Paraphaenocladius exagitans subsp. exagitans

Paraphaenocladius exagitans exagitans (Johannsen, 1905)

and Paraphaenocladius impensus impensus (Walker, 1856)

(Fig. 13)

Material examined. USA: IOWA: Allamakee Co., Yellow River State Forest, 43.174434, -91.221148, 13.x.2017, em. 23.xi.2017, J. van der Linden, ex Marchantia polymorpha (1♀, P. exagitans exagitans, ANC); same but em. 2.xii.2017 (1♀, P. exagitans exagitans, USNM); same but em. 1.i.2018 (1♀, P. impensus impensus, USNM); same but em. 15.i.2018 (1Ô, P. exagitans exagitans, ANC); same but em. 29.i.2018 (1♀, P. impensus impensus, ANC); same but em. 12.ii.2018 (1♀, P. exagitans exagitans, USNM); OREGON: Lane Co., Blue River, 44.1535, -122.328, 28.iv.2022, em. 29.iv–7.v.2022, leg. M. W. Palmer, ex Marchantia sp. (2ÔÔ, 1♀, 1 larva, P. exagitans exagitans, USNM; 1Ô, P. exagitans exagitans, ANC).

Biological notes. The nominate subspecies of P. exagitans is known from Ontario and throughout the USA as well as China and Japan. It has previously been reared from moist soil at the margins of rivers, streams, seepage, and springs, but none of the published label data mention herbivory (Saether & Wang 1995). Paraphaenocladius impensus s. str. is recorded from Manitoba, Minnesota, Greenland, and northern Europe, where it has been found in moist soil at the margins of lakes, springs, meadows, and alder carr; there have likewise been no observations of herbivory in this species (Saether & Wang 1995).

Nematoceran larvae were found mining in thalli of the liverwort Marchantia polymorpha L. (Marchantiaceae) in Iowa in mid-October 2017. The habitat was a ditch in which the soil varied from saturated to submerged in several centimeters of standing water. The larvae initially formed digitate mines with short radiating galleries, and later formed brown blotches. Unlike Metriocnemus mines, which have frass scattered throughout, these mines were mostly clean with frass accumulating around the rim of the entry hole. The first adult of P. exagitans emerged on 23 November from an oblong, dark brown cocoon, composed of frass, which was formed on the surface of a thallus (Figs. 13a, b). Its pupal exuviae were left on the surface of the cocoon. The frass appeared to have been produced by the larva feeding on the surface of the thallus. Two other adults of P. exagitans and three of P. impensus emerged over the next few months. Two larvae were observed mining in fresh thallus growth in late December. Unfortunately, none of the larvae were preserved, and we are unable to confirm that either Paraphaenocladius species feeds as a thallus miner. The photographed larvae are all Sciaridae, and although this rearing effort did not produce any adult sciarids, in 2022 JvdL reared adult sciarids from similar larvae found mining liverwort thalli at another site in Iowa. The sciarid larvae pupated inside their mines.

A collection of Marchantia cf. polymorpha in Oregon in late April 2022 produced four adults and a larva of P. exagitans exagitans, along with eight adults of Boreochlus persimilis. Although some empty mines of an unknown insect were seen in a nearby colony of this plant, none were observed in the collected sample, and since the rhizoids of the collected plants were thick and encompassed a substantial amount of mucky organic matter, it is unclear what these midges were feeding on as larvae. The repeated rearing of Paraphaenocladius adults from collections of Marchantia suggests more than a casual association with this plant, but further investigation is required to determine whether the larvae feed as thallus miners, as secondary inhabitants in sciarid mines, as external feeders on the thallus surface or concealed in the rhizoids, or some combination of these.