Interactions between natural enemies and herbivores may be affected by application of fertilizers and different cultivars. We investigated the functional response of the predatory gall midge, Aphidoletes aphidimyza Rondani (Diptera: Cecidomyiidae) larvae to the nymphs of the melon aphid, Aphis gossypii Glover (Hemiptera: Aphididae), reared on two commonly grown cucumber cultivars in Iran (Khasib and Karim) treated with different vermicompost/soil ratios (0:100, 10:90, 20:80, and 30:70%). Based on logistic regression analysis, A. aphidimyza revealed a type II functional response to the aphid in all treatments. Attack rates and handling times of A. aphidimyza larvae on different vermicompost/soil ratios ranged from 0.076 to 0.140 h(-1) and 0.969 to 1.164 h on Khasib and from 0.092 to 0.123 h(-1) and 0.905 to 1.229 h on Karim, respectively. Furthermore, increasing the density of the melon aphid on both cultivars amended with vermicompost/soil ratios resulted in increased prey consumption by the predator. Density of trichomes increased when plants received higher concentrations of vermicompost. So, trichomes may be responsible for different attack rates and handling times of A. aphidimyza on both cultivars.