The mosquito family Culicidae is the most common disease vector in the Diptera order. Almost all of them are bloodsuckers that cause a lot of major diseases, such as malaria, yellow fever and elephantiasis. The development of resistance in mosquitos to pyrethroid, organophosphate, and carbamate created the need for alternative insecticides. Therefore, 3^rd instar Culex pipiens larvae were treated with the median lethal concentrations (LC₅₀) of imidacloprid (IMI) and tannic acid. Biological tests were performed by following up the treated insects from the 3^rd larval instar till the adult stages. The biological test showed that tannic acid was more effective than imidacloprid by increasing the larval period and decreasing both pupation percentage and pupal duration. The histopathological effect of the tested compound on the larvae's midgut epithelium was studied using light and transmission electron microscopy. A histological study showed that the treated larvae had cytopathological alterations of the midgut epithelium, muscular layers, epithelial cells and internal organelles. Both IMI and tannic acid showed nearly the same effect on the larvae histology. Based on these results, tested compounds could be used in the Integrated Pest Management (IPM) programs for greater effectiveness.