Field experiments were carried out in cotton felids at Manzala district, Dakhlia Governorate during two successively seasons 2008 and 2009 to evaluate the release of C. undecimpunctata for controlling certain piercing-sucking insects infesting cotton fields. The obtained results in untreated area revealed that were four peaks for the piercing-sucking insects in cotton fields during the two seasons of study at Manzala district. The predator prey ratio in this area was 4.56, in 2008 season, while it was 4.64 in 2009 season at the beginning of study, then these ratios increased gradually and reached their maximum 1: 19.6 in 2008 and 1: 16.78 in 2009. In the treated area with C. undecimpuctata the ratio exceeded the critical ratio (1: 10) in the following dates: the third week of June, in third week of July, in the second week of August and in the end of August during 2008 season. The number of this coccinellid predator needed to justify the ratio were 2500, 2000, 2000 and 2500 individuals, in the four dates of releasing respectively. While the predator prey ratio exceeded the critical ratio (1:10) in the following dates: in the second week of June, in the first week of July, in the first week of August and the end of August during 2009 season. The number needed from this predator to justify the ratio were two thousands individuals in the
four dates of releasing respectively. The results showed clearly the possibility of using C. undecimpunctata as a biological control agent against the piercing-sucking insect in cotton crop.