Mermithids are obligate parasites of invertebrates, and most of them are parasites of insects (^{Poinar 1975}, Poinar and Stockwell 1988, Poinar and Curcic 1992). ^{Poinar (1975)} found that most of the mermithid parasitisms in Hemiptera are probably accidental infections; however, high mortality levels due to infections of the genus Hex-amermis by mermithids were recorded in Asian delphacid pests (Auchenorryncha: Delphacidae) (^{Waloff and Jervis 1987}). ^{Helden (2008)} cited the first extant nematode parasitizing adults of two other species of Auchenorrhyncha in Europe, the leafhopper Macustus grisescens (Zetterstedt 1828) (Cicadellidae), and the planthopper Javesella dubia (Kirschbaum, 1868) (Delphacidae). In North America mermithids have already been recorded parasitizing three species of Delphacidae and 26 species of Cicadellidae from Kentucky, USA (^{Sperka and Freytag 1975}). In this work, we present the first example of mermithid infection in adults and larvae of Cicadellidae for South America.

Sampling was carried out at the Estación Experimental Agropecuaria, INTA Bella Vista (28° 28' 00" South - 59° 03' 00'' West) located in Corrientes, Argentinean province during 2013-2015. Insect hosts were collected from weeds around a Citrus sinensis (L.) Osbeck orchard using an entomological sweep net. The collected specimens were preserved almost immediately in tubes with 70 % ethanol. Due to this preservation method, it has not been possible to identify the mermithids to species level, as they could not reach the adult stage.

Host species were identified based on ^{Young (1977)} and prepared following the technique proposed by ^{Mejdalani (1998)}. The parasitized specimens were deposited in the Entomological Collection of Museo de La Plata, Buenos Aires, Argentina.

Adults and larvae of the species Hortensia similis (Hemiptera: Cicadellidae) were detected carrying mermithid nematodes (Fig. 1). The parasites were found partially emerged from the host, mainly through the ventral side of the body (Fig. 1b). Of the 2133 captured specimens of H. similis, only six nymphs of fifth instar and one adult female presented parasitism. This low rate of parasitism (0.33 %), agrees with that presented by ^{Sperka and Freytag (1975)} who found a mermithid parasitism rate of only 0.3 % from 60 000 Auchenorrhyncha specimens collected. We believe that this low rate of parasitism is due to the vegetation physiognomy, along with the climatic and soil conditions (e.g. heavy rains, deluge) of the sampling site.

Figure 1 Different aspects of parasitism of Hortensia similis by a mermithid a. Nymph with a partially emerged mermithid; b. Adult with a partially emerged mermithid (ventral view); c. Post-parasitic juvenile manually removed from the host. Scale bar a and b: 3 mm, c: 5mm.