Invasive plants are generally thought to have negative impacts on native communities. Rarely, however, are the impacts of invasive plants on native insects examined, limiting our understanding of the broader effects of invasive plants on native habitats. In this study, I document the use of purple loosestrife, Lythrum salicaria L. (Lythraceae), an aggressive wetland invasive, by Hemileuca sp. (Saturniidae), a native silkmoth. This buckmoth is a unique ecological variant of the more widespread buck moth, Hemileuca maia (Drury), and is of conservation concern in some areas. Hemileuca sp. was regularly observed feeding on L. salicaria in 2003 and 2004 at two sites in southern Wisconsin. Densities of late instars were as high as 12 individuals/m² and caused significant defoliation of L. salicaria with an average 34% loss of foliage relative to undamaged plants. Experimental manipulations of Hemileuca sp. caterpillars showed that herbivory could decrease seed production as much as 66%. In a laboratory assay, Hemileuca sp. feeding on L. salicaria had significantly lower pupal mass and lower survivorship to pupation (24%) relative to larvae feeding on Salix foliage (90%). Moreover, in two-choice preference assays, larvae preferred Salix over Lythrum foliage. Populations of Hemileuca sp. were subject to intense parasitism (up to 93%) by a tachinid parasitoid, Leschenaultia flavipes (Bigot). This combination of factors suggests there may be significant negative selection on Hemileuca sp. feeding on L. salicaria and that invaded habitats may be ecological sinks for the native herbivores.