Compositions for combating ectoparasites, and particularly head lice and their nits comprise a low viscosity linear polysiloxane combined with a higher viscosity linear polysiloxane and at least one spreading agent. The low viscosity linear polysiloxane has a viscosity of less than 10 cSt and is present in a weight percentage of 30-40% of the formulation, while the higher viscosity linear polysiloxane has a viscosity of greater than 90 cSt and is present in a weight percentage of 35-65%.