Rhopalum Nicaraguaense Cameron, 1904:66, ♀. ♀, San Marcos, Nicaragua (leg. Prof. C.F. Baker). - Amarante, 2002Amarante, S.T.P. 2002. A synonymic catalog of the neotropical Crabronidae and Sphecidae (Hymenoptera: Apoidea). Arquivos de Zoologia (São Paulo) 37:1-139. (distribution); Leclercq, 2002Leclercq, J. 2002. Hyménoptères Crabonides Craboniens des Amériques du genre Rhopalum Stephens, 1829. Notes fauniques de Gembloux 48:3-115. (distribution); Amarante, 2005Amarante, S.T.P. 2005. Addendum and corrections to a synonymic catalog of Neotropical Crabronidae and Sphecidae. Papéis Avulsos de Zoologia 45 (1):1-18. (distribution); Rasmussen & Asenjo, 2009Rasmussen, C. & A. Asenjo 2009. A checklist to the wasps of Peru (Hymenoptera, Aculeata). ZooKeys 15:1-78. (distribution).
Rhopalum opacum Rohwer, 1915:521, ♂. Holotype ♂, Quirigua, Guatemala, February, 1912 (U.S.N.M., No. 15084, leg. W.P. Cockerell). - Cockerell, 1946Cockerell, T.D.A. 1946. New species and subspecies collected in a month in Guatemala. Journal of the New York Entomological Society 54 (3):203-206. (distribution); Amarante, 2002Amarante, S.T.P. 2002. A synonymic catalog of the neotropical Crabronidae and Sphecidae (Hymenoptera: Apoidea). Arquivos de Zoologia (São Paulo) 37:1-139.:33 ( listed as synonym of Rhopalum nicaraguaense Cameron, 1904); Leclercq, 2002Leclercq, J. 2002. Hyménoptères Crabonides Craboniens des Amériques du genre Rhopalum Stephens, 1829. Notes fauniques de Gembloux 48:3-115.:73 ( listed as synonym of Rhopalum nicaraguaense Cameron, 1904); Amarante, 2005Amarante, S.T.P. 2005. Addendum and corrections to a synonymic catalog of Neotropical Crabronidae and Sphecidae. Papéis Avulsos de Zoologia 45 (1):1-18.:8 ( listed as synonym of Rhopalum nicaraguaense Cameron, 1904).