Euparthenos is a monotypic moth genus in the family Erebidae erected by Augustus Radcliffe Grote in 1876. Its only species, Euparthenos nubilis, the locust underwing, was first described by Jacob Hübner in 1823. The adults resemble some of the underwing moths of genus Catocala, which are fairly close relatives, in color, pattern, and the habit of resting on tree trunks. But E. nubilis can usually be immediately recognized by the four concentric black bands per hindwing, as opposed to one or two (at most three) in Catocala. Color morphs of E. nubilis with altered pattern are known, however, and these may be hard to recognize without detailed examination.

This moth has light grey forewings with a pattern of darker grey and brown lines and shading. The hindwings are yellow orange with the aforementioned four black bands each. The wingspan is 56–70 mm (over 2 to almost 3 inches.

The species is found in North America from Maine, Ontario and Quebec, south to northern Florida, west to Nebraska and Arizona. Adults are on wing from April to September. There are two generations per year. The larvae feed on Robinia (locust tree) species, such as black locust (R. pseudoacacia). The adults like to drink the juice of fermenting fruit, and are attracted to lights.

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