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The melon aphid, Aphis gossypii Glover (Hemiptera: Aphididae), is one of the most important pests of agricultural products in Iran. In this study, susceptibility of melon aphid that collected from seven regions (Yazd, Varamin, Mohammadshahr, Esfehan, Karaj, Hashtgerd and Ramsar) was compared to pirimicarb and malathion after 24 hours in 2009. Bioassay experiments on adults were carried out by using leaf dipping method on squash (Cucurbita pepo) leaf in different concentrations of insecticides in laboratory conditions (25 ± 0.2 ºC, 60 ± 5%, 16: 8 [L: D] h). Control treatment was dipped in distilled water. Each test was replicated three times. The results showed that LC₅₀ values of malathion for Ramsar, Karaj, Mohammadshahr, Hashtgerd, Varamin, Esfehan, and Yazd populations were 162.99, 159.19, 117.94, 79.96, 38.81, 36.32, and 20.25 ppm, respectively. Also, LC₅₀ values of pirimicarb for Yazd, Mohammadshahr, Kararj, Hashtgerd, Ramsar, Esfehan, and Varamin populations were 1414.16, 1359.34, 1057.62, 970.33, 881.87, 806.14, and 601.98 ppm, respectively. According to results, aphids of Varamin and Yazd populations had the most susceptibility to pirimicarb and malathion, respectively. Also, aphids of Yazd and Ramsar populations had the least susceptibility to pirimicarb and malathion, respectively. Regarding the fact that Yazd and Ramsar populations had the least susceptibility to pirimicarb and malathion, respectively, there is a possibility that melon aphids in these regions are susceptible to other organophosphate and carbamate (similar mode of action) pesticides. Therefore, in the pest management program, careful chemical control measures for melon aphid in these regions is recommended.  

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The toxicity of thiamethoxam and pirimicarb on Diaeretiella rapae (Mc'Intosh) as a parasitoid of Lipaphis erysimi (Kaltenbach), exposed to residues on glass vials, was investigated under laboratory conditions at 23 ± 2 °C, 70 ± 5% RH and 16:8 h (L: D). Moreover, in order to find the negative impacts of both insecticides on the efficiency of D. rapae, the functional response of the exposed parasitoids were assessed. The newly emerged parasitoids were exposed to LC₂₅ of the insecticides and distilled water as control. Host densities of 2, 4, 6, 8, 16, 32 and 64 were placed on canola seedlings in a transparent cylindrical container and were offered to the treated parasitoids. Type of functional response was determined by means of logistic regression method and the parameters, attack rate (a) and handling time (T_h) were calculated by nonlinear regression model using SAS software. The parasitoid exhibited type II functional response in all experiments. Attack rates in control, pirimicarb and thiamethoxam were 0.057 ± 0.01,0.059 ± 0.013 and 0.040 ± 0.01 h^-1, and handling times were 1.097 ± 0.1, 1.86 ± 0.02, 2.81 ± 0.296 h, respectively. Maximum rates of parasitism (T/T_h) were estimated 21.87, 12.9, 8.53 aphids, respectively. These observations suggest that pirimicarb with less harmful effects is the preferred candidate for controlling the mustard aphid.