I guess you could look into tea tree oil, etc essential oils which are known for flea prevention. Garlic etc,
I use Neem oil for fleas and tics.
Thing about neem is it is cheap and it lingers on the dogs hair so to prevent fleas or tics jumping onto the dog. It also works really good better than any other natural remedy i tried. It has a good effect on the hair and skin.
You can simply put it on raw or mix it with warm water + washing liquid and spray it on the dog, or add it too shampoo.
I generally wash the dogs with it once a month and spray it on weekly in summertime. It definitely works for my dogs.
Can spray bedding, carpets and outdoor areas too, and it is non toxic so safe to use.
If your trying to get pregnant or get the dog pregnant i wouldn't use it as it is a birth control in India a country where it is famous for it's anti bacterial and anti parasite qualities. After using it years i stopped for few months, when getting gf pregnant and we were successful. lol. So no lasting effect on that front. good to wear gloves too as it stinks like sulfur.
Let's make Mongrels great again