Newly Documented Life History of Tenthredo mellicoxa Provancher (Hymenoptera: Tenthredinidae) from Hydrophyllum virginianum L. (Boraginaceae)

Abstract

the most species-rich genus of Symphyta (sawflies) with more than 1000 described species worldwide—roughly 154 of these found in the Nearctic Region (Taeger et al. 2018, Goulet 2020). Despite their larger body size relative to many sawflies, surprisingly little is known of the biology of this genus in North America—we estimate the life histories of greater than 90 percent remain unknown (Baine et al. 2019). Published host records are documented from a wide diversity of plant families, and while several species appear to be highly specialized, many Palearctic Tenthredo are considered to be mainly polyphagous (Goulet 1996). Herein, we reveal for the first time the life history of T. mellicoxa Provancher, 1878 (Fig. 2) from Virginia waterleaf (Hydrophyllum virginianum L.) (Boraginaceae). As the first member of the T. rufopectus species group with a known life history, we hope this discovery will offer clues to the biology of related Tenthredo species, i.e., T. rufopectus (Norton, 1860), T. nimbipennis Cresson, 1872, T. ruma MacGillivray, 1923, and T. fernowi Goulet and Smith, 1995. As T. mellicoxa appears to be a Hydrophyllum specialist, we predict its kin may also be dietary specialists. In late April 2022, in Brown Co., Kansas (Hamlin, No-man’s Land Farm, (39.90625° lat, -95.58656° long)), TAM observed an abundance of adult T. mellicoxa flying around a large stand of Virginia waterleaf in a mesic, mixed deciduous forest (Fig. 1). Where the adults were abundant, Hydrophyllum, just shy of blooming, stood about 0.5 m tall and was the dominant understory plant. Adult Tenthredo (Fig. 2) were readily collected with a sweep net as they skimmed low above the waterleaf or as they perched on the upper surface of waterleaf leaves. TAM netted four adult females and placed them into separate 50-dram vials, each bearing a single Hydrophyllum leaf. All four females successfully oviposited. Observed details of the life history are as follows. Ova were inserted between the lower epidermis and mesophyll in nonvascular areas of the waterleaf leaf tissue (Fig. 3). The captive oviposition rate was remarkably consistent across the four captured females; each laid 12–15 eggs, with oviposition slowing 48 hours post collection and ceasing altogether 72 hours post collection. Eggs hatched 9–10 days post oviposition and first instar larvae immediately began feeding solitarily within the interior of the leaf blade leaving ovate holes behind (Fig. 4). The cloudy white bloom of early instars was replaced by the characteristic gray dorsum of later instars in the third instar. Like other Tenthredo and related genera, when not actively feeding larvae often coiled themselves on the undersides of the leaves, and when disturbed, would drop from the leaf surface. The fifth, prepupal instar did not form a silken cocoon, but rather was found to shelter in an earthen cell, presumably overwintering until pupation and subsequent eclosion the following spring. The average time from oviposition to prepupa was 21 days. The description of the immature stages of Tenthredo mellicoxa are as follows: Ova PROC. ENTOMOL. SOC. WASH. 124(4), 2022, pp. 884–888