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Fieldwork was performed at the experimental field of the Institut of Forage Crops, Pleven, Bulgaria during the period 2006-2009. To measure the abundance and sex ratio of Empoasca pteridis moving in alfalfa stand, net sweeping was carried out weekly during the growing season. Samples were taken along the diagonals of a crop from the plant growth to the harvest of each regrowth in the flowering stage. It was found that the population growth of leafhoppers in alfalfa reached a peak just before harvest, and the peaks increased from the first to the third harvest. The sex ratios of E. pteridis successively decreased from the first to the fourth growth cycle. In addition, the first, second and third growth cycles were initially strongly female-biased, and the sex ratio decreased over time successively and occupied the lowest value immediately before cutting. In the fourth growth cycle, the sex proportion was equalized. Significant impact on female density had the temperature factor and the predominance of females was a consequence of the higher temperature and negligible amount of rainfall. It was found also that the sex ratio of E. pteridis in the third growth cycle was characterized by a high homeostaticity and stress resistance and is the most stable in relation to the changing environment.