A method for controlling Dipteran larvae or a method for inhibiting the development of larvicidal resistance, controlling resistant populations and reducing resistance levels in Diptera by introducing a larvicidally-effective amount of a combination of a strain of Bacillus thuringiensis subspecies israelensis and a strain of Bacillus sphaericus into an environment containing Dipteran larvae; and a composition of the combination are disclosed. Preferably both strains are non-genetically modified.