Laboratory experiments were carried out in the Insectary of Economic Entomology Department Faculty of Agriculture, Mansoura University study the relationship betweenthe developmental stages of the predator, Rodalia cardinals (Mulsant)and the required thermal units at threeconstant temperatures, (15, 25 and 30˚C) on the third nymphal instar of Icerya purchasi Maskell. The results indicated that, the duration of the predator was longer at 15˚C. As temperature increased from 15 to 30˚C the developmental aspects of R. cardinals decreased. The lower thermal threshold for thedifferent stages were 10.5, 8.5, 5.3, 5.2, 5.4, 6.9, 0.8 and 1.3˚Cfor egg stage, larval stage, pupal stage, preoviposition period, oviposition period, postoviposition period, adult female longevity and adult male longevity, respectively. The heat units required for the development of eggs was 56.2 DD unite, while for larvae, pupae, adult female and adult male were 211.3, 114.7, 969.3 and 664.4, consecutively. The results revealed that, the longevity of the predator adult stage decreased with increasing the temperature degrees. The highest consumption rate per female was at 15 ˚C comparing with the other temperature degrees. The highest number of deposited eggs per female was 25 ˚C with the value of 443.71 eggs/ female. Obtained results provide essential information for predicting the field population of the predator R. cardinals, releasing time on certain mealybug species for controlling this pest and it's recommended to be an item of integrated pest management programs in Egyptian field designed to control certain mealybug species.