Metriocnemus eurynotus (Holmgren, 1883)

Eiseman, Charles S., Namayandeh, Armin, Linden, John Van Der & Palmer, Michael W., 2023, Metriocnemus erythranthei sp. nov. and Limnophyes viribus sp. nov. (Diptera: Chironomidae: Orthocladiinae): leafminers of monkeyflowers, speedwells, and other herbaceous plants, with new observations on the ecology and habitats of other leaf-mining Chironomidae, Zootaxa 5249 (1), pp. 41-68 : 51-54

Metriocnemus eurynotus (Holmgren, 1883)

( Figs. 6–7 )

Material examined. USA: IOWA: Winneshiek Co., Decorah, Van Peenan Spring at Van Peenan Park, 43.312834, -91.776010, 14.iv.2022, em. 6.v.2022, leg. J. van der Linden, ex thallose liverwort (1Ô, 1 pupa, 1 larva, ANC) ; same location, 10.v.2022, preserved 16.v.2022, leg. J. van der Linden, ex Impatiens sp. (1 larva, USNM) ; same but preserved 21.v.2022 (2 pupae, ANC) ; same but em. 21.v.2022, (1♀, ANC) ; same but em. 23.v.2022 (2♀, 2 pupae, 2 larvae, ANC) ; same but em. by 27.v.2022 (2♀♀, ANC) ; same location, 17.v.2022, em. 21.v.2022, leg. J. van der Linden, ex Impatiens sp. (1Ô, 1 pupa, 1 larva, USNM) ; same but em. 23.v.2022 (1♀, ANC) ; same but em. by 27.v.2022 (2♀♀, 1 pupa, ANC) ; same location, 17.v.2022, leg. J. van der Linden, ex Veronica sp. (1 larva, USNM) ; same but em. 29.v.2022 (1♀, 2 pupae, 1 larva, USNM) ; OREGON: Lane Co., Blue River , 44.1535, -122.328, 3.vi.2022, leg. M. W. Palmer, ex Petasites frigidus (2 larvae, USNM) .

Biological notes. It appears that nothing has been published previously about the larval habits of this Holarctic species ( Saether 1989, 1995). John van der Linden (JvdL) was examining 5- to 10-cm tall sprouts of jewelweed ( Balsaminaceae : Impatiens sp. ; very likely I. capensis Meerb. ) on the margin of a rocky spring-fed pool in Iowa in early May when he noticed mines in cotyledons of some of the plants ( Fig. 6a ). Chironomid larvae were present inside the mines and on the surfaces of the cotyledons and appeared to move freely between these niches ( Figs. 6b–h ). The mines, which were only observed in Impatiens cotyledons and not in the developing true leaves, consisted of short, irregular tunnels into the tissue emanating from a central blotch. Tears or holes in one or both epidermises in the central blotch area allowed larvae to enter and exit the mines. In some older mines, the central area’s epidermis had fallen away completely, resulting in a hole in the cotyledon. In captivity, larvae were observed to move around in the mines and feed on mesophyll. The mines contained sparsely scattered green or greenish-brown pellets or rods of frass. One feeding larva with similarly colored material in its gut was observed depositing excrement in the mine. As many as five larvae were observed inside or on the edges of one mined area on a single cotyledon (video at https:// youtu.be/Tt4 SYMH 3U5U). Larvae interacted vigorously, especially upon physical contact or very close proximity, to which they responded with thrashing movements or by appearing to bite or nip each other’s bodies ( Fig. 6i ).

Most captive larvae soon exited the mines permanently. They switched to feeding externally on the cotyledons ( Fig. 6j ). In containers holding multiple larvae, two or three individuals fed communally on a cotyledon and, in some cases, consumed it entirely before reaching maturity. Full-grown larvae pupated exposed on the cotyledon remnants or on the moist paper towel bedding or sides of the containers ( Fig. 6k ). Nine adults emerged ( Fig. 6l ). Several other individuals reached pupation. However, they failed to emerge as adults, with some darkened, mature pupae crawling around the rearing container actively, only to perish a short time later.

Larvae of M. eurynotus were not observed to initiate new mines in pristine cotyledons. Although no other insects were observed feeding on or in them, we cannot exclude the possibility that there was some initial damage to the cotyledons that allowed the larvae to enter them. A week after the larvae were first discovered, more M. eurynotus larvae were found on leaves of Veronica sp. in the same rocky spring, both moving around on the leaf surfaces and feeding within leaf mines of M. erythranthei ( Fig. 12a ; videos at https://youtu.be/n9J6RJ0-DnI and https://youtu.be/qQ6k2eWBz_I). One male was also reared from a thallose liverwort collected from this site in midApril, although the larva was never actually observed in this case. There were mines in the liverwort, at least some of which appeared to be agromyzid in origin. It is conceivable that the M. eurynotus individual fed as a larva inside these. Before starting the rearing, the undersides and rhizoids of the thalli were thoroughly washed and massaged in water to remove the substrate. The pupa of this individual was found loose in the rearing container on 5 May, and examination of the thalli at this time revealed that a few of them showed evidence of heavy external feeding; no possible source for this was found other than the M. eurynotus , and the damage was similar in appearance to the external feeding on the jewelweed cotyledons.

In Oregon, Mike W. Palmer ( MWP) collected larvae of M. eurynotus on leaves of Petasites frigidus along with M. erythranthei and the Metriocnemus species discussed below.

Figures

FIGURE 6. Metriocnemus eurynotus associated with Impatiens sp. cotyledons. a. cotyledon with mine containing two larvae; b–f. larvae in mines; g. two larvae in mine; h. three larvae in mine; i. two larvae interacting; j. older larvae feeding externally; k. pupa on cotyledon surface; l. reared adult female.

FIGURE 7. Metriocnemus sp. “Oregon”, larva (a–c). a. habitus; b. antenna; c. labrum and premandible; d. mandible; e. mentum; f. posterior portion of the larva.

FIGURE 12. Secondary inhabitants in leaf mines of Metriocnemus erythranthei on Veronica sp. a., larva of M. eurynotus (upper left) and two larvae of M. erythranthei (center and upper right); b., larva of Limnophyes sp. (prob. L. viribus) (lower left) in vacated mine.