*Listroderes* of North America
Weevils in the genus Listroderes originate in Argentina, but have been introduced into many other parts of the world including Australia, New Zealand, South Africa, Asia and North America. In the USA they have been found in most states south of Kentucky and Oklahoma. They are commonly observed on iNaturalist, with over 500 observations as of November 2020, putting them as one of the top 10 most observed weevil taxa in the USA.
Morrone 1993 [1] synonymised L. difficilis with L. costirostris. Until now, this was the work that I was aware of and was following for my identifications. Under this framework there are only two species in North America (L. costirostris and L. apicalis), which are relatively straightforward to distinguish. Easy peasy and I was merrily putting all North American non-apicalis as L. costirostris.
However, I recently started encountering things which began to unsettle me. @victorengel's comment finally inspired me to pursue this further, and so tonight I managed to find Morrone 2002 [2]. In this paper, Morrone shows that the male genitalia of L. costirostris and L. difficilis are sufficiently different to reinstate L. difficilis as a valid species. Regretably, he offers no external characters to differentiate the species and ominously states that "females of these species are impossible to separate".
Unfortunately, it gets even worse. Most populations of Listroderes outside of Argentina are parthenogenetic, with males either rare or unknown. At this point, therefore, I do not see how we can tell which species these populations belong to.
There are colour variations in the North American Listroderes which might prove to be species-specific (e.g. compare between almost wholly dark specimens with a pale chevron anterior of the elytral declivity [3-7] and specimens with bicoloured elytra with a strong contrast between the pale disc and darker lateral and apical regions [8-12]). I'd love to know if Charlie O'Brien had some way of separating the two (his comment on BugGuide [13] suggests he might), but otherwise there's a substantial amount of work that would be required to get to the place where we might be able to separate these species.
[1] Morrone JJ 1993. Systematic revision of the costirostris species group of the weevil genus Listroderes Schoenherr (Coleoptera: Curculionidae). Transactions of the American Entomological Society 119(4): 271--315. http://www.jstor.org/stable/25078579
[2] Morrone JJ 2002. On the species of the Listroderes costirostris complex (Coleoptera: Curculionidae: Cyclominae). Neotropica 48:70--72 [Not available online]
[3] https://inaturalist.org/observations/63416192
[4] https://inaturalist.org/observations/64152878
[5] https://inaturalist.org/observations/64073412
[6] https://inaturalist.org/observations/37372842
[7] https://inaturalist.org/observations/62375166
[8] https://inaturalist.org/observations/63931051
[9] https://inaturalist.org/observations/63749064
[10] https://inaturalist.org/observations/44851684
[11] https://inaturalist.org/observations/26503036
[12] https://inaturalist.org/observations/62109047
[13] "L. costirostris is currently restricted to CA and rarely from western AZ. All other North American vegetable weevil specimens are currently L. difficilis [that name will be changed soon], and are misidentified in most collections as L. costirostris. --C.W. O'Brien" https://bugguide.net/node/view/248280
Comentarios
"females of these species are impossible to separate"
Comments like these should be read as, "the author does not know how toe separate...". :) Sometimes two species will seem nearly identical but then someone finds a character to easily distinguish them. Of course, that's only possible if you have a set accurately separated to pore over.
if there's not a node for the complex, I'd suggest one be created, and we can put identifications there.
BTW, do you know why Cyclominae are called underwater weevils? I had no idea they were associated with water somehow.
Many species of Listronotus are partly aquatic, or are associated with plants usually found in swampy areas. I assume that this connection is what has lead to the name "underwater weevils". However, most other Cyclominae are not at all associated with water, and I think the name is somewhat misleading.
Thank you @sdjbrown for this informative post. Very interesting.
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