Without synthetic pesticides and herbicides there would be many more starving people in the world. However, herbicide-resistant GM crops enable the use of less toxic general herbicides. Furthermore, herbicides can be applied later than usual to herbicide-tolerant GM crops, so that weeds have longer to develop, providing habitat for insects. Such crops also need less ploughing to remove weeds, which helps conserve soil microfauna and microflora as well as reducing soil erosion. These predicted results have been borne out in both field trials and in commercial crops.
Plants typically produce their own pesticides, especially in response to perceived threats.
For example, when a potato is exposed to light, the risk of attack by insects is increased, so it turns on the production of solanines, which are potentially highly toxic pesticides. In the future, GM potatoes might be grown which produce pesticides that are equally effective against predator insects but far less toxic to humans. That, of course, was why researchers at the Rowett Institute have been adding genes that encode for insecticidal lectins to potatoes. In the near future it is expected that GM techniques will be used to produce Cassava without these toxins. A more first-world concern is allergies to nuts.