Alligator weed, Alternanthera philoxeroides (Mart.) Griseb. (Amaranthaceae) is an aggressive invader in waterways and pastures throughout most of the southeastern U.S.A., and California. Three insect biological control agents have provided substantial control of aquatic A. philoxeroides in the warmer parts of its range in the U.S.A. However, these insects are less effective against A. philoxeroides in riparian-terrestrial habitats. In this study, the host specificity of Disonycha argentinensis Jacoby, 1991, was investigated to determine its suitability as a biocontrol agent of A. philoxeroides in the U.S.A. No-choice and multiple-choice tests were conducted using 19 test plant species. Complete development of larvae took place only on A. philoxeroides and on a native plant, Alternanthera maritima (Mart.) A. St.-Hil. Larval survival on A. philoxeroides (0.55 ± 0.05) did not differ from A. maritima (0.35 ± 0.15), but development time (days) from first instar to adult was faster on A. philoxeroides (21.78 ± 0.67) than on A. maritima (27.64 ± 0.70). No-choice oviposition tests showed that fecundity was higher on A. philoxeroides (400.6 ± 115.2 eggs) than on A. maritima (93.5 ± 23.5 eggs), but egg viability (proportion) did not differ between A. philoxeroides (0.40 ± 0.14) and A. maritima (0.60 ± 0.19). In multiple-choice tests, D. argentinensis oviposited indiscriminately on all plant species tested. Collectively, these results indicate that the native A. maritima is within the ecological host range of D. argentinensis and further tests (e.g., field studies) are necessary to determine if predicted non-target impacts will be transitory or persistent.