Abstract Pseudaletia unipuncta (Haworth) is a pest in Azorean pastures that causes up to 8% yield loss mainly during summer and early autumn. The objective of this research was to develop robust sampling plans based on larval spatial distribution. The number of larvae in three 60-m2 plots on S. Miguel Island (Azores) was determined. Plots of 0.25 m2 were the most accurate unit for estimating the population size. The relationship between mean and variance fitted both Taylor’s power law and Iwao’s patchiness regression model. Relationships to determine optimum sample sizes for fixed levels of precision based on both models were developed, but demanded a heavy sampling cost for the usual precision values (0.1–0.25). To reduce sampling effort, two sequential sampling plans were developed and compared, one based on Taylor’s parameters and the other based on Iwao’s parameters. For a precision of 0.25, Taylor’s sequential sampling plan led to an average 76% reduction of the sampling effort compared with sample s...