I am a pharmacist and I truly agree with fifs2. Do not use an insectiside as a preventive measure, because the lice get ressistant. Exactly as it happened with the first antibiotics discovered. Most strings died, but the few that become resistant grew as a new-resistant infestation.
There is a good preventive, vinegar mixed with water. I do not remember the proportions, I am sure you can find it on-line. The principle of the use of the vinegar is that the vinegar is a weak acid, so it changes the surface of the hair and the lice cant "walk" on the hair. Try pouring the mix of water and vinegar on clean, wet hair and let the hair dry with the air. And when the hair is dry, it doesnt smell like vinegar.