Midrib and petiole tunnelers in great water dock (Rumex britannica)

The initial observations I am assigning to this species occurred in March 2022, when I found exit holes in midribs or petioles of senesced basal leaves of Rumex britannica. One of these holes proved to be the terminus of a short tunnel that connected to a small chamber inside the midrib/petiole, which contained a few coarse grains of frass (images 4-01 and 4-02). Later, on 7 May at the same site, I noted "a number of tiny, very dark gray or black [adult] weevils" perched on the newly sprouted, living R. britannica basal leaves. "The basal leaves were pretty fresh and reddish-green and mostly not fully unfurled yet" (quotes from my journal).

I examined a few of these living basal leaves more closely a few weeks later in late May. Their midribs and petioles contained several short rows of tiny, neatly round holes in the epidermis, the rows often accented by bright red discoloration of the plant tissue. Black flecks of frass decorated the epidermis surrounding the holes. Within or just underneath the holes, intact yellow eggs or spent black eggshells could be found, some of them topped by a small collection of additional frass (presumably placed there by the mother weevil) (images 1-01 through 1-08). The eggs were similar in color and shape to apionine weevil eggs depicted in Gourlay (1989).

In one such affected area, I found a very tiny weevil larva, apparently just recently hatched, flanked by two spent eggshells (one of these eggshells is shown in image 1-07). A shallow subepidermal mine led away from another such row of holes, and inside the mine was a very young weevil larva accompanied by scattered grains of bright green frass (image 1-09). Dissecting the midrib and petiole tissue further, I found two or three short linear tunnels each containing a young weevil larva, similar in appearance to the others, along with more grains of green frass (images 1-10 through 1-12). I also found a few young Lepidoptera larvae in blotchy cavities they had excavated in the midrib and petiole tissue (see "Lepidoptera" section, below), but these did not show a clear connection to any of the rows of epidermal holes.

On 16 June, I scrutinized the midribs and petioles of another batch of living R. britannica basal leaves from the same site. In these I found more examples of the shallow mines and epidermal holes. I wrote: "[The mines] tended to be only a few centimeters long and dead end in an elliptical area with some blackish material in it; I dissected at least two such endpoints...and found absolutely no disturbance to the [deeper layers of midrib/petiole tissue further underneath] them, indicating these were [apparently not] places where the larva suddenly turned into a borer. Some of these mines originate in what I’m fairly certain are the weevil egg laying areas." The mines I dissected were empty, and my photos show slits or irregular tears in the epidermis covering at least two of the elliptical endpoints, suggesting the responsible insects had evacuated the mines at these endpoints.

Also on 16 June, one of the basal leaf midribs I dissected contained a relatively mature weevil larva in an irregular cavity, accompanied by brownish grains of frass and two immature parasitoid wasps. One of the wasps was a pupa resting near the weevil larva, while the other was a very tiny ectoparasitoid larva attached to the weevil larva's integument near its head (images 3-01 through 3-04). The weevil larva later crawled out of the dissected midrib piece and came to rest on the wall of the rearing container, where it was consumed by the ectoparasitoid larva. Two adult wasps emerged in the rearing container in the ensuing weeks, and the larger and later-emerging of these, which was apparently the ectoparasitoid, is shown in image 3-08.

In late June and early July, several similar-looking, diminutive adult apionine weevils matching the description of P. punctinasum in Bright (1993) emerged from the midribs and petioles collected on 16 June. One of these (image 3-05) was identified to species on BugGuide.net (Belov 2026). This individual "emerged from a ~45mm piece of midrib I isolated in a rearing container...There was a small, weevil-sized chamber in the midrib interior with a short necklike tunnel leading to the exit hole, but I didn’t see any frass accumulation or obvious internal tunneling leading to the chamber. The chamber and short tunnel to exit hole were identical to what I found in the winter collection of overwintered [senesced] midribs/petioles" (quote from journal). The exit hole and inner midrib/petiole chamber of one of the emerged adults are shown in a photo (image 3-06) and a sketch (image 3-07) below.

I found young larvae of this miner/borer tunneling in the interiors of basal leaf midribs of R. britannica in late May. They were pale greenish-white in color, with black head capsules and brown, narrow prothoracic shields (images 5-01 through 5-06).

By mid-June, their feeding had progressed to include short blotchy excursions into portions of the leaf blades adjacent to the midribs (images 6-01 through 6-08). On or about 23 June, I wrote in my journal: "Dock 1: 2 areas of significant frass expulsion along midrib, including one of about 25 square mm; white cocoon spun between leaf and bag. Dock 4: 5 areas of mining + minor frass accumulation, but the frass accumulation is only a fraction of what happened in Dock 1; attached to fibers on [cut end of] petiole, one fuzzy thing that could be a cut-out case from the leaf blade mine as with [M.] Palmer’s find on Erigonium [sic]." In this entry, "Dock 1" and "Dock 4" refer to two separate rearing bags housing basal leaves inhabited by the caterpillars, and "[M.] Palmer’s find on Erigonium [sic]" refers to the moth observed by M. Palmer in Oklahoma and listed in couplet 3b on page 1775 of Eiseman (2025), which mines in Eriogonum midribs and leaf blades and eventually builds a "portable case from an elongate piece excised from the mine" (ibid.). I did not make any further documentation about the possible case attached to the petiole in my rearing container.

Referring again to my journal entry, the "white cocoon spun between leaf and bag" eventually gave rise to a parasitoid wasp. It may have been the work of a parasitized caterpillar.

Unfortunately, I was not able to rear any adults of this moth.