Abstract The weevils Neochetina eichhorniae and N. bruchi were released in the USA for waterhyacinth control during the early 1970s and have since been used in many other countries. Although successful control has been reported, questions regarding the efficacy of these insects remain unresolved. This lack of consensus may be attributed to the subtle impacts of these agents on plant health and vitality or to unrealistic expectations on the part of evaluators. Previous studies that anticipated impacts to pre-existing mats were designed to document declines of established plant populations. This approach overlooks important consequences that occur during plant colonization, because suppression of growth can be as important as reduction of existing populations. We investigated the effects of Neochetina spp. on the expansion of waterhyacinth mats and the subsequent colonization of the surrounding water surface under conditions designed to simulate incipient infestations. Varying numbers of weevils (0 to 4000, in increments of 1000 weevils) were released in early April onto 10-m 2 mats at two sites, one in north Florida and one in south Florida, during 1992 and 1993, respectively. Results were more striking at the northern site. Maximum leaf area was 122 cm 2 in the control vs 65 cm 2 in the 4000-weevil plot. Maximum leaf length was 70 cm vs 51 cm. Plant coverage at the southern site increased only half as fast in the 4000-weevil plot as in controls, and this disparity was even greater at the northern site. Weevil infestations at both sites produced smaller, less intertwined mats and impeded growth of the plants. Weevil dispersal out of the plots at both sites caused weevil numbers to equalize among treatments by midsummer. However, suppression of plant growth in plots receiving the most weevils persisted several months beyond the term during which differences in weevil populations were evident, especially at the northern site. Release of weevils earlier in the year or multiple releases throughout the year might suppress growth even further.