What could be a negative impact of planting crops that have been engineered to make their own pesticides?

Many genetically engineered crops are used which have genes that toxins that kill the pest insects that feed on them. The most popular crops are Bt crops that are engineered to produce toxin Bt toxin which kills pests when they feed on these crops.  

The negative impact of planting crops that have been engineered to make their own pesticides is that they kill those insects which are helpful in the pollination of the crops. So these engineered crops kill non- target insects which are beneficial for nature.

Like one species of butterfly called Monarch butterfly is decreased abruptly because its larvae are killed by the toxin produced by these engineered crops.  

prism b but im not for sure