Abstract Whitebrush, Aloysia gratissima (Gill. and Hook.) Troncoso (Verbenaceae), is a weed of rangelands in the southwestern United States. The genus Aloysia probably originated in southern South America, with the center of evolution in Argentina; A. gratissima is also native there. We traveled 81,550 km in Argentina, Paraguay, and Brazil over a 6-year period searching for natural enemies that could be introduced into North America for biological control of A. gratissima and other weeds of rangelands. We found 82 species of insects and four plant pathogens on various species of Aloysia, in addition to the 36 species of insects previously reported from the plant genus. Promising candidates for further research for introduction included a new species of rust fungus, Prospodium tumefaciens Lindquist that produces stem galls, a probable new species of buprestid stem-boring beetle in the tribe Agrilini, a large crown-boring cerambycid beetle probably in the genus Calocosmus, a scale in the genus Cerococcus, an oecophorid moth Timocratica sp. that feeds on bark, and a twig-girdling cerambycid near the genus Arenicia. Only the rust fungus was abundant enough in South America to cause much damage, but the other species might cause greater damage in North America if their own parasites were eliminated before release.