The seasonal dynamics of Tetranychus urticae Kochon clover and cotton crops were investigated at Qaha research Station, Qalubyia governorate, Egypt during two successive years, 2013 and 2014. Infestation of clover and cotton started in January and June, respectively. The monthly collected number and percent occurrence of eggs and most movable life stages gradually increased to reach peaks in March and September then decreased in next month(s) at the end of the season of each clover and cotton, respectively. Eggs were the most abundant life stage of the total stages collected in each month and adult females predominated males and most other movable life stages monthly collected throughout the clover and cotton season in each year. Generally, the annual number of each life stage of T. urticaeinfested each of the clover and cotton was higher in 2014 than 2013 and on clover than cotton in each year. Laboratory rearing of T. urticaeon leaf discs of each clover and cotton at three different temperatures (15, 25 and 35˚C) and a relative humidity of 69±10 % showed that change of temperatures greatly affected development and reproduction of this mite. At 15˚C, no eggs were laid and development of the whole life cycle was arrested. However, development from egg to adult was accelerated by increasing temperature from 25 to 35˚C which shortened the durations of development of immature stages, life cycle, adult female reproductive period (longevity) and life span. Fecundity and fertility were higher at 25˚C than 35˚C on each of clover and cotton and were the highest on clover at 25˚C.