Answer:
1) They can kill Other Unintended Organisms
2) Some of the Pests might be Resistant to the Pesticide, Meaning the Pesticide no Longer Works on the Pests
3) They can Bioaccumulate in the Food Chain
Explanation:
1) Bugs need for the proper growth of plants are often killed by pesticides, often from wind blowing them onto other plants for the bugs
2) Its the same princincle as super bacteria, some bugs will likely survive the pesticide, often because of a mutation, and pass that on as it is advantageous
3) This is the most talked about issue when it comes to pesticides as it is largely blamed for endangering the eagle population in the US - with DDT in particular being pointed to