The poison ivy sawfly, Arge humeralis Beauvois, was previously known to feed only on poison ivy, Toxicodendron radicans (L.) Kuntze. However, in 2009, larvae were discovered in southern Florida feeding on poisonwood, Metopium toxiferum (L.) Krug and Urban. To better characterize the host range of A. humeralis, we compared larval performance on T. radicans and M. toxiferum. Adults oviposited on both plant species, and days to pupation and survival to adulthood were similar. With the exception of longer 1st and 4th stadia on T. radicans, stadia were also similar. Most mortality occurred during pupation. The use of M. toxiferum by A. humeralis indicates a wider range of hosts and of ecosystems than previously known for this insect.