Piesma quadratum is the only vector of beet leaf curl - virosis affecting sugar beet. The sugar content of beets infected by beet leaf curl is reduced by an average of 9% and the root weight is decreased by an average of 58.4%. P quadratum appeared in Poland in the beginning of this century after migration from the territory of Germany. This bug has one generation a year. Its occurrence is characterised by 2-3-years-long gradations in periods of every 9-11 years. Chemical control of beet leaf bug caused disturbance of this rhythm. Recent gradations of the pest occurred every 17 years.