Boric acid is a fantastic insect killer, and it is safe to use - it is less toxic than table salt (that doesn't mean that you should mix it with your food, though).
Do some internet research first to determine how to use it for your intended application. Remember that it does not kill on contact and it does not serve as an immediate kill spray. For ants (my intended application), it must be mixed with sugar and hot water (1 tsp sugar, 1 Tbsp boric acid, 4 oz water - heat until it dissolves). Drip this on sealed surfaces, or on cotton balls, and ants will be drawn to this in hoards. After a few days of gluttonous feasting, ants disappear.