Spiny bollworm, Earias insulana Boisduval. (Lep.: Noctuidae), causes huge losses to cotton crops each year. As a relatively hardy species of insect, E. insulana tolerates a wide range of environmental conditions and so is prevalent in many regions of the world. Effect of sex pheromone traps to control spiny bollworm by mass trapping method was studied. Investigation of monthly changes in captures in relation to temperature carried out during 2012 in the Darab region of the Fars Province of Iran. The experiment was carried out in randomised complete blocks design with four treatments and four replications. The treatments used application of the sex pheromone trap at the rates of 16, 20, 24 and 30 traps/ha. The captured male moths were collected and counted every three days. Analysis of the variance of results showed significant differences between time and trap number in the trapping values of E. insulana. Moth populations were observed in the second week of September. The highest and lowest captured insects per hectare were 30 and 16 traps/ha, respectively. The peak captured E. insulana adult males in the sampling period was on 18 November. These results enable forecasting of seasonal E. insulana population peaks, providing additional information vital for the development of a successful, integrated pest-management programme for spiny bollworm.