There are short and long answers to this. The short answer is: yes. In many cases, insecticides can harms plants since they are indeed poisonous. One of the big things going on in genetic engineering right now is trying to make plants resistant to insecticides. That way we can kill bugs easily and not harm the plants. This is a mixed bag: there are no known harmful effects of GMO foods, and there have been many many studies. But, sometimes this means farmers use too much pesticide and can harm the land. But without these pesticides, we would have more famines and food shortages. It's a challenging problem for sure, and one that is still being worked on.