Functional response of Exochomus nigromaculatus (Col.: Coccinellidae) to different densities of Aphis nerii and Aphis craccivora ALIREZA NAZARI Faculty of Agriculture, Arak Islamic Azad University Daneshgah Ave. Arak, Iran. P.O. Box: 38135-567 JALIL HADJI-ZADEH & AHAD SAHRAGARD Faculty of Agriculture, Guilan University Km. 10 of Rasht- Qazvin Road, Tehran, Iran. P.O. Box: 41335-3179 BAHRAM TAFAGHODINIA Iranian Research Organization for Science and Technology #71, Forsat St., Engelab Ave., Tehran, Iran. P.O. Box: 15815-3538 http://www.interyet.com Abstract: - The functional response of the Ladybird beetle, Exochomus nigromaculatus, was examined at different densities of Aphis nerii and Aphis craccivora under laboratory condition (30 ±1 °C and 65 ±5% RH, with a 14-10 L/D photoperiod). For each experiment, a piece of host plant and a given density of prey and one predator were placed in a Petridis. After 24 hours, the total numbers of prey attacked was counted and experiments conducted in 5 replicates. The results revealed that, 4 th instar larvae an adult of e. nigromaculatus exhibited a type  functional response to density levels of 4, 8, 16, 32, 64, 96, and 128 aphid preys. A non-linear regression analysis (Least squares procedure) was used to estimate searching efficiency and handling time of predator. Searching efficiency and handling time of four instar larvae of the predator reared on A. nerii were 1.24 and 0.006, and for those reared on A. craccivora were 1.17 and 0.009, respectively. Searching efficiency and handling time of Adult predators reared on A. nerii were 1.12 and 0.007, and for those reared on A. craccivora were 1.01 and 0.008, respectively. The difference in prey species seems to have a contributing factor in determining the rate of predation, whereas searching efficiency in both stages of predator with prey of A. nerii was higher than A. craccivora. . Key-Words: Functional response, predator, prey, Exochomus nigromaculatus, Aphis nerii, Aphis craccivora 1- Introduction The number of prey attacked by predators is often a function of prey density [12] and [4]. The functional response has been defined as a change in the number of prey attacked as prey density changes. Several types of such functional responses have been recorded: Linear increase (type Ι); an increase decelerating to a plateau (type ΙΙ); sigmoid increase (type ΙΙΙ) and a dome shaped response (type ΙV) [4] [13] [2] and [8]. When behavioral components of predator- prey interactions are interpreted with respect to some quantitative model, the resultant descriptive parameters of the model can be applied to predictions about the dynamics of predator-prey associations [2]. An important element of such association is the “Functional