Hydrellia wirthi is very similar to several species of the griseola species group, particularly H. bicarina Deonier, H. philippina Ferino, and H. spinicrus Cresson. Externally, we cannot consistently distinguish between these species and rely primarily on structures of the male terminalia to differentiate between them. Like H. bicarina and H. philippina, the fused surstyli are deeply and narrowly cleft medioapically and there are also lateral clefts, forming short, lateral, digitiform surstylar processes (Fig. 6). In addition, these species have a conspicuous carina at the basal merger of the surstyli. In H. bicarina, this carina is deeply bilobed with a fin-like and a rudder-like lobe. In H. philippina there is a single, rudder-like carina, and in H. wirthi, the carina is shaped like a single, vertically elongate, keel-like process (Fig. 7). The surstylar carina is best observed in lateral view and is sometimes extended and visible in dried specimens.
Although the postgonites are similar in H. wirthi and H. bicarina, the apical half in lateral view is narrower in H. wirthi, and the base of the robust, tooth-like seta is short and the tooth-like seta is long and acutely pointed. In H. bicarina the apical half of the postgonite is generally wider and the base of the enlarged seta is L-shaped and elongate, and the enlarged, apical seta of the postgonite is much smaller and is bluntly rounded apically.
Another species of Hydrellia, H. griseola Fallén, has also been reported to be a pest in rice in the United States (DeOng 1922, Lange et al. 1953, Grigarick 1959). In Europe as early as the mid 19th Century, this species was identified as a pest in other gramineous hosts of economic importance, such as barley (Lilljeborg 1861) and later in oats (Balachowsky and Mesnil 1935). This is the only cosmopolitan species of Hydrellia, and Deonier (1998) reported it from 42 gramineous genera and numerous nongramineous genera. In North America, H. griseola bears the common name of "smaller rice leafminer" (in Europe, this species is sometimes known as the barley miner). Externally, especially in coloration and setation, H. griseola and H. wirthi are very similar. Males of H. griseola, however, do not have a basal, surstylar keel, and there are no lateral, digitiform surstylar processes.
We have proposed the common name of "South American rice miner" because this species was first described from specimens collected in Peru (Korytkowski 1982). The type locality is in Peru (Lambayeque: Ferreñafe), and the species has since been found in Colombia (Pantoja et al. 1993, Pantoja and Salazar 1993, Salazar et al. 1993). In 2001, we discovered this species in Costa Rica, and only in 2004 was it found in Louisiana and Texas. Deonier (1971, 1998), who comprehensively treated the Nearctic species of the genus Hydrellia, did not report this species. Thus we are of the opinion that it is probably a recent invasive species, perhaps in the last 10-15 yr, and that it most likely came from South America. Affected fields in 2004 and 2005 in Louisiana include Acadia, Cameron, Jefferson Davis, Allen, St. Martin, Vermilion, Concordia, and Tensas parishes and in Texas in Calhoun, Colorado, Jackson, Jefferson, Matagorda, and Wharton counties. (from Mathis et al. 2006)