Hi Clara. The yellowing on the leaves of your peace lily is likely from a watering issue or a light issue. More than likely, I'm guessing that it may be an issue of overwatering which is common with houseplants. Before you water again, make sure to let the soil dry out and be sure the pot has drainage so that the excess water can drain away. Those insects you mention may be fungus gnats that can fly around as a result of the soil being too wet. Again, just let the soil dry out and those will do away. You're welcome to email me some photos if you have any.