Stem borer (Diptera: Agromyzidae) in Phlox (#0376) [0376]

Order	Diptera
Family	Agromyzidae[M,L,P]
Lower taxon	Phytomyzinae
No. spp. involved	One confirmed [1-C]
Feeding mode	Stem borer
Host plant	Wild blue phlox, Phlox divaricata (Polemoniaceae)

Tunnels in stems of the host. Tunnel walls are distinctly ragged as with other dipteran borers. Pupation is external, off the plant, with the puparium very small (~2mm in length) and yellow. The population of this borer observed by the author in 2023 suffered very high larval mortality, with roughly 66-75% of examined larvae having been killed by a parasitoid wasp or otherwise dying of natural causes before reaching the puparium stage. Some of the parasitoid wasp larvae spun cocoons in the tunnels not far from their hosts' remains, as shown in the images below.

IMG#: 0376-01
Dead larva in stem. (Photo date: June 30, 2023)

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IMG#: 0376-02
Parasitized borer larva in stem. The parasitoid is visible just below the borer larva's cephalopharyngeal skeleton, clinging to its integument. (Photo date: June 21, 2023)

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IMG#: 0376-03
Parasitized borer larva in stem. The parasitoid is visible on the borer larva's body to the right of center.

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IMG#: 0376-04
Another evidently parasitized larva. Note the object (a parasitoid life stage?) adhering to the larva's posterior end.

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IMG#: 0376-05
Larva, posterior end, with apparent parasitoid life stage attached.

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IMG#: 0376-06
Larva, posterior end, rear view, with apparent parasitoid life stage attached.

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IMG#: 0376-07
Detail, rear view of larva's posterior end, with apparent parasitoid life stage attached. Note also the characteristics of the posterior spiracle that is more or less in focus to the right of center.

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IMG#: 0376-08
Another borer larva, removed from its tunnel in the host stem; ventral (top panel) and dorsal (bottom panel) views.

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IMG#: 0376-09
Anterior (top panel) and posterior (bottom panel) spiracles of larva.

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IMG#: 0376-10
Larva from stem of host. (Photo date: June 22, 2023)

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IMG#: 0376-11
Several views of a larva. Top left: larva's anterior end, showing cephalopharyngeal skeleton; top right: detail, front of cephalopharyngeal skeleton, showing teeth on mouthhooks; bottom left: anterior spiracle; bottom right: rear half of larva, showing posterior spiracles in dorsolateral view.

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IMG#: 0376-12
An affected stem containing remains of an agromyzid borer larva (right bracket) with the parasitoid larva that consumed the agromyzid nearby (left bracket), in a cocoon it has spun in its host's tunnel.

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IMG#: 0376-13
Remains of borer larva from previous photo (0376-12).

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IMG#: 0376-14
Parasitoid larva in its cocoon from previous photo (0376-12).

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IMG#: 0376-15
A second example of a tunneled stem containing the remains of the borer larva with its parasitoid in a cocoon nearby. Here, the most visible remnant of the agromyzid larva is the cluster of calcareous discs leftover from the interior of the larva -- an internal body part the parasitoid was evidently unable or unwilling to eat. For more about such discs, see Ellis (2023). Just to the right of the discs in this image is the parasitoid larva, faintly visible through the thin wall of its spun cocoon.

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IMG#: 0376-16
A cluster of calcareous disks, all that's left of the body of an agromyzid larva consumed by a parasitoid.

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IMG#: 0376-17
Larvae of this borer exit the stem to pupate. This image shows the exit hole left by a fully mature larva when it vacated the stem in order to pupate in the soil.

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IMG#: 0376-18
Distinctive feeding pattern of an agromyzid larva in its tunnel in the host stem.

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IMG#: 0376-19
Tunnel in lower stem of host.

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IMG#: 0376-20
Puparium, formed outside the stem.

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IMG#: 0376-22
Anterior spiracles of puparium.

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IMG#: 0376-23
Posterior spiracles of puparium.

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IMG#: 0376-24
Puparium, with millimeter ruler for scale.

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IMG#: 0376-25
Puparium, after overwintering, with the cephaloskeleton of the final-instar larva appressed to the inner wall of the puparium on the anterior end.

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IMG#: 0376-26
Detail, anterior end of puparium after overwintering, showing cephaloskeleton of final-instar larva visible in the puparium interior.

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Specimen data for images

Coll. 06/29/23, photos taken 06/29/23-06/30/23 (01, 03, 12-17, 19); coll. 06/21/23, photos taken 06/21/23-06/22/23 (02, 04-11, 18); coll. 06/29/23, puparium found outside of stem in rearing container on 07/01/23, photos on 07/02/23 (20-24); coll. 06/29/23, puparium by 07/xx/23, puparium photographed after overwintering on 03/17/24 (25-26).

References

Ellis, W.N. 2023. Agromyzidae. In Plant Parasites of Europe [website]. Retrieved October 26, 2023 from here.

Page created 10/26/23. Last update: 11/19/24