Species of the genus Chaetarthria Stephens, 1833 (Coleoptera, Hydrophilidae) are small, oval beetles living near or in various bodies of water (Smetana 1988). Although not much is known about their biology, they are most commonly found burrowed in the mud or sandy banks in the day near rather calm water, and are most active at night, as suggested by long series collected at light traps (Miller 1974). The genus is characterized not only by its small size and its ability to contract into a loose ball but also, most distinctly, by the two large excavations on the first abdominal segments (Smetana 1988). Formerly, it was the only genus in the subfamily Chaetarthriinae (or tribe Chaetarthriini, depending on the classification followed, e.g. Bousquet et al. 2013), until Short & Fikáček (2013) elevated its rank to subfamily, positioning the genus as the sole representative of the tribe Chaetarthriini sensu nov. and incorporating other genera (i.e., Ancaena and Crenitis in Canada) and another tribe, Anacaenini sensu nov., within Chaetarthriinae.
Currently, the genus comprises over 40 species worldwide with the vast majority found in the New World (Smetana 1988). In Canada, it is represented by three species: Chaetarthria atra (LeConte, 1861), known from Quebec and Nova Scotia; Chaetarthria nigrella (LeConte, 1861), recorded in British Columbia; and Chaetarthria pallida (LeConte, 1861), previously only documented in Saskatchewan (Bousquet et al. 2013). Chaetarthria pallida can be distinguished from its congeners by its pale dorsum and the plates that males bear on the inner surface of their protibias. Identifying females can be more challenging, especially in comparison with several paler species, but their geographical range and the micropuncture pattern of the pronotum will reliably distinguish them, at least in northeastern America (Miller 1974).
The presence of C. pallida (originally described as Cyllidium pallidum LeConte, 1861) was documented by Wickham (1895) in Ontario and Quebec, but no vouchers were available to confirm these records during both the generic revision by Miller (1974) and the Canadian Hydrophilidae revision by Smetana (1988), leading to the rejection of their occurrence. However, Smetana (1988) suggested that the species was likely to occur in Ontario, a theory that is now confirmed.
Miller (1974) delineated three allopatric groups with several morphological distinctions. While he retained these groups within the species limit, he noted that they might eventually emerge as distinct subspecies or even species. According to his characterization, the present specimen belongs to his "central group" as it falls within its geographical boundaries and exhibits the typical micropunctures on the elytra. Additionally, he mentioned that the type of Cyllidium nigriceps LeConte 1861 (currently considered a junior synonym of C. pallida) belongs to that group of species; if his species groups are confirmed to be separate species, that name would gain priority for the species occurring in eastern Canada.
The only collected specimen (Fig. 1) was found in Athan Park, a large urban park in the Ottawa region. The author deployed a white cross-vane trap, coupled with a UV blacklight (Fig. 2), within a forest of poplar and linden trees that had been heavily damaged by a previous storm. This capture method has proven highly effective in gathering significant numbers of small to medium-sized beetles, especially on warm nights. Moreover, it can be easily adapted to utilize various attractants, including UV lights, as demonstrated here, as well as pheromones, kairomones, or even alternative mixtures such as fermenting baits or ethanol/vinegar solutions.
Examination of the material collected in this single trap allowed the author to identify a small, distinctive beetle that matched the description of C. pallida according to Smetana's (1988) revision. Given the complexity of the Hydrophilidae family, the author sought confirmation from Dr. Matthew Pintar, who verified its identity, marking not only the first record for the species and genus in Ontario but also a significant finding for the northeast. The proximity of the collection event to the province of Quebec, coupled with the absence of unique ecosystems in the collecting area, suggests that the species likely occurs in Quebec as well, potentially overlooked in previous inventories as it has been the case with numerous other small, less traditionally attractive beetles (e.g. Bédard et al. 2024).
Many thanks to Dr. Matthew Pintar for confirming the identification of the specimen.
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Fig. 1. Chaetarthria pallida (LeConte, 1861) collected in the Ottawa region (#NBC004678)
Fig. 2. White cross-vane trap, paired with UV lights, in Athans Park forest blowdown (Ottawa, ON, Canada)
