Abstract The spatio-temporal distribution of adult Sitona humeralis Stephens (Coleoptera: Curculionidae), a key pest of alfalfa, was studied using sweep netting for two years in alfalfa fields. The first objective was to analyze the distribution of this insect within alfalfa and the second objective was to develop sampling plans based on fixed levels of precision for estimating S. humeralis adult populations. The following models were used to analyze the data: Taylor's power law and Iwao's patchiness regression. Our results document that in both years, Iwao's patchiness provided a better description between variance and mean density. Taylor's b and Iwao's β were both significantly >1, indicating that adults had aggregated spatial distribution. Iwao's α was significantly <0, indicating that the basic distribution component of S. humeralis is the individual insect. Optimal sample sizes for fixed precision levels of 0.10 and 0.25 were estimated. The optimum sample size fluctuated throughout the seasons and depended upon the weevil density and desired level of precision. This generally ranged from 3 to 7×15 sweeps and 16 to 46×15 sweeps to achieve precision levels of 0.25 and 0.10, respectively. With respect to the optimum sample size, the developed fixed-precision sequential sampling plans were suitable for estimating weevil density at a precision level of D=0.25. The sampling plan presented here provides a tool for research on pest management decisions of S. humeralis.