The goldenback fly, Pterallastes thoracicus Loew (Diptera: Syrphidae), is an uncommon eristaline flower fly occurring in central and eastern North America. Its previously published range extends north from Nebraska to Connecticut and south from Mississippi to South Carolina, United States of America.
The genus Pterallastes Loew contains four valid species. Three species are Palearctic in distribution (P. bomboides Thompson and P. bettyae Thompson in China, P. unicolor (Shiraki) in Japan), whereas Pterallastes thoracicus is Nearctic. In North America, Pterallastes is easily diagnosed from similar genera by the dense yellow pollen and pile covering the thorax, in combination with the wing vein R4+5 with a dip into wing cell r4+5 (Skevington et al. Reference Skevington, Locke, Young, Moran, Crins and Marshall2019; Fig. 1). Palearctic members of the genus can be distinguished from other genera by the following combination of characters: eye bare, arista bare, wing vein M1 processive anteriorly, wing cell r1 open to the wing margin, metasternum bare, and hind femur without an anterobasal patch of short, dense black setulae (Thompson and Rotheray Reference Thompson and Rotheray1998). Pterallastes is currently placed within the tribe Milesiini and the subtribe Temnostomina. A recent molecular phylogeny of eristaline Syrphidae recovers Pterallastes in a clade with other members of Temnostomina but places Hemilampra Macquart and Neoplesia Macquart, members of the tribe Brachyopina, as sisters to Pterallastes within this clade, rendering Temnostomina paraphyletic (Moran et al. Reference Moran, Skevington, Kelso, Mengual, Jordaens and Young2022).
Figure 1. ♀ Pterallastes thoracicus (CNC2063426) from St. Williams Conservation Reserve, Norfolk, Ontario: A, head in anterolateral view; B, dorsal habitus; and C, lateral habitus. All scale bars represent 2 mm.
Due to a lack of behavioural and habitat observations associated with collected specimens, current knowledge as to Pterallastes’ natural history is limited. Skevington et al. (Reference Skevington, Locke, Young, Moran, Crins and Marshall2019) report P. thoracicus adults being found in hardwood forests and a single specimen being collected from a cypress swamp in Mississippi (Canadian National Collection of Insects, Arachnids, and Nematodes 2024, Jeff_Skevington_Specimen26324). Collection specimen data suggest that the species is active from May to October across its range, although a majority of specimens were reported from May to August. Skevington et al. (Reference Skevington, Locke, Young, Moran, Crins and Marshall2019) describe hilltopping behaviour (defined in Skevington Reference Skevington and Capinera2008) observed in Pennsylvania, United States of America and notes floral visitation to Ceanothus Linnaeus (Rhamnaceae), Solidago Linnaeus (Asteraceae), and Viburnum Linnaeus (Viburnaceae). Larvae for the genus are unknown.
The purpose of this scientific note is to document the first known records of Pterallastes thoracicus in Canada, to describe the known Canadian habitat in as much detail as possible, to collate all available natural history observations on the species, including floral records, and to update the overall known range of the species using community science data. Much of the natural history information shared here derives from community science observations, demonstrating the scientific relevance of the monitoring efforts of iNaturalist participants.
Materials examined
Collection abbreviations:
Canadian National Collection of Insects, Arachnids, and Nematodes (Ottawa, Ontario): CNC
University of Guelph Insect Collection, Guelph, Ontario: DEBU
Cleveland Museum of Natural History, Cleveland, Ohio: CLEV
CANADA: Ontario: Elgin County, Newport Forest, (42° 37′ 52″ N, 81° 46′ 43″ W), 31.vii–11.viii.2005, Malaise trap, A.K. Dewdney, (1♂ DEBU, debu00406294/CNC2063424); Norfolk County, St. Williams Conservation Reserve (42° 42′ 06.12″ N, 80° 26′ 56.4″ W), 27.viii.2023, hand-netted, B. Gallant, A. Dabrowski (1♀, DEBU, CNC2063425); same data except (42° 42′ 03.3″ N, 80° 26′ 50.4″ W), 01.x.2023, hand-netted, blooming Symphyotrichum, A. Dabrowski, B. Gallant (1♀, DEBU, CNC2063426).
Specimens were photographed using a Leica M205-C stereoscope and the Leica Application Suite X software (LASX; Leica Microsystems GmbH, Wetzlar, Germany), using the focus-stacking option, version 5.1.0.25593.1. Individual images were then focus-stacked outside LASX using ZereneStacker, version 2020-05-22-1330 (Zerene Systems LLC, Richland, Washington State, United States of America). Adobe Photoshop 2015, version 20150529.r.88 (Adobe, San Jose, California, United States of America) was used for photo retouching and plate assembly.
These three specimens represent the only known Pterallastes thoracicus records for Canada. The first Canadian specimen was a male collected in 2005 from a Malaise trap situated along a river landing in the Newport Forest in West Elgin, Ontario, Canada. This record was included in an unpublished report on the area (Dewdney Reference Dewdney2011) but was overlooked in Skevington et al.’s (Reference Skevington, Locke, Young, Moran, Crins and Marshall2019) flower fly field guide. In 2023, two specimens were collected via handnet in late August and early October from a Carolinian forest in St. Williams Conservation Reserve, Norfolk County, Ontario. As the only known records for Pterallastes thoracicus in Canada, an understanding of their natural history conditions may reveal important information for future targeted monitoring efforts.
The Newport Forest property is a 44.5-ha (110-acre) area of mixed habitat, featuring lowland and upland forest, meadow, floodplains, and ephemeral wetlands. The site is part of the Brantford Soil series, consisting of Orthic Regosols soils composed primarily of silt and clay, with variable drainage on moderate slopes (Schut Reference Schut1992; Ontario Ministry of Agriculture, Food, and Rural Affairs 2023). Although not considered a significant groundwater recharge area, the site is listed as having a highly vulnerable aquifer (Ontario Ministry of the Environment, Conservation and Parks 2023). The area features a hogback ridge, with the Thames River to the north and an abandoned creek bed (the former Blind Creek, now an ephemeral wetland) to the south. The unique landscape elements contribute to diverse microhabitats and mature ecological systems. Significant biological inventory efforts for the area confirm high diversity and rare flora and fauna species (Dewdney Reference Dewdney2011).
The habitat complexes within the Nursery Tract of the St. Williams Conservation Reserve properties support exceptional biodiversity, with documented reports of multiple rare species and habitat types (Draper et al. Reference Draper, Gartshore and Bowles2003; Ontario Ministry of Natural Resources 2005). The property’s eastern corner is dominated by Carolinian forest, riparian zones, and a wetland complex associated with Dendrick’s Creek. The area in which the P. thoracicus specimens were collected is composed of alluvial Gleyed Melanic Brunisols soils, which contain a high percentage of silt and are reflective of wet conditions (Ontario Ministry of Agriculture, Food, and Rural Affairs 2023). The site is a significant groundwater recharge area and is listed as a highly vulnerable aquifer (Ontario Ministry of the Environment, Conservation and Parks 2023). The St. Williams Wetland is a designated provincially significant wetland, hosting both swamp and marsh habitat (Ontario Ministry of Natural Resources and Forestry 2024).
Specimen CNC2063425 was collected at the edge of a mixed-wood Pinus strobus Linnaeus (Pinaceae) forest, along a walking trail flanked on both sides by riparian vegetation (Table 1) as it transitions to shallow swamp habitat (Fig. 2C). Specimen CNC2063426 was collected from edge habitat transitioning from a Picea abies (Linnaeus) H. Karsten (Pinaceae) strip to a dry meadow edge to cultural agriculture plots (Fig. 2B; Table 1).
Table 1. Plant survey of St. Williams conservation reserve collection points. Data were collected from a preliminary plant survey conducted 5 July 2024 at CNC2063425 (site A; 42° 42′ 06.12″ N, 80° 26′ 56.4″ W) and CNC2063425 (site B; 42° 42′ 03.3″ N, 80° 26′ 50.4″ W) collection sites
Figure 2. Site images of the eastern extent of St. Williams Conservation Reserve: A, satellite imagery of portions of St. Williams Wetland; B, panoramic image of CNC2063425 collection site, facing south; and C, panoramic image of CNC2063426 collection site, facing south.
In addition to the three Canadian records discussed in the present paper, 50 specimens of P. thoracicus were sourced from the Canadian National Collection of Insects, Arachnids, and Nematodes database (2024), and 55 records are located in various institutions in the United States of America, as reported from the Global Biodiversity Information Facility (2024). None of the physical specimens from the United States of America were examined in the present study, and the authors defer to existing species determinations.
As of 12 July 2024, 372 observations of P. thoracicus by 246 unique observers were reported in iNaturalist (2024). The authors confirmed species identification using the distinctive yellow pollen and pile covering the thorax, in combination with the dip in vein R4+5. All community identifications of P. thoracicus were deemed correct, suggesting that the species is distinct enough from local syrphid fauna that community scientists and iNaturalist identification software can identify individuals from suitable images with confidence.
In total, 480 records of Pterallastes thoracicus across North America are reported from various databases (Fig. 3). One specimen in the Canadian National Collection of Insects, Arachnids, and Nematodes, seven records from various institutions, and 10 observations reported in iNaturalist did not include collection coordinates, were considered duplicate entries, or had unsatisfactory positional accuracy, and therefore were excluded from mapping in the present study.
Figure 3. Range map of Pterellastes thoracicus records across central to eastern North America: A, Newport Forest specimen CNC2063424; and B, St. Williams Conservation Reserve specimens CNC2063425 and CNC2063426. Geographic coordinates were sourced from the Canadian National Collection of Insects, Arachnids, and Nematodes, the Global Biodiversity Institute Facility, and iNaturalist databases. Figure was created using SimpleMappr with the geographic projection (epsg:4326; Shorthouse Reference Shorthouse2010).
This paper details the first records of Pterallastes thoracicus in Canada. Independent survey efforts in southern Ontario now reveal three records since 2005, often in proximity to wet or moist mature-forest sites, officially extending the range of P. thoracicus to Canada. Previous institutional records note P. thoracicus for the states of Alabama, Connecticut, Georgia, Illinois, Kansas, Kentucky, Maryland, Mississippi, Missouri, Nebraska, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee, Virginia, West Virginia, and Wyoming, United States of America (Skevington et al. Reference Skevington, Locke, Young, Moran, Crins and Marshall2019). The inclusion of community science observations extends the species’ range to the states of Arkansas, Delaware, Indiana, Iowa, Louisiana, Minnesota, New York, Oklahoma, Texas, and Wisconsin, United States of America (iNaturalist 2024). Overall, many iNaturalist observations are positioned within the modelled range for Pterallastes thoracicus (using MaxEnt; Skevington et al. Reference Skevington, Locke, Young, Moran, Crins and Marshall2019), but several records from Wisconsin and Minnesota are outside the predicted range. These recent observations, including our reported Canadian records, indicate possible range expansion for this species and highlight the need for range modelling that includes contemporary data.
The nearest observations to both Canadian Pterallastes records occur on the south side of Lake Erie along the Ohio shoreline. The nearest observations to the Newport Forest specimen (CNC2063424) is an institutional record (CLEV specimen CMNHENT0062021) and iNaturalist observation (87751549) approximately 107 km to the southeast in North Madison, Ohio. The nearest observation to the St. Williams Wetland specimens is an iNaturalist observation (187449708) less than 95 km to the southeast in Erie County, Ohio. In contrast to Canadian records, more than 30 community observations of P. thoracicus are reported approximately 10 km from the south shore of Lake Erie. These recent iNaturalist observations highlight existing southern P. thoracicus populations that may function as a source population for an emerging northern range expansion into Canada; they also indicate a need to refine range modelling for the species using community science data.
Community science contributions are capable of generating novel natural history information. Although not fully explored in the present paper, community science observations capture in situ interactions between target organisms and their habitat. Little is documented in terms of P. thoracicus behaviour, floral associations, or site use, but the iNaturalist images assessed as part of the present study did reveal new insights. Many of the iNaturalist observations document individuals raising their forelegs, behaviour that is displayed in both sexes (iNaturalist 2024). This behaviour can be seen in other syrphid taxa and is thought to be a behavioural mimicry attempt but is not otherwise reported for P. thoracicus. Floral associations assessed by the authors have been expanded to include 38 additional plant genera across 16 families (Table 2). This new information suggests P. thoracicus is a fairly generalist nectar feeder and visits nonnative varieties. Collating observational data for this species using iNaturalist records demonstrates the potential for community scientists to capture unique ecological information and to contribute to the natural history understanding of our regional flower flies.
Table 2. Tentative Pterallastes thoracicus floral associations sourced from iNaturalist observations
Acknowledgements
The authors thank Steven Paiero, curator of the University of Guelph Insect Collection, for sharing specimen data. They also thank the Thames Talbot Land Trust for access to Newport Forest records and the many people involved in documenting the site’s diversity. The participation and contributions of iNaturalist users and community scientists are celebrated. And of course, the authors thank the Land for its gifts and commit to honouring the responsibilities and the lives sacrificed for this work. The specimens are collected from the treaty lands of the Mississaugas of the Credit and traditional territories of the Anishinaabe, Attawandaronk, and Hodinöhsö:ni’ (Haudenosaunee) Confederacies. As settlers to this area, the authors acknowledge the ways academic institutions can reinforce colonial violence and challenge our colleagues to prioritise meaningful collaboration with our Indigenous partners and the Land.
Competing interests
The authors declare that they have no competing interests.
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