How are insects a pest to the ecosystem?

They are a natural part. There is no danger while nature's delicate balance is maintained. An overabundance of any particular one could present some problems. It has been speculated, without their natural predators & pesticides, we would drown in them, but that's only speculation. The Earth would then lose it's atmosphere. Pesticides today are safer-they are designed to break down & structural pesticide applicators are limited to a certain distance & agricultural, FDA is strictly regulated. In addition, companies are using more & more Eco-friendly products to keep them from taking over which many well educated people say is a possibility.