The impact of injury by larvae and adults of Sitona discoideus Gyllenhal on lucerne production was studied in two consecutive seasons at two adjacent sites near Christchurch, New Zealand. The root-feeding larvae were found to be more damaging than the adults; in the first cut in the 1982–83 season in a three-year-old stand, larvae reduced dry matter yield by 18%. In the wetter following season in a one-year-old stand, second and third cut losses of 43 and 30% dry matter production, respectively, were recorded. In both seasons, the lucerne had apparently substantially recovered from larval damage by the time of the last cuts. In the drier 1982–83 season, two cuts were taken compared with four in the 1983–84 season. Adult feeding was found to reduce second cut wet yields by 20–30% in the 1982–83 season but had no measurable effect in the 1983–84 season. In both seasons, manipulated ranges of larval densities showed that damage appeared only when larval populations were in excess of a distinct larval population density threshold. At larval densities less than this threshold, yield was unaffected; above the threshold, yield losses were independent of larval density. Such a yield response curve was found to be adequately described mathematically by a simple arctan model; this approach allowed the threshold to be further defined and overall estimates to be made of the yield losses arising from larval damage. The threshold occurred at about 1200 larvae/m2 in the dry season and about 2100 larvae/m2 in the wet season. This suggested moisture sensitivity and the possibility that the crop may enter a damage-induced dormancy related to that observed during a severe drought.