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For desert locusts, Schistocerca gregaria (Forskal) (Orthoptera: Acrididae), the hopper density threshold of gregarization remains poorly documented. Field sampling was carried out in traditional seasonal breeding areas of Mauritania during two successive years without invasion to approximate the gregarization threshold. Hopper densities were assessed at numerous sampling sites. Vegetation was also sampled to characterize the habitats. Hopper behavior was analyzed in situ with the help of a behavioral circular arena to test our assumptions on empirical locust phases determination based on physical appearance (coloration and behavior) following FAO guidelines. The results provided a critical density value around 2.45 hoppers m(-2), above which gregarious hoppers were expected to be seen more frequently in nature. Hopper density was confirmed as the main factor explaining the presence of gregarious individuals. The level of involvement of vegetation parameters such as plant density, basal area, volume, distance between plants, greenness, or combinations of these indicators was low in explaining the observation of gregarious hoppers compared with hopper density. Vegetation cover and height were the only vegetation characteristics that could enhance the prediction of phase status with hopper density. The hoppers’ phase determined from their behavior observed in the arena was similar to that characterized through FAO guidelines phase assessment, making consistent the field sampling method. Additionnally, the use of this arena illustrated that the grouping behavior of hoppers is a gradual response to density. This study can be seen as a step forward in the estimation of hopper density thresholds of gregarization in the field. This should improve the decision making for intervention during preventive control operations.