Mortality of a herbivorous insect is greater on non-indigenous congeneric compared to native rush in coastal salt marsh

Within endangered coastal salt marsh of south-east Australia, the non-indigenous rush Juncus acutus L. (Juncaceae) is displacing the native rush Juncus kraussii Hochst. (Juncaceae), with concurrent changes to the structure and composition of insect species assemblages. Here we test hypotheses that the abundance of a common and widespread sap-sucking herbivore, the planthopper Haplodelphax iuncicola Kirkaldy (Hemiptera: Delphacidae), differed between the non-indigenous J. acutus and the native J. kraussii, and that these differences were due to differential survivorship. Surveys at two salt marshes revealed that there were significantly fewer H. iuncicola on J. acutus than J. kraussii, and that J. kraussii at sites not invaded by J. acutus supported more than double the number of H. iuncicola than J. kraussii at invaded sites. A field experiment enclosing H. iuncicola on both Juncus species revealed complete mortality of planthoppers on the non-indigenous rush J. acutus in about 2weeks, whereas there was greater than 80% survivorship on the native rush J. kraussii. Measurements of plant architecture showed that J. acutus is structurally different to the native rush J. kraussii, with thicker, taller and less densely packed stems, suggesting that J. acutus might therefore provide unsuitable habitat or food resources for H. iuncicola. These results suggest that J. acutus does not play a functionally similar role to J. kraussii for native insect assemblages.