DEET (chemical name N,N-diethyl-meta-toluamide) is used as the active ingredient in many insect repellents. Insect repellents that contain DEET offer the best protection against mosquito bites.DEET is designed for direct application to skin to repel insects, rather than kill them.EET (chemical name N,N-diethyl-meta-toluamide) is used as the active ingredient in many insect repellents. Insect repellents that contain DEET offer the best protection against mosquito bites.