Parasitism by water mites in native and exotic Corixidae: are mites limiting the invasion of the water boatman Trichocorixa verticalis (Fieber, 1851)?

The water boatman Trichocorixa verticalis verticalis (Fieber 1851) is originally from North America and has been introduced into the southern Iberian Peninsula, where it has become the dominant Corixidae species in saline wetlands. The reasons for its success in saline habitats, and low abundance in low salinity habitats, are poorly known. Here we explore the potential role of water mites, which are typical parasites of hemipterans, in the invasion dynamics of T. v. verticalis. We compared infection levels between T. v. verticalis and the natives Sigara lateralis (Leach, 1817) and S. scripta (Rambur, 1840). No mites were found in saline wetlands where T. v. verticalis is highly dominant. Larvae of two mite species were identified infecting corixids in habitats of lower salinity: Hydrachna skorikowi and Eylais infundibulifera. Total parasite prevalence and prevalence of E. infundibulifera were significantly higher in T. v. verticalis compared with S. lateralis and S. scripta. Mean abundance of total infection and of E. infundibulifera and H. skorikowi were also higher in T. v. verticalis. When infected with H. skorikowi, native species harbored only one or two parasite individuals, while the smaller T. v. verticalis carried up to seven mites. When infected with E. infundibulifera, native species harboured only one parasite individual, while T. v. verticalis carried up to 6. Mite size didn't differ among host species, suggesting that all are suitable for engorgement. Both mite species showed a negative correlation between prevalence and salinity. T. v. verticalis susceptibility to parasitic mites may explain its low abundance in low salinity habitats, and may contribute to the conservation of native corixids. The success of T. v. verticalis in saline wetlands may be partly explained by the absence of parasitic mites, which are less halotolerant