showing the anterior spiracles in silhouette against the exit window cut by the larva prior to pupating.

Image #: 244-23 Anterior spiracles of puparium, in silhouette against the exit window cut by the larva prior to pupating.

Image #: 244-24 This photo shows the same specimen as the previous photo (0244-23), but from the opposite side, a view of the exterior of the inhabited node. The posterior end of the black puparium is visible to the right of center. Just to the left of center, the exit window for the adult-to-be is a lighter shade of gray than the surrounding tissue, a result of light being transmitted through it in this partially backlit view.

Image #: 244-25 Detail of exit window from previous photo (0244-24).

Image #: 244-27 Reared adult, fronto-lateral view.

Image #: 244-29 Reared adult, dorsolateral.

Image #: 244-35 Dorsal-anterior portion of puparium, which split off from the rest of the puparium upon the successful emergence of the adult fly.

Specimen Data for Images

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Coll. 07/09/21, photographed same day (01); coll. 06/28/21, photographed same day (02-08); coll. 08/11/21, damage and larva photographed 08/13/21 (09-17, 31) and larva's fresh puparium photographed on 08/14/21 (18-19, 34); coll. 02/11/22, photographed same day (20); coll. 04/02/22, photographed 04/04/22 (21-25); coll. 02/11/22, em. 03/15/22 (26-28); coll. 04/02/22, em. 04/14/22 (29-30); coll. 04/02/22, adult em. by 04/16/22, puparium photographed on 10/27/23 (33, 35).

References

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Lonsdale, O. 2021. Manual of North American Agromyzidae (Diptera, Schizophora), with revision of the fauna of the “Delmarva” states. ZooKeys 1051: 1–481. https://doi.org/10.3897/zookeys.1051.64603 [return to in-text citation]

Page created: October 27, 2023. Last update: March 10, 2026